Marine Biology PDF Notes

TOPIC PAGE NO. UNIT-1 MARINE ENVIRONMENT AND ITS LIFE 2 Chapter 1: Divisions of marine environment (2) Chapter 2: Life in the ocean-an account (plankton, nekton, bentho...

TOPIC PAGE NO. UNIT-1 MARINE ENVIRONMENT AND ITS LIFE 2 Chapter 1: Divisions of marine environment (2) Chapter 2: Life in the ocean-an account (plankton, nekton, benthos) (4) Chapter 3: Plankton (classification, harmful, beneficial effect) (6) Chapter 4: Nekton (types, adaptations, marine mammals) (12) Chapter 5: Benthos (15) UNIT-2 MARINE PRODUCTIVITY AND ENERGY FLOW 19 Chapter 1: Influence of Environmental factors on Marine Habitat (19) Chapter 2: Primary production (24) Chapter 3: Geographical variations in productivity (27) Chapter 4: Energy flow, Food chain, Food web and ecological Pyramid (29) UNIT-3 INTERTIDAL ECOLOGY 33 Chapter 1: Rocky shore 33 Chapter 2: Sandy shore 36 Chapter 3: Muddy shore 40 UNIT-4 SUB-TIDAL ECOLOGY 43 Chapter 1: Mud banks 43 Chapter 2: Coral reefs 44 Chapter 3: Red-tide phenomenon 48 Chapter 4: Bioluminescence 51 UNIT-5 MARINE BORDERED ECO SYSTEMS 52 Chapter 1: Estuaries 52 Chapter 2: Mangroves 59 UNIT-6 MARINE ORGANISMS 62 Chapter 1: Marine mammals 62 Chapter 2: Seaweeds 70 Chapter 3: Fouling and boring organisms 74 1  MARINE BIOLOGY Unit 1: Marine Environment and its life Chapter 1: Divisions of marine environment Introduction The ocean currently covers 71% of the earth’s surface. Around two thirds of earth’s land area is found in the Northern Hemisphere which means the ocean covers 61% of the total area. The rest (39%) is land however the southern Hemisphere the ocean covers of the area as much as 80% of the area and the rest (20%) is land. Marine ecosystem is the largest aquatic system of the planet which includes oceans, coral reefs, and estuaries. Since it is a single large and a complex system, it is very difficult to deal with it as a whole. Therefore the oceanographers have divided the ocean into many zones according to physical characteristics, mainly based on depth, light and temperature. The two major zones of the ocean are the entire sea floor, or bottom region of the sea, called the benthic realm and the watery region above the sea floor is called the pelagic realm. Each of these is further subdivided into many different zones based on environmental conditions. Pelagic realm The pelagic realm is further subdivided vertically into five zones viz. epi pelagic, mesopelagic , bathypelagic , abyssopelagic and hadalpelagic zones. ii. Epipelagic zone It covers I surface of the ocean and extends upto 200m depth (all light rays are seen here initially) It is also called as the photic zone or euphotic zone. ii. Mesopelagic zone –It extends from 200 to1000 meters depth. It is also called as disphotic zone as; only blue light is seen here. It is also referred to as the “twilight zone”; its lower boundary in the tropics is the 10º C isotherm. iii. Bathypelagic zone – This zone extends from 1000 upto 4000 meters deep (aphotic zone; no light reaches this depth, there is total darkness); It lies between the boundaries of water with 10 and 4º C isotherm layers. iv. Abyssopelagic zone – It lies below 2000 and extends upto 6000 meters depth (aphotic zone). v. Hadalpelagic zone – has a depth of 6000-10000 meters (aphotic zone). The epipelagic photic euphotic zone is the ideal place for about 90% of all ocean life to live because of warm temperature and sunlight.This is the only zone to support plant life because it has the light needed for photosynthesis. As this region supports diverse plant life, variety of animals such as zooplankton, crustaceans, mollusks, sharks, sting rays, mackerels, tuna, seals, sea lions, sea turtles, etc., are abundant here. Therefore this region has to plant life though some sunlight penetrates through mesopelagic zone, it is not enough for photosynthesis. However this zone has octopus, squid, and hatchet fish. There animals tolerate cold temperatures, increased water pressure and darkness. Some fish have extra big eyes to help them see, while others produce their own light called bioluminescence using special organs in their bodies called photophores. Most fish do not don’t chase their food but either wait for it or stalk it. Some have sharp fangs or big mouths to help them catch their food. The bathypelagic and hadalpelagic zones, do not have as many fish as the earlier zones. The coloration of the fish is black or red, and have bioluminescence (used to lure prey). The shape of the fishes living have is globular (round is shape and no streamlining). Most the weak swimmers live here and the fishes are mostly small but some are large. The eyes of the fishes are almost small or absent. This zone is also called midnight zone or dark zone. This zone has a very intense water pressure which can be as great as two tons per square inch. Just like the mesopelagic zone, there are no plants and fewer animals which include vampire 2 squid, giant squid, amphipod, slime stars, snake dragon fish, anglerfish, oarfish and gulper eel. The sperm whale dives to these depths search of food. Only about 1 % of all ocean species live in this zone, and some do not have eyes. The Hadal zone covers the deepest parts of the ocean. This zone is totally dark and cold, with intense pressure. Creatures found here have adapted themselves to the darkness by reducing the use of eyesight. Fishes occurring heredo have eyes, and they are usually enormous, which indicates enough flashes of bioluminescent light to keep their eyes from totally deteriorating. Benthic realm The term benthic, a means the bottom ranging from the deepest parts of the ocean to the tide influenced areas. The most productive region of the benthic zone is the area over the continental margin, which is unaffected by the tides. Many groups and varieties of animals live here, a few are worms, sea pens, crustaceans, stars, and protozoa. The life in this zone is mostly made up of bottom dwellers which get most of their food from dead and decaying organisms. Therefore most of the organisms in the benthic zone are scavengers because they depend on dead flesh as their main food source. The benthic environment is further divided based on depth into five zones as given below. A cross section of the ocean, from the shore line to the deepsea, showing the location of major habitats. iii. Intertidal zone – the area between the lowest low tide and highest high tide markings, it is sometimes called the littoral zone. ii. Sublittoral zone – from the lowest low tide mark to the shelf break, about 200 m deep. This area essentially coincides with the continental shelf. iii. Bathyal zone – from the shelf break to 4000 m. This area coincides with the continental slope and rise. iv. Abyssal zone – from 4000 to 6000 m. This includes the average depth of the deep ocean floor. v. Hadal zone – sea floor deeper than 6000 m. This includes the trenches, the deepest part of the sea floor. 3 Chapter 2: Life in the ocean-an account Introduction The organisms occurring in the ocean are generally divided into pelagic and benthic organisms based on their selection of habitats. Pelagic organisms Pelagic organisms are those that live or occupy the pelagic realm of the ocean (living in the water column). They represent only two percent of marine species. The pelagic zone can be divided into photic and aphotic zones, a distinction that is especially important for photosynthetic organisms. The photic zone is the shallower part of the ocean that receives enough sunlight to support photosynthesis. This zone is about two hundred meters deep in the clearest waters, and as shallow as three meters in turbid coastal waters. The aphotic zone is where there is no light to support photosynthesis, and extends from the bottom of the photic zone to the ocean floor. Pelagic organisms include plankton, which float along with currents, and nekton, which are active swimmers. The marine habitat is said to be the ‘mother of life’ as the earliest known life forms are marine. The average depth of oceans is about 3,800 meters, which means that they represent 99 percent of the living space on the planet. Despite having 99 percent of the planet’s living space, only 250,000 of approximately 1.8 million described living species (14 percent) are marine. While the oceans lack diversity at the level of species, they are home to members of thirty-one of the thirty-four animal phyla, about twice the number of phyla that are found on land or in freshwater. Because of its vastness and humans’ inability to easily visit deep waters, the oceans remain the least studied habitats on earth. Plankton Plankton are divided into phytoplankton, which include photosynthesizing species such as algae, and zooplankton, which are consumer species. Zooplankton consist largely of copepods (tiny crustaceans). Although plankton generally drift with ocean currents, some plankton have limited mobility. For example, certain zooplankton species move towards the water surface at night to feed, when there is less danger of predation, and return to deeper waters during the day. This type of migration is termed vertical migration. Although most planktonic species are small, some are large, such as Sargassum and jellyfish. Nekton Nekton is active swimmers that use diverse means to propel themselves through the water. Some species swim using fins, tails, or flippers. Other species, such as cephalopods, move by shooting out jets of water, known as jet propulsion. Nektonic species include fish, octopus, sea turtles, whales, seals, penguins, and many others. Many nektonic species eat high in the food chain, although there are plankton-eating species (e.g., some fish) and herbivorous species (e.g., sea turtles) in addition to carnivorous ones (e.g., seals and killer whales). In the aphonic zone, there are only heterotrophic organisms that are supported mostly by organic material that rains down from the euphonic environments above. These animals live in darkness, with the exception of light produceing animals (bioluminescentones). It is common for sea creatures (especially animals living is the intermediate depths) to house luminescent bacteria within their tissues, which are able to produce light for communication, as a lure to attract prey, or to light their bottom surface to conceal their silhouette against the dimly lighted background from above. Anglerfish are deep-sea predators that attract prey near their mouths by dangling a bioluminescent lure in front of their head. The density of organisms in the deep sea is low. Because of this low density, a long period of time can pass between meals, or between encounters with the opposite sex. To deal with the problem of infrequent meals, deep-sea creatures are often gigantic compared to shallow- water relatives. Large size allows for storage of food reserves that sustain the animals between meals. Predatory fish also have large mouths and stomachs that allow them to take full advantage of any meal, regardless of size. 4  Benthic organisms Organisms that live in, on, or near the ocean floor are appropriately called benthic organisms. There represent 98 percent of all marine creatures. They occur in such familiar marine habitats as intertidal rocky shores, mud flats, sandy beaches, coral reefs, and kelp forests. The main primary producers in benthic habitats are macroscopic seaweeds that grow attached to the bottom or microscopic algae that grow within the tissues of animals such as corals, sponges, and bryozoans. Benthic animals include mobile creatures such as fish, crabs, shrimp, snails, urchins, sea stars, and slugs. Additionally, there are numerous animals that never move around as adults. These sessile animals include barnacles, sponges, oysters, mussels, corals, gorgonians, chrinoids, hydroids, and bryozoans. The commonness of sessile animals in the bottom suggests that it is a successful way of life. Their way of life combines facets of plant and animal lifestyles. Sessile invertebrates are plant like in that they obtain some of their energy from sunlight (the animals themselves do not photosynthesize, but they house photosynthetic symbionts), they are anchored in place, and they grow in a modular fashion just as the branches of a tree do. They are animal-like in that they capture and digest prey and they undergo embryonic development, often involving metamorphosis. In fact, nearly all benthic animals start life in the pelagic realm, drifting around as planktonic larva, dispersing to new habitats as they develop and feed. After a few hours to weeks of pelagic living, they sink to the ocean floor to complete life as adults. Being stuck in one place presents special challenges for sessile animals, including food acquisition, predator avoidance, and mating. Sessile animals feed by having symbiotic algae and by filtering organic particles from passing water currents. Like plants, sessile animals use structural and chemical inimize against predators, and have tremendous regenerative abilities to recover from partial predation events. Most benthic animals reproduce via external fertilization. Sperm and eggs are spawned into the water column and fertilization occurs outside the body of the female. Surprisingly, sessile barnacles must copulate to achieve internal fertilization. These animals increase their reproductive success by being hermaphroditic, thus assuring that any neighbour is a potential mate; being gregarious to assure a high density of mates; and by having a penis long enough to deliver sperm to an individual seven shell lengths away. Certain deep-sea habitats can be highly diverse. In the deep-sea vents, for example, chemosynthetic bacteria (rather than photosynthetic species) form the basis of the food chain. These bacteria obtain energy from chemical sources such as hydrogen inimize instead of from sunlight. 5 Chapter 3: Plankton Introduction The term ‘plankton’ was first used by a German Scientist, Victor Hensen (1887). The word plankton is derived from the Greek word ‘planktos’, meaning for wanderer or drifters. Plankton form the major component of pelagic life in the oceans and on which all the marine life depends. These organisms are free floating with limited locomotory powers and are transported horizontally with the mercy of the prevailing water movements such as tides, waves and currents. Plankton are often used as indicator of environmental and aquatic health because of their high sensitivity to changes such as eutrophication and pollution with their short life span. Based on Nutrition Plankton are classified into phytoplankton and zooplankton based on nutrition. Phytoplankton Phytoplankton are the free floating organisms of the sea that are capable of photosynthesizing organic matter as their food i.e. primary producers. These are the autotrophic organisms, as they bear chlorophyll and synthesise organic matter, and hence are the major primary producers in the marine ecosystems. Phytoplankters are microalgae, which include diatoms, dinoflagellates, blue-green algae, silicoflagellates, etc. Zooplankton Zooplankton are the various free floating animals, i.e. heterotrophic or primary and secondary consumers. All the animal components of the plankton are called zooplankton. They are heterotrophic in nature as they depend on the already formed organic matters for their source of food. Their food materials include phytoplankton, smaller or microzooplankton and detritus. Zooplankton encompass many different groups of animals that range in size from microscopic crustaceans to jellyfish which measure a few feet across. As many zooplankton can feed on tiny phytoplankton, and are intern eaten by larger zooplankton, fish, or even whales, zooplankton form an important and intermediate link in the food web between primary producers and the higher trophic levels. E.g., copepods, foraminiferans, siphanophores, eggs and larvae of fishes, veligers of molluscs etc. Based on life History Zooplankton are further classified into two types based on their mode of life viz. holoplankton and meroplankton. Holoplankton Holoplankton are organisms that spend their entire life as plankton. These are the permanent planktonic organisms (both phytoplankton and zooplankton) covering the whole spectrum of plankton sizes and types. E.g. most phytoplankton, some seaweeds, copepods, salps, jelly fishes etc. Meroplankton Meroplankton are organisms that spend only part of their life in the plankton. These are temporary planktonic organisms. These include the larval forms of majority of benthic invertebrates and nektonic forms. Often benthic organisms have an early planktonic stage in the life history followed by metamorphoses into an organism which settles to the bottom. There include species of seaweeds and kelps, and also crabs, shrimps, lobsters, clams, oysters, and worms among many others. The meroplanktonic forms, such as eggs and larvae of fin fishes are collectively called as ichthyoplankton. Based on size Based on size, these organisms are classified as follows into seven groups:. 1. Megaplankton - are organisms above 20 cm in size 2. Macroplankton - are organisms in size range of 2-20 cm 3. Mesoplankton - fall between 0.2&20 mm in size range 6  4. Microplankton - are organisms in size range of 20to200 µm 5. Nanoplankton – are very small organisms ranging from 2-20 µm in size. 6. Picoplankton - are minute organisms of 0.02-2.0 µm in size 7. Femtoplankton – are still smaller organisms of 0.02-0.20 µm in size. Of the seven size groups, megaplankton, macroplankton microplanktonand mesoplankton are normally captured by using standard plankton nets. Hence, they are called net plankton. The nanoplankton, picoplankton and femtoplankton cannot be collected by using plankton nets and these organisms can be obtained only by centrifuging samples of seawater, filtering water samples on fine filters such as Millipore filters, or allowing them to settle down from water samples. Phytoplankton The free-floating plant plankton are called phytoplankton and wide range of photosynthetic organisms are included under this category. Generally, phytoplankton are grouped under algae. These are usually single celled (unicellular, filamentous, or chain forming species) that inhabit the euphotic zones / epipelagic zones of surface waters in open oceans and coastal environments. The primary importance of these plants is due to their ability to capture the solar energy to make food. Phytoplankton are the prime producers of oxygen and contribute a significant portion of the oxygen found in the air we breathe. They are the base of the food pyramid. Most aquatic organisms of the higher trophic levels depend on these microscopic single-celled organisms for food and oxygen. Without them, marine animals would die off and the atmosphere would lose a major source of oxygen. There are several types of phytoplankton viz., diatoms, dinoflagellates, blue-green algae, etc. Diatoms (Class : Bacillariophyceae) Diatoms with their characteristic yellow-brown pigment that masks their green chlorophyll are also called golden algae. They are unicellular and either solitary or chain forming. The cell contents are enclosed in a unique glass (pill box), which is called as frustules and have no visible means of locomotion. The frustule is made of two parts, much like a petridish /petriplate, one valve fitting over another. The upper part (largest part) is called as epitheca and the smaller part is called as the cell wall (frustule) is made of silicon dioxide. The valves or the frustules are highly ornamented with species-specific designs, pits and perforations, which make the frustule a lot lighter in weight and also provide a place for materials to move in and out of the cell. Diatoms may occur singly or they may occur in chains of various kinds. Many species have flotation mechanisms (spines, internal oil droplets or disc shaped). Some of these are holoplanktonic and some are not planktonic at all i.e benthic. When conditions are bad they dieand sink. The cell decomposes and the frustule breakes up and mixes with sand and mud. This combination of sediments and glass frustules makes the siliceous ooze called diatomaceous earth. It is mined by human beings and used as filtering as well as insulating materials. Diatoms belong to two orders viz. Centrales and Pennales. Some bloom forming species of diatoms are known to produce harmful chemicals (i.e domoic acid) which got concentrated in animals which feed on such plankton (i.e. filter feeders). Large amounts of domoic acid consumed by mammals such as seals, sea lions and human beings by eating the shell fishes caught from the polthese may exhibit erratic inimize and finally die. , If the accumulation of domoic acid exceeds certain level in human beings it is known to cause amnesia and this poisoning is called Amnesic Shellfish Poisoning (ASP). ASP can be a life-threatening syndrome and is characterized by both gastrointestinal and neurological problems. Gastroenteritis usually develops within 24 hours of the consumption of toxic shellfish; symptoms include nausea, vomiting, abdominal cramps, and diarrhea. In severe cases, neurological symptoms also appear, normally within 48 hours of toxic shellfish consumption. These symptoms include dizziness, headache, seizures, disorientation of focus, short-term memory-loss, respiratory difficulty, and coma. Diatoms species such as Pseudonitzschia sp. And Nitzscia 7 pungens produce this acid.. This type of biotoxin is not destroyed even after cooking and can cause illness in human beings who have consumed sea food which has accumuted excess levels of this acid. Some species of diatoms can be mass cultured under controlled conditions and used as live food for larvae in shrimp and finfish hatcheries. E.g. Skeletonema costatum, Chaetoceros spp., etc. Reproduction Diatoms reproduce mainly by simple fission i.e. each diatom divides into two halves. Each half will then develop a new inner valve so that the typical box like structure of cell is recreated. Since this process continues through several generations, the average size of the diatom population gets decreased. As a result, individuals of diatom species vary in size among the population i.e. some are large and many are small. The very tiny ones can no longer undergo divisions and at that time they cast-off both upper and lower valves and become a structure called an auxospore. This spore, develop new valves of upper and lower with the original size of the parent cell size of the diatom species. This process is called restoration of cell size. Though this is the general mechanism of diatom reproduction and reestablishment of size, there are some diatoms which undergo division of cells without reduction of valve size and hence, without auxospore formation. Dinoflagellates Dinoflagellates are unicellular and very abundant next to diatoms. They have characteristics of both plants and animals. Like plants, they prepare their food materials by converting sunlight and nutrients in water into food and however, like animals, many varieties of dinoflagellates eat microscopic particles of organic matter found in the water. Some dinoflagellates even eat each other, the condition of which is known as phagotrophy. They have two whip-like appendages, called flagella, which provide some mobility. They lack an external skeleton of silicon but are impregnated with armored plates of the carbohydrate, cellulose. These generally small organisms usually solitary and rarely chain forming ones They reproduce by simple fission, as the diatoms. Each daughter cell of diatom retains half the original cellulose armor and forms a new part to replace the missing half without any reduction in size; hence, successive generations do not change in size. Some dinoflagellates are also capable of producing toxins that are released into seawater. At times, dinoflagellates become extremely abundant, the condition is known as bloom (2-8 million cells per liter), and the toxins released by these forms may affect other organisms of higher trophic levels, causing mass mortality. Such extreme concentrations, or blooms of dinoflagellates are called red tides and are responsible for massive localized mortality in fishes in various places. Some dinoflagellates have non-motile stages called Zooxanthellae, which are symbionts in the tissues of many invertebrates such as corals, sea anemones, and giant clams. The dinoflagellate species like Noctiluca scintillans is highly bioluminescent. Other Phytoplankton Constituents of the nanoplankton and picoplankton size classes (sometimes collectively called nanoplankton) include a number of photosynthetic microalgal organisms. These organisms are important in primary productivity as well as in oceanic food webs, which has only been recently realized. The important groups among these organisms are the prochlorophytes, the haptophytes (Coccolithophoridae, Haptophyceae) and the blue-green algae, also called the cyanobacteria (Cyanophyceae). The cyanobacteria are prokaryotic cells that possess chlorophyll-a, but this is not in plastids and occurs in single cells, filaments, or chains. Cyanobacteria are abundant in the tropics, where they occasionally form dense mats of filaments and discolour the water (red tide) brownish or saw-dust coloured by Trichodesmium erythraeum. The abundant haptophytes are the coccolithophores, easily distinguished by the tiny calcareous plates (coccoliths) on their outer surface. They have complex life histories, with several morphologically different cells present in the same species, and several modes of reproduction. Coccolithophores are now recognized as a major source of primary production in many ocean areas. Other less abundant microalgae include the silicoflagellates (Chrysophyceae), the cryptomonads (Cryptophyceae), and certain motile green algae (Chlorophyceae). 8  Pelagic bacteria or bacterioplankton, are also found in all oceans. They are most abundant near the sea surface and are now thought to equal or exceed the total biomass of phytoplankton. They are usually found in association with organic particles in the water column, collectively called Particulate Organic Carbon (POC), or on various gelatinous zooplankton pieces known as marine snow. They decrease markedly with depth, and their role in the microbial loop of the oceanic food web is now clearly inimize. Zooplankton Zooplankton comprise many microscopic and macroscopic animals represented by almost all the major taxa of the Kingdom Animalia. The zooplankton are ubiquitous. They eat other smaller sized zooplankton as well as phytoplankton. Almost all zooplankton are invertebrates , however some belong to the vertebrate groups ( e.g. salps).Zooplankton include both unicellular ( protozoans) and multicellular forms. Zooplankton play an important role in the aquatic food web, having the potential to affect water transparency, levels of suspended algae (phytoplankton),and the fishery. Many economically important fish depend on a diet of zooplankton during some stages in their life cycle. Some of the zooplankton species include those of copepods and krills. The krills (Euphausiids) are serving as an indicator organism for the presence of baleen whales in the polar waters. Holoplankton Holoplankton (permanent plankton) The holoplankton are composed of forms representing nearly every phylum of the animal kingdom with the exception of the sponges, bryozoans, and phoronids. Among the echinoderms, sea cucumbers belonging to two species Pelagothuria and one species of Planktothuria are planktonic throughout their life. Although all other phyla are abundantly represented in the holoplankton, no planktonic animal plays a vital role in the economy of the sea as do the crustacea of the phylum Arthropoda. However, the copepods which rank first in most parts of the ocean are important in food chain and serve as very good live food organisms is shell and finfish hatcheries. The euphausiids are of equal or greater importance as food for the larger plankton-feeding animals such as baleen whales and penguins. They also have direct commercial consequence as food for human beings. Taxonomic groups of some marine holozooplankton Phylum Sub groups Examples Protozoa Dinoflagellates Noctiluca Foraminifera Globigerina Radiolaria Radiolaria Ciliates Favella Cnidaria Medusae Aruelia Siphanophores Pyrosoma Ctenophora Pleurobrachia Chaetognatha Sagitta Annelida Tomopteris Mollusca Limacina Clione Arthropoda Cladocera Penilia avirostris Copepods Calanus sp. Euphausids Euphausia superpa Chordata Appendicularia Oikopleura dioica Salps Salpa 9 Meroplakton Meroplankton (temporary plankton) The abundance of temporary plankton is dependent upon the spawning habits of the benthic organisms. This variation in time of spawning of the various benthos i.e some spawn during different times of the year or some spawn throughout the year, the occurrence of meroplankton is always there in the seawater. Further, meroplanktonic forms are abundant mostly in the inshore waters or neritic waters as this region is in close proximity to the littoral region, where more benthos is known to live. Also as this region is known to be rich in phytoplankton, these meroplankton larval forms will be able to get sufficient food and metamorphose quickly into adult benthos. Further, the length of larval period is also an important aspect which determines the abundance of the meroplankton in the neritic zones, and this may vary from a few hours, as in the tube worm Spirorbis, to a period of perhaps four or five months, as in Emerita, the sand crab. These forms are more diverse than the holoplanktonic forms as the benthic population constitutes more than 98% of total marine organisms. Almost all the benthic species will have planktonic larval stages as meroplankton and many species are known to have series of larval stages before they become adult. For example, some decapod crustaceans ( lobsters) are known to have more than 18 different larval stages and most have more than one larval stage. Apart from marine benthos, more nektonic species spawn their eggs during different times or throughout the year and their eggs and larval forms occur throughout the year in the seawater as meroplankton. Adaptations of plankton Plankton possess a wide variety of floatation mechanisms to lead a planktonic life in the continuously moving surface of the sea water. The mechanisms involved are mainly to keep them float on the surface by reducing the rate of their sinking by increasing the surface of resistance and also by reducing their weights Floatation by increasing surface of resistance Plankton, show several important morphological and ecological adaptations essential for their survival. They show different structural / morphological adapatations to remain on the well lighted surface waters by reducing the sinking rate. These forms show inimiz body shapes to reduce the sinking by increasing the surface of resistance. For this many plankton have flattened body shapes, for example some are bladderlike, ribbonlike, needlelike, or branched types. Larvae of eels and ten-pounder fish, called leptocephalus , which are of flat or leaf like in shape. Certain phytoplankton like Chaetoceros, which possess hair-like setae on its cell and zooplankton, phyllosoma ( larva of lobster) have long projected appendageswhich help these forms in creating surface of resistance, hence aids in floating in the surface. Floatation by reducing over weight For reducing the weight of the organisms so as to live as plankton in the surface layers of the water, these organisms simply change their body fluid composition in order to make the body less dense than the seawater without affecting the salt balance/ osmotic conditions of the planktonic organisms. The most common way for this is the replacement of heavy ions in the body fluids with lighter ones. This is seen in Noctiluca, by having ammonium chloride in its internal body fluid with the specific gravity of 1.01 against the seawater value of 1.025. Similarly, plankton species like salps, ctenophores, and heteropods are known to eliminate heavy ions like SO42- from their bodies and replace them with osmotically similar but lighter chloride ions. Some plankton will have more water contents in their body to reduce the specific gravity, thereby increasing the buoyancy e.g. jelly fish. Some animals like foraminiferans have many perforations in their test or shells so as to reduce the weight of the animal to increase the buoyancy. Some are having oil globules or droplets in their body to reduce the specific gravity so as to increase the buoyancy e.g. fish eggs and diatoms. Another mechanism that is employed to reduce density is the possession of a gas float of some sort. Some siphonophores have air filled sacs called pneumatophores in its body. In the case of the blue-bottle (Physalia physalis, commonly known as Portuguese man- of- war), its float is so large which projects above the surface of the sea like a boat-sail and trails its tentacles below. This type of animal floating at the air-sea interface is termed neuston. 10  Importances of Plankton Plankton are known to have both beneficial as well as harmful effects. Phytoplankton Beneficial effects  Phytoplankton are very much important in the aquatic environments as these are the basis of the life in any aquatic system. Beingthe main primary producers in the marine ecosystem, they are often referred to as the grasses or pastures of the sea and also they form the base of the aquatic food chain, upon which all the marine life depends. About 80 % of the oxygen on the earth is known to be produced by these marine phytoplankton.  They are also playing a major role in the cycling of biogeochemicals in the marine environment and hence are of great significance on the aspects of global warming.  Phytoplankton cycles major nutrients in aquatic habitats.  Phytoplankton are used as indicators of water quality as these are very sensitive to even slight change in the environmental quality.  Phytoplankton species are known to serve as indicators of some commercially important fisheries.For example, very high abundance of diatom , Fragillaria oceanica in plankton samples indicates the presence of oil sardine fishes (Sardinella longiceps) in that location. Similarly, the abundance of diatom species Hemidiscus hardmanianus indicates the presence of lesser sardines which form the choodai fishery in the west coast of India.  They are also known to serve as a major live food item for fish larvae in the hatcheries. E.g. diatoms(Chaetoceros, Skeletonema), silicoflagellates (Isochrysis galbana) and green algae (Chlorella). Harmful effects  Some red-tide causing dinoflagellates ( Gonyaulax and Gymnodinium ) are toxic to the organisms of higher trophic levels of the aquatic systems when they form bloom. They are responsible for localized mass mortality of fishes in the marine ecosystem. Besides the mass mortality of fishes, they are also responsible for the transmission of some diseases to human beings particularly, PSP, when the shell fish harvested from the red-tide affected coastal waters is consumed by human beings. Zooplankton Beneficial effects Zooplankton play a pivotal role in the aquatic food chain and by forming an intermediate link , they transfer the energy to the higher trophic levels as they graze on phytoplankton. Most of the zooplankton form a very good food source for the larvae and adult of commercially important marine fishes. It is worth mentioning here that the abundance of jelly fishes in the sea is considered as a menace or hindrance to the fishing operations as they are known to clog the fish nets. Apart from this, areas rich in jelly fishes are also invariably observed to be devoid of fishes and that way they serve as the indicator of poor fisheries.. The presence of rich shoal of herrings and mackerels is indicated by the abundance of copepod species Calanus. The abundance of krill (Euphausia superba) indicates the presence of baleen whales to the whalers (those who catch whales) Harmful effects Some zooplankton are also known to have some adverse effects on the fishery as they are the voracious predators on the fish eggs and larvae, which may lead to poor fishery of that location. E.g., Sagitta sp. (arrow worm). 11 Chapter 4: Nekton Introduction Nekton comprise all the fast and free swimming animals of the pelagic waters. The term “nekton” was coined by Ernst Haeckel (1890) and it is derived from the Greek word nekton, which means “swimming”. These nekton are provided with efficient locomotory organs, called fins, and hence they are not at the mercy of currents. Most nekton are carnivores (flesh eaters) and predators (organisms that kill and eat other animals) and some are scavengers (flesh eaters that don’t kill what they eat). Often, predators will eat dead animals if they are available. A few nekton are carnivorous filter feeders (animals that filter large volumes of water to obtain their food, which is usually zooplankton) e.g. baleen whales. The nektonic animals include chordates and invertebrates such as molluscs and arthropods. There are no plants under this category. These animals are of different sizes and able to migrate freely throughout the oceans. They posses a variety of feeding habits and most of them feed on planktonic organisms (both phytoplankton and zooplankton) found in the marine and fresh waters. The locomotory organs are not only used to maintain their positions against the water movements for quite some time but also used to pursue prey, escape from enemies and for undertaking long distance migrations. These animals are having streamlined body with slime cover to reduce resistance while moving through the water The musculature, nervous system and vision are the special adaptations found in the nektonic animals, which help them to lead nektonic life. Oxbow theory TYPES OF NEKTON Types of nekton There are three types of nekton viz. chordates, molluscs and arthropods. The majority of the nekton species consists of carnivorous organisms which find their prey over a wide area. ii. Chordates Nekton belonging to the chordates form the largest group, having bones or cartilage. This group includes sharks, bony fish, whales, porpoises, dolphins, seals, turtles, snakes, and sea birds. There are more than 25,000 species of fishes, higher than any other group of vertebrates. iii. Fishes Fishes represent the major constituent of the nektononic vertebrates. Based on the habitat they inhabit, these are grouped into holoepipelagic and meroepipelagic. Holoepipelagic fishes are those that spend their entire lives in the epipelagic region. These fishes include most of the tropical and subtropical fishes which often lay floating eggs and have epipelagic larval life. The meroepipelagic fishes spend only part of their life in the epipelagic region and visit this region to find their prey. They may spawn in inshore or fresh water regions. Herbivorous species like the anchovy (Engraulis encrasicolus), however, play a major role in upwelling areas of the sea where enormous quantities of fish can be harvested. ii. Reptiles The nektonic marine reptiles include sea snakes and turtles. These are cold blooded animals. Though the turtles are true pelagic animals they often visit the shores for breeding.Their limbs are modified to form paddles, which enable them to swim fast, and they can also drift motionless. The leathery turtle (Dermochelys) and the hawk- billed turtles (Chelonia inimize) are carnivores, and known to feed on inimi, crustaceans, cephalopods and fishes, while the green turtle (Chelonia mydas) feeds largely on seaweeds and eel-grass. The sea-snakes are represented by more than sixty species distributed abundantly in the Indo-Pacific region. Important genera include Enhydrina, Pelamis, Microcephalophis and Laticauda. They feed mainly on the fish. They are viviparous and mostly venomous. 12 iii. Seabirds (Aves) Seabirds (Aves): Seabirds, which are warm blooded nektonic animals, play a significant role in the food chain dynamics of the marine ecosystem. Especially their interaction with marine fishes and fishing is regarded as very important.”Seabird” is rather a loose term traditionally used to cover those birds which obtain at least part of their food from the sea by travelling some distance over or under its surface. They typically breed on offshore or coastal areas like cliffs, dunes, skerries or remote islands. There are 274 species of seabirds, comprising mainly penguins, albatrosses, fulmars, petrels, shearwaters, pelicans, cormorants, skuas, ducks, terns and auks. Seabirds are characterized by long life (high adult survival rate), higher age at first maturity and breeding, slow reproductive rate and intense care for the offspring. iv. Mammals Mammals: The mammals include the seals, dugongs, manatees, dolphins, porpoises and whales, of which the whales are the most important. These are the largest members of the nekton. The blue sulphur-bottom whale, a plankton feeder, attains a length of 25 metres and a weight of 60-80 tons. The toothed whales (sperm whale, Phinimi catodon, and killer whale, Orcinus orca) are predators feeding on squids and large fish. The whalebone whales (baleen whales) on the other hand feed on plankton, mainly sieving euphausids and others from surface waters. Invertebrates Molluscs : Nektonic molluscs include octopus and squid. Unlike clams and oysters, squids have no external shells.They have a shell inside their bodies called a pen. There are about 375 species of squids. They possess 10 arms, of which two are longer than the others and are called tentacles. Squid range in size from less than an inch to more than 60 feet in length. They have long, tubular bodies and short heads. They swim very fast using a kind of jet propulsion to move. Squids have some unique adaptations. Some can change colour, some use bioluminescence to create light, and some shoot ink to cloud the water and escape from predators. They usually travel in groups and can be found in the euphotic zone and the twilight zone. There are two large groups of octopus. The cirrata or finned octopuses live in the deep sea at depths between 1000 and 24,000 feet. About 85% of octopuses are in the incirrata group. They have no fins and live in shallow water in caves or crevices. Octopuses have no shell at all, not even an inner one. They have eight tentacles. The tentacles have suction cups on them and are used to hold onto prey. The tentacles also have taste sensors that let the octopus to know whether grabbed ones are worth eating. It has a sharp beak on its mouth which is used for cracking shells. Some species may also inject prey with a toxic substance. Because it has no shell, an octopus can squeeze into very small spaces. Octopuses live alone and, like the squid, some species can shoot ink and change colours. ii. Arthropods: Among arthropods, shrimps are the major nektonic organisms. Adaptation of Nekton ii. Buoyancy : Most fish have a gas-filled swim bladder. Most fish can regulate the amount of gas in the bladder and thus control their buoyancy. Gas filled cavities (lungs) help all air-breathing nektonic animals to float. Other means used by marine mammals to increase buoyancy are bone reduction and the presence of a layer of lipids (fats or oils). Large amounts of lipids are also present in nektonic fish that do not have swim bladders (sharks, mackerels, bonito). In addition to these static means of producing buoyancy, some nektonic animals have hydrodynamic mechanisms for producing buoyancy during movement. Examples are pectoral fins and flippers and the presence of a heterocercal tail. ii. Musculature: The fishes those living in the deep sea areas have suitable adaptations to withstand the prevailing high pressures and to the dark conditions. These fishes are fragile and weak with soft and loose muscle. 13 iii. Colour: They develop black or dark brown colourations to inimize the problem of predation. iv. Eyes: Many deep sea nekton have reduced eyes or no eyes. v. Bioluminescence: Some fishes have bioluminescent organs in their body to attract their prey as well as to find their mates (e.g. angler fish) 2. Reptiles : Turtles possess a special adaptation to marine life by having buccal respiration in which a highly vascularised mucous epithelium takes up oxygen from water in the mouth. As in the case of turtles, the mucous epithelium of the buccal cavity in sea-snakes is known to be supplied with numerous capillaries which enable these snakes to take oxygen form the water. This would explain the records of sea snakes seen resting in the bottom and remaining submerged for several hours. 3. Seabirds : Seabirds swim at the sea surface and under water; they use their webbed feet, their wings, or a combination of wings and feet. They float by using fat deposits in combination with light bones and air sacs developed for flight. Their feathers are waterproofed by an oily secretion called preen, and the air trapped under their feathers helps keep the birds afloat, insulates their bodies, and prevents heat loss. When diving, the birds reduce their buoyancy by exhaling the air from their lungs and air sacs and pulling in their feathers close to their bodies to squeeze out the trapped air. The underwater swimmers such as cormorants and penguins have thicker, heavier bones, and penguins have no air sacs Marine Mammals I. Temperature Maintenance Ø Large size – less loss of body heat (reduces surface to volume ratio) Ø Insulating layers of blubber or fat Ø Adaptations of the circulatory system for reducing heat loss ii. Respiratory modifications for diving Ø Have the ability to hold their breath for extended periods of time making deep dives possible. Ø They are able to dive with their lungs empty avoiding problems of buoyancy and bends (nitrogen bubbles in the blood) Ø Their blood is rich in haemoglobin and other respiratory pigments, and the muscles are rich in myoglobin (another respiratory pigment in the muscles). Ø During deep dives, sphincters in arteries shut off blood to parts of the body so it only goes where it Ø Is needed. Ø The heart slows, and the muscles can tolerate a greater oxygen debt than terrestrial animals. Ø When the animal surfaces and breathes, a very rapid O2-CO2 exchange occurs. iii. Osmotic Adaptations Ø The kidneys reabsorb water and they excrete a very concentrated urine 14 Ø Fatty insulating layers may also play a role in water storage. Ø Most water intake comes from the fish they eat. Oceanic nekton also have adaptations for locomotion, surface resistance, defense (cryptic coloration), sensory systems (echolocation). Some have osmotic adaptations to allow for absorption of fresh water Chapter 5: Benthos Introduction All the organisms living or inhabiting in the bottom regions of the aquatic environments are termed benthos.These benthos/benthic organisms live in a variety of bottom environments of the aquatic ecosystems and are as diverse as the plankton.These bottom living organisms have direct contact with the substrate, which limits the distribution of these organisms.The factors such as the composition, size of the particles, its firmness or resistance to penetration, its lability and the food they contain, are all known to have major influence on the distribution of organisms. Classification of Benthos-Based on size Benthic organisms include both plants (phytobenthos) and animals (zoobenthos) and no vertebrates are represented as true benthos. Based on size Life in the benthic region is also classified based on size viz. macrobenthos, meiobenthos and microbenthos. Macrobenthos are organisms that are larger than one millimeter like oysters, starfish, lobsters, sea urchins, shrimp, crabs and coral. Meiobenthos are between one tenth and one millimeter in size. Organisms in this group include ciliates, annelids, kinorhynchs, copepods, etc., Microbenthos are very tiny organisms like bacteria and ciliates. They are smaller than one tenth of a millimeter. Types of meiobenthos Meiobenthos are also called as interstitial organisms (mainly fauna) which live in the spaces between the sand grains or particles of the substratum or live on the individual particles. However, meiofauna is the preferred term to refer to the organisms that live interstitially. Meiobenthos and meiofauna are synonymous terms to each other. These organisms include all animals that pass through a 0.5 mm screen but are retained on a 62 µm screen mesh. This term meiofauna is mainly based on size of the benthic organisms and it does not explain about the location of the organisms or their movements within their habitats. In order to indicate their location and to refine the classification of these forms, some other terms have been introduced as, endobenthic, mesobenthic and epibenthic. Endobenthic organisms are the meiofaunal sized organisms which move within the sediment by displacing particles (i.e as they are the bigger than the interstitial spaces of the sediment(sand) particles). Mesobenthic organisms are the meiofaunal organisms living and moving within the interstitial spaces of the sand grains. Epibenthic organisms are those that living at the sediment-water interface. Based on the mobility Based on mobility or movement, benthos are grouped into sessile or attached, sedantary and vagrant. Sessile organisms are those that do not have any mobility, attached or fixed firmly with the substratum or bottom of the aquatic environments and rely on currents or other mechanisms to bring food to them.Plants such as benthic algae (seaweeds) and sea grasses and animals such as corals, other attached forms such as barnacles, oysters, etc. are the typical examples for sessile organisms. Sedantary benthos include all the slow moving animals like snails and nudibranchs.Vagrant benthos are those that have locomotory powers and they can move very rapidly on the substratum (e.g. shore crabs). Only animals are included under this category. Based on the mode of life Based on the mode of life i.e. whether they inhabit on the surface and on the substratum or into the substratum, benthic organisms are divided into epifauna and infauna. Epifaunal organisms are those that animals which live on the 15 substratum, the infaunal organisms are those that animals live into the substratum. These terms are applied regardless of whether the bottom sediment is unconsolidated (soft) or hard rock. Organisms that penetrate or burrow into the unconsolidated bottom sediments are called burrowers, where as organisms those that penetrate or bore the hard rock or substrate materials are called borers, These organisms exhibit a variety of feeding behaviour, some scavenge across the substrate or bottom for organic debris, some are aggressive carnivores, others plough through the bottom sediments and ingesting unselectively, still others burrow into the sediment and take in food through a filtering system of inhalant and exhalent siphons (bivalves, Scrobicularia sp, and Teredo sp.). Benthic Plants(Phytobenthos) Bottom living plants include varieties of algae and angiosperms or flowering plants, particularly sea grasses. The algae include the Cyanophyta (blue-green algae), Chlorophyta (green), Phaeophyta (brown) and Rhodophyta (red), in increasing order of diversity. All range from microscopic to large except the blue-green algae, which are all microscopic. They commonly have filamentous character and occur in clusters or mats. Green algae include several calcareous species that are important contributors to the sediment substrate, especially in the shallow waters of the low latitudes.The brown algae include the largest varieties of seaweeds i.e. the kelps, which are very abundant in shallow cold waters Most of them are of commercial value as they contain algin, a gelatinous material that is used as an emulsifier in ice-creams, paints, drugs, and cosmetics. The red algae are represented by more species of seaweeds than all other classes of algae. They may inhabit relatively in the deep waters of the subtidal regions of the sea. Some are calcareous and an important constituents of low-latitude reefs. There are very few common types of marine grasses. Some of these occupy the intertidal marsh environment. For example, Juncus sp, is known to occupy the higher level and Spartina sp occupy the lower level of the intertidal marsh regions. The sea-grasses are known to occupy the sub-tidal regions, which are included by the species such as Thalassia spp. (turtle grass), and Halodule spp. live in shallow waters of the low-latitudes. In the higher latitudes, Zostera spp (eel- grass) are the dominant type of seagrasses. Benthic Animals(Zoobenthos) Marine benthic animals comprise the most diverse major group of marine organisms, with about 150,000 species known. The majority of these are epifaunal in nature. It is appropriate to discuss the benthos in detail based on their life styles, as they exhibit variety of life styles and diversity of the organisms. Sedentary or Vagrant epifauna This group of animals is the largest in terms of both the number of individuals and the diversity of types. They have two common primary features: all live on the surface of the sea floor and all have at least some ability to move. They may live on rigid substrate, firm sand or soft mud. They range in size from microscopic to over a meter in length. While some move very slowly, others move very quickly. The smallest and probably the slowest of the benthic animals are the single celled animals, which include the foraminifera. These tiny animals are typically less than a millimeter in diameter with different shaped tests or shells and most of which are multichambered. Most foraminiferans tests are composed of calcium carbonate, although some are comprised of sediment particles held together by organic material. They are a significant contributor to marine sediment. Mobility is nearly nonexistent in foraminiferans; however, the individual foraminiferan can extend its protoplasm through pores in the test and then contract it to pull the test, thus moving slowly. They feed by engulfing particulate organic matter. Most vagrant benthos are shelled macro-invertebrates, although there are some worms thatcrawl over the substrate. The shelled invertebrates are dominated by three groups viz. arthropods (crabs and lobsters), molluscs (clams, gastropods, chitons, and octopuses) and echinoderms (sea urchins, sand dollars, star fishes and sea cucumbers). Except the octopuses, the crabs and lobsters are the largest and fastest of the vagrant benthos. These jointed- leg animals can move rapidly; they use this ability along with their hard exoskeleton for protection from enemies. In addition, they have some swimming ability, using either their tails and/or specially adapted legs. Many species in this 16 group live in the shelter of rocks, ledges or other cover. They are scavengers and will eat almost anything that is available. Nearly all vagrant benthic molluscs have external shells and move slowly, on the order of millimeters or centimeters per minute. Some, such as the chitons, are completely protected by their shell and will move across a hard substrate rasping and scraping food from the surface. A few gastropods move slowly through the sediment, ingesting whatever material they encounter, with little or no selection. They digest the nutrient material and excrete the mineral sediment in the form of pellets which become an important contributor to the volume of sediment. Generally, these animals have thick, heavy shells to protect them from predators and from the enemies. Most have somewhat bulbous shapes, those that live on soft sediment may have special adaptations in shell morphology to prevent sinking into the mud. e.g. Murex sp with spines. The echinoderms, or spiny-skinned animals have bulky shapes, such as the various sea-urchins, sea cucumbers, and many of the starfish. They have numerous appendages in the form of sucker feet or spines that are used for locomotion. The sea urchins live on hard substrates where they feed on debris attached to the rocks (or) they may live on the unconsolidated sediment. Sand dollars have the poorest mobility but they can move slowly by the whisker-like feet that surround their body. These animals may be partially in faunal in that they can burry themselves just under the sediment surface for protection. Sessile Epifauna Many organisms are attached to the substrate throughout their maturity and have no mobility at all. Included are both solitary organisms and colonial ones, with as many as hundreds of individuals merged into large condominium-style skeletal complexes. Some can torn up and moved, then reattached and still carry on, where as others expire when uprooted. Virtually, all sessile epifauna are filter feeders, relying on currents to carry their food to them. There is a variety of sessile solitary invertebrates that attaches to hard substrates, typically bedrock. These include barnacles, oysters, some brachiopods, and mussels; sponges, sea anemones, and sea-lilies. Although all are macroscopic, there is some range in size, from sponges only a few millimeters across to large sea lilies with arms that may be meters long. Brachiopods and mussels (Pelecypoda) are both bivalves and filter feeders. Brachiopods (lamp shell) attach with a stem like foot that extends from near the hinge line that holds the shells together. Mussels are about the same size and they attach themselves to a hard surface with strong thread like structures, called byssus threads which develop at the hinge line. The mussels are especially well attached and can withstand vigorous wave and current action. Anemones (Coelenterata) and lilies (Echinodermata, crinoid) belong to different phyla and have markedly contrasting anatomies, but there are some similarities in their feeding activities. Both have multiple appendages that serve as the primary food gathering mechanisms. Anemones are carnivores; they grasp their prey, then envelop and digest it. Some have sticky substances or toxins in their appendages to aid in capture and submission of their pray. By contrast, the sea lilies have sophisticated system of circulating plankton and organic debris to their centrally located mouth. Barnacles are crustaceans (Arthropoda) that exist in two different forms, both sessile. Encrusting barnacles have their calcareous shells attached directly to the hand substrate, their soft part extended during feeding and retracted for protection. These are the barnacles that encrust boats, bridges, and other marine structures. The goose-necked barnacles have fleshy, stalk like structures that attach to the substrate and emanate from the shell that contains the soft parts of the organism. Both feed by removing small particles of organic debris from the water. There are also colonial varieties of sessile benthic animals. Primary members of this group are the corals, sea whips, sea fans and bryozoans. Bryozoans are small and may be encrusting or delicately branching. They have calcareous external skeletons forming lacy structures that have given this phylum the nick name “moss animals”. The corals, sea whips and sea fans are all coelenterates, the same phylum as the sea anemones. Sea whips and fans do not have hand, articulated 17 skeleton and so they disintegrate upon expiration, where as the corals have massive calcareous skeletons that house numerous individuals. All feed by filtering plankton and organic debris from the water. Infaunal Organisms This group includes various meiofauna and macrofauna such as snails, clams, worms, sea urchins, and crustaceans. Some groups are entirely infaunal, such as the tusk shells (scaphopods). Infaunal organisms occupy two different models of life. Some graze or plow through the sediment (sediment destabilizers) and others construct extensive burrow complexes that they occupy and in which they move about (sediment stabilizers). There are also those that burrow or bore near the surface and simply occupy that place; they do not move from place to place unless uprooted by waves, currents, or other organisms. Grazing or plowing organisms include some sea urchins, snails, and clams. These organisms have shells that are stream lined for this type of activity. In the case of the sea urchins, they have short, stubby spines. Such animals ingest large quantities of sediment, extract the organic debris, and them excrete the sediment in pellet form. A few types like the ghost shrimp, have great burrowing abilities and may be found over 2 m beneath the substrate. They take in suspended particles and digest the organics, then excrete the mineral sediment. This feeding style is also used by the numerous clams that burrow near the surface. They have an inhalant siphon and an exhalent siphon that are used for circulating the water through their digestive systems. Numerous varieties of worms also occupy this mode of life. These types of infaunal organisms typically move only when they are exhumed from their burrow. The types of meiofaunal organisms are represented by a broad range of invertebrate phyla. These meiofaunal organisms have a size range similar to that of some of the smaller mesoplankton and the microplankton. These include the members of the phyla Ciliophora, turbellarians of the Platyhelminthes, Gastrotricha, Kinorhyncha, Tardigrada, Annelida, and Arthropoda. These organisms are very abundant only in the intertidal beaches and their biomass decreases with increasing depth in the oceans. The abundant groups are nematodes and harpacticoid copepods. These meiofauna forms a very good food source to most of the macrofaunal deposit feeders like larger polychaetes, holothurids, fishes such as young ones flat fishes, gobies and mullets. The role of these as food of the macrofaunal organisms mainly depends on the nature of sediments. That is the muddy sediment is known to harbour more meiofaunal biomass in the top layer, which is more accessible to the predators than the sandy sediment. These infaunal meiobenthos are also known to exhibit a variety of feeding habits viz. herbivores- feeds on the attached diatoms; detritus feeders, suspension feeders and predators. Suspension feeders are quite rare, due to lack of plankton availability. They prefer to feed on bacteria and microalgae attached to the sand grains. Large bodied animals and sessile benthos are poorly represented in the marine meiobenthos, for example, members of the phyla Echinodermata and Cnidaria. The members of the phyla such as Phoronida, Pogonophora, Porifera, Ctenophora, Hemichordata and Chaetognatha are totally absent in the meiobenthic communities. There are some organisms, such as certain clams and sponges that can bore into solid rock or shells. This is done through a combination of physical rasping and chemical reaction between substances secreted by the organisms and the substrate. 18 Unit 2: Marine Productivity and Energy flow Chapter 1: Influence of Environmental factors on Marine Habitat Introduction The sea covers about 70% of the earth’s surface and is a great reservoir of life. Among the three major habitats of the biosphere, marine realm provides the largest inhabitable space for living organisms. The study of organisms in relation to oceanic environment is known as marine ecology. The factors affecting the marine habitat are enlisted as follows:- Temperature Ocean is the largest store house of the sun’s heat and it occupies much space. This stored heat of the ocean is able to regulate the temperature of the world. The extremes of temperature range from – 3 to 40º C, while in the Indian seas the temperature ranges between 18 to 25º C at the surface. Seasonal variations of temperature in tropical waters are not much. There is always a direct stratification in sea and the temperature of the bottom water of the deep sea may be about – 1º C. The density of the sea water increases with decrease in temperature. Similarly the solubility of oxygen also increases generally with a lowering of temperature. Seawater temperature affects marine organisms by changing the reaction rates within their cell(s). Although each species has a specific range of temperature at which it can live, the warmer the water gets the faster the reactions and the cooler the water the slower the reactions. An organism's response to water temperature is considered to be cold blooded (or poikilothermic) or warm blooded (homiothermic) depending on their ability to control their internal body temperature. If any species is moved out of its temperature tolerance range, it may die in a short time although temperatures on the cool side of the range are easier for organisms to tolerate than temperatures on the warm side because cell reactions just slow down in the cold but may speed up over six times the normal levels for each 10º C of heat. Cold blooded (poikilothermic) marine organisms lack any body temperature regulatory controls. These include marine plants, invertebrates, most fish and marine reptiles. These species each have their specific temperature tolerance range within which they must live (some are adapted to polar temperatures, some to tropical temperatures). Some have a narrow range (and are thus very restricted) and some have a wide range (and are thus less restricted). Warm blooded (homiothermic) marine organisms have some type of internal temperature controls to maintain a constant body temperature. These organisms include a few fish, all sea birds and mammals. This ability allows these warm blooded marine species to migrate over vast distances through various temperatures without problems and include some of the animals on Earth with the longest migrations. Marine animals show a varied response to temperature. The stenothermal animals like reef corals, salps and heteropods are always found around 20º C. Eurythermal animals like Cardium and Arenicola are able to withstand wide ranges of temperature. Temperature difference in the sea though not very conspicuous yet acts as effective barrier for the distribution of animals. Marine animals present certain structural variations in relation to temperature. The number of vertebrae in fishes of colder regions is more. The fish species of flounders have 35 vertebrae in the warmer regions while in the colder regions they may have up to 50. The cold water forms also show an increase in size. This is because it takes a longer time for the cold water forms to attain sexual maturity and thus they get a chance to grow till then. There are however a few exceptions to this rule. The sea urchin, Echinus esculentus, and the gastropod Urofolpinx cinerea, show a larger maximum size in warmer waters. There is also an increase in respiratory rates in many marine organisms. In Mytilus edulis , the respiratory rate increases with temperature up to the optimum limit and then it slowly decreases. A similar behaviour is found in Calanus finmarchicus and Emerita sp. 19  Salinity The salinity of the open ocean at about 300 metres depth is about 3.5%. There is a slight variation in salinity in some seas, as in the Mediterranean where it is 3.9% while in the red sea it is 4.6%. The salinity of the sea is due to the two elements sodium and chlorine which account for 80% of the salts of the sea. The composition of chemicals contributing to seawater salinity is given in the following table. In the sea water, cations and anions are not balanced against each other. As a result, sea water is weakly alkaline (pH 8 to 8.3) and strongly buffered. This factor possesses much biological importance. The various salts of the sea are indispensable to the marine life. Animals absorb and utilize many substances like Ca, Na, K, Mg, S, C l etc. They also use many inorganic materials like Na, Mg, Ca, and silicic acid to build their bodies. A few animals even use and store rare elements. Strontium sulphate is utilized by some radiolarians. Bromine and iodine is stored by horny corals and vanadium is used by ascidians. An increase or decrease of salinity brings about changes in the specific gravity of the sea water. All marine animals are affected by changes in specific gravity. Only some animals like the teleostean fishes have the swim bladders which are used for hydrostatic control. Animals with hard skeletal materials of calcium and silicon face the problem of sinking. These animals however have various adaptations developed to keep themselves afloat, which include reduction of calcium contents by having perforations in the shell of foramnifers and thin shell in radiolarians. Some pelagic molluscs have thin and uncalcified shells, which aids in floatation. Osmotic properties of the seawater present another problem to some of the marine animals. Most of the marine animals are isotonic with seawater and when they come across any change in salinity, they are put to much difficulty. The stenohaline animals have a restricted distribution. These animals are usually found in the open oceans far away from estuaries and below the level of tidal variation and only a few metres below the surface. Euryhaline animals include the coastal forms found between tide levels. Arenicola, Mytilus, Sagitta and Oikopleura are some good examples. Some animals like the shore crab, Carcinus, can tolerate lower salinity at higher temperature. The younger organisms have lesser tolerance for lower salinity than their adults. Salinity has a profound effect on the respiratory activities of marine animals. The respiratory rates increase with a reduction in salinity. The animals spend much of their energy in osmoregulation, when salinity falls and this leads to a higher rate of oxygen consumption and higher respiratory rates. The highest respiratory rates are found in estuarine forms like Hydrobia, Carophium and Pygospio. More calcium carbonate is deposited in the skeleton of molluscs, crustaceans and other animals living in the water with a high salinity content. The molluscs found living in lower salinity have thinner shells. Animals can tolerate lower salinity when the temperature is high. LIGHT The availability of light in the marine ecosystems plays a major role , mainly in the process of photosynthesis by plant communities in the sea. Light that is falling or penetrating into the seawater is absorbed by the microscopic as well as macro vegetation like seaweeds and synthesise organic matter. This is called primary production, with which the consumers of the higher trophic levels of the marine food chain depends for their food. When there is good light, then there will be a good amount of plants and hence good organic matter, which determines the distribution of animals in the sea. The vision of the marine animals is also controlled by the light availability and also light plays an important role in the breeding cycles of the many marine animals. 20 Of all the light rays emitted by the sun is not fully reaching the ocean surface and only 50% of it striking the ocean surface. The rest of the sun light is scattered, absorbed or reflected back into the atmosphere. The reflection of the same depends up on the various factors like angle of incidence of sun light on the ocean surface, seasons of the year, the time of the day and the location or latitude. In the equator, the sun light radiation is fairly constant throughout the year when compared to the higher latitudes. That is at near the poles, the availability of sunlight is continuous during the summer and a continuous darkness in winter seasons. Of all the light falling on the surface , only 1% of the falling light will be penetrating down the sea and the remainder is absorbed and is scattered by the particles suspended in the water. The penetration of light also varies with the wave lengths of the light. Not all the rays are penetrating deep in the water and some of them are absorbed in the top few meters of the water column. Red light is absorbed in the top few meters depth whereas the blue light rays penetrates up to a depth of about 150 meters in the sea. The penetration of light is also influenced much by the turbidity or clarity of the water. If the water is more turbid in nature, the most of the light will be absorbed in the top few meters of depth whereas in the clear waters it will penetrate to the deeper levels of the sea i.e about 1000 m depth. Zonation of sea on the basis of intensity of light On the basis of the penetration of light, the ocean is vertically divisible into three zones:- a. Euphotic stratum – This extends from 0-80 meters. This may also be called as “illuminated zone” of “Producing zone”. This zone is very rich in phytoplankton and primary consumers and secondary consumers. b. Dysphotic stratum – This zone extends from 80 to 200 meters and is weakly lighted. Animals here are mostly secondary consumers and a few primary consumers. Plant life is rare. c. Aphotic stratum – This zone extends below 200 meters where light is completely absent. The producers are absent as photosynthesis cannot take place. The animals in this region are secondary consumers and also feed on other animals. The only light available in this zone is mainly of light produced by the bioluminescent animals (living light or cold light). Harmful Effects of light Light which is very essential for the photosynthesis of plants and generally beneficial to animals may sometimes be harmful. The violet and ultra-violet parts of the spectrum have harmful effect on animals. In order to avoid these harmful effects, many animals become nocturnal. By taking up a nocturnal habit, these animals also avoid the predators which can easily spot them in good light. Diatoms react to excessive light by clumping. Many animals avoid excessive illumination by diurnal migrations. Animals like corals which cannot move about, open their polyps only in dim light and close them when there is bright light. Intertidal animals also become nocturnal. Ligia oceanica comes out from crevices only in night. Mytilus edulis closes its shells in excess of light and opens only in night. Chiton also lives in areas where shade is available. It is negatively phototropic when young. As regards the zooplankton, the young stages do not live at the same level as their adults. Pressure Pressure affects marine animals in various ways. The deposition of calcium is difficult at heavy pressure. Carbon dioxide accumulates in high pressure and makes calcium carbonate more soluble. Animals that have a great vertical range in the 21 sea are referred to as eurybathic. On the other hand, animals which are limited to a narrow range of depth are called stenobathic animals. The fish, Chimaera and the snail, Turris are stenobathic. Waves The waves are caused by wind. They have their maximum effect in the intertidal zones. The maximum height that is normally reached by the waves in the oceans is 17 metres. When the waves strike, the impact is very heavy and can even turn huge stones. The force of impact of the waves is roughly about 1.5 kg. per cm2. Animals unless well attached will easily be dislodged from the substratum and thrown away or dashed against rocks. Wave action is also beneficial to animals. It helps in the mixing of oxygen with water. The breaking waves that cause spray, wet many animals and save them from drying. The shape of reef corals is modified by wave action. TIDES The tides are caused by the gravitational pull of the moon and sun on the waters of the ocean. As the earth is continuously rotating, the water is heaped in some areas causing a rise in water level in some places and reduction in other areas. The tides may appear once in 12 and half hours. These tides are called diurnal tides. The lunar tides are almost double the size of the solar tides and normally mask them except on two days viz. full moon and new moon when the pulls work together and increase the height. This is the reason why tides on the full moon and new moon days have larger amplitude. Those on full moon will be larger than on new moon. The tidal range in most of the places is in between 5 and 7 metres. The ecological effect of tides depends on two factors: (1) the duration of exposure or immersion, and (2) the time at which the exposure occurs. If the duration of exposure is very long, it will affect the intertidal fauna very much by causing desiccation and osmotic problems. Exposure during midday and that too in summer will also affect the fauna adversely. Currents The sea is in continuous circulation. Air temperature differences between poles and equator set up strong winds which create definite currents in the oceans. Due to currents, the cool dense water from the pole moves towards the equator and it gradually becomes warmer as it reaches the equator. From the equator, water again moves towards the pole and becomes cooler and dense as it reaches the pole. Because of the circulation, oxygen depletion or stagnation so common in freshwater lakes, is rare in the oceans. Currents transport food materials and remove waste materials. Currents also distribute the planktonic larvae to the different parts of the world. They also break the temperature barriers in the ocean by constantly circulating the waters. Currents can be divided into three kinds:- a. Density currents b. Tidal currents c. Wind currents Density currents are produced by the heating action of sun on the ocean water. As the warm water rises up and spreads on the surface, the cold water sinks down for the cold region and spreads downwards. Tidal currents are produced by the gravitational pull of moon and sun on oceanic waters. Wind currents are caused by the effect of winds blowing on the waters in a slanting manner. These currents affect only the surface waters. Dissolved Gases The concentration of dissolved oxygen and carbon dioxide are very important for marine life forms. Although both oxygen and carbon dioxide are a gas when outside the water, they dissolve to a certain extent in liquid seawater. Dissolved oxygen is what animals with gills use for respiration (their gills extract the dissolved oxygen from the water flowing over the gill filaments). Dissolved carbon dioxide is what marine plants use for photosynthesis. 22 The amount of dissolved gases varies according to the types of life forms in the water. Most living species need oxygen to keep their cells alive (both plants and animals) and are constantly using it up. Replenishment of dissolved oxygen comes from the photosynthetic activity of plants (during daylight hours only) and from surface diffusion (to a lesser extent). If there are a large number of plants in marine water mass then the oxygen levels can be quite high during the day. If there are few plants but a large number of animals in marine water mass then the oxygen levels can be quite low. The amount of dissolved gases in seawater depends more on the types of life forms (plants and animals) that are present and their relative proportions. Dissolved Nutrients Nutrients, like nitrogen (N), phosphorous (P), and potassium (K), are important for plant growth. The level of dissolved nutrients increases from animal faeces and decomposition (bacteria, fungi). Surface water often may be lacking in nutrients because faeces and dead matter tend to settle to the bottom of the ocean. Most decomposition is thus at the bottom of the ocean. In the oceans, most surface water is separated from bottom water by a thermocline (seasonal in temperate and marginal polar regions, constant in tropics) which means that once surface nutrients get used up (by the plants there), they become a limiting factor for the growth of new plants. Plants must be at the surface for the light. Nutrients are returned to surface waters by a special type of current called 'upwelling' and it is in these areas of upwelling that we find the highest productivity of marine life. Silica and iron may also be considered important marine nutrients as their lack can limit the amount of productivity in an area. Silica is needed by diatoms (one of the main phytoplankton that forms the base of many marine food chains). Iron is just recently being discovered to be a limiting factor for phytoplankton. pH pH is a measure of the acidity or alkalinity of a substance and is one of the stable measurements in seawater. Ocean water has an excellent buffering system with the interaction of carbon dioxide and water so that it is generally always at a pH of 7.5 to 8.5. Neutral water is a pH of 7 while acidic substances are less than 7 (down to 1, which is highly acidic) and alkaline substances are more than 7 (up to 14, which is highly alkaline). Anything either highly acid or alkaline would kill marine life but the oceans are very stable with regard to pH. If seawater is out of normal range (7.5-8.5), then something would be horribly wrong. 23 Chapter 2: Primary production Introduction All heterotrophic life in the aquatic systems depend on the production of organic matter by primary producers. In the near-surface waters, the primary producers are green plants which convert carbon dioxide and water into organic matter using sunlight as the energy source (photosynthesis). In simplified form, the photosynthetic reaction is written as follows: 6CO2 + 6H2O + sunlight → C6H12O6 (carbohydrate) + 6O2 In this reaction, carbon dioxide and water in the presence of sunlight are converted to simple sugars and oxygen. The energy required for metabolic activity is derived by reversing this reaction (respiration), i.e., oxygen and sugar react to release energy, carbon dioxide and water. In the case of the primary producers (also referred to as autotrophs), if photosynthesis exceeds respiration there is a net gain in biomass. While photosynthesis is the primary pathway used to create organic matter, the required energy can also be obtained through chemical reactions (chemosynthesis). In the ocean, this pathway (chemosynthesis) occurs at deep sea hydrothermal vents where primary producers obtain their energy through the oxidation of hydrogen sulfide, released by the hydrothermal solutions, to sulphur and sulphate. There are several types of productivity. Primary productivity is the conversion of inorganic compounds into organic compounds. Gross primary productivity is the total amount of organic material synthesized during photosynthesis or chemosynthesis. Net primary productivity is the difference between the gross productivity and the amount of organic material used during respiration. Net productivity = Gross productivity - Respiration Primary productivity can be determined in a number of ways. The various methods include light and dark bottle method, 14C- method, through chlorophyll-a estimation, plankton biomass estimation, etc. There are direct and indirect methods of estimating primary productivity. Among all the methods, light and dark bottles are simplest and commonest one. The light and dark bottle metyhod was propounded by Gaarder and Gran (1929).This is an indirect method of estimating primary production and in which the amount of liberation of oxygen during the process of photosynthesis is measured and productivity is calculated. 14C - method is a direct way of estimating primary production, In this method, the amount of carbon assimilated during the process of photosynthesis is measured and calculated the productivity. This method was first propunded by Steeman Nielsen (1952). Light and dark bottle technique A water sample is collected from a particular depth in the ocean. The oxygen content of the water sample is measured. The water from the sample is then placed in two bottles, one is transparent (light bottle) and the other is opaque (dark bottle). The two bottles are submerged at the original depth of the water sample for a period of time. Both photosynthesis and respiration will occur in the light bottle while only respiration will occur in the dark bottle. The bottles are then retrieved and the oxygen content of each bottle is measured. Gross photosynthesis = increase in oxygen in light bottle + decrease in oxygen in dark bottle Net photosynthesis = increase in oxygen in light bottle 14 C method 24 A known amount of radioactive carbon in the form of bicarbonate is added to a water sample. The uptake of carbon by the primary producers is determined by measuring their radioactivity. Phytoplankton Phytoplankton are free-floating microscopic plants which are the primary producers of the oceanic system. In this method, either the number of plankton or the total weight of plankton per unit volume or unit area is measured. Factors influencing primary production The factors that affect primary productivity are (1) the availability of light, (2) the availability of nutrients and (3) the rate of grazing by primary consumers (herbivores). 1. Light One of the most obvious variable factors influencing primary production is the amount of solar energy reaching the surface of the sea. It is dependent on the altitude of the sum and the changing weather patterns. The light that penetrates the water is rapidly absorbed by inorganic and dead organic matter present in it. Thus, nearly 80% of the total solar radiation is absorbed in the upper 10 metres, and only 1% of incident visible light reaches 120 meters in clear tropical waters and 10 to 20 meters in turbid inshore waters of the incident light that falls over the sea, only 0.02 to 2.0% or 0.1% on an average alone is utilized for the production of organic matter. Since photosynthesis is primarily dependent on light energy, the nature of penetration of sunlight in the seawater is of prime importance in governing the productivity of an area. That is when the light strikes the surface of the water, a certain amount of light is reflected back. The amount of light refection depends upon the angle at which the light strikes the surface of the water. If the angle of incidence is low, large amount of light will be reflected. On the other hand, if the angle is to nearly 90o (perpendicular to the horizontal surface of the water) the greater will be the penetration and the lesser will be reflection. Light that is reflected is lost to the system and hence maximum penetration is the most desirable for maximum production. In the tropical regions of the earth, the sun is directly overhead at midday (or) virtually perpendicular to the sea surface, giving an angle for maximum penetration of light into the water column. In the temperate regions, the sun may be directly overhead during the summer months, but may be far from this position at the other times of the year. Also in the polar regions, the sun is absent during the winter or is so low to the horizon that no light can penetrate the water. The presence of ice in these areas also reduces light penetration into the water. The portion of light that enter the water column is subject to further reduction from two additional processes acting on it within the water. The first is reflection from various suspended particles in the water column. Suspended living or dead particles intercept the light and either absorb it (or) reflect it back to the surface. This light is unavailable for use and further reduces available light. Secondly, water itself absorbs light, making it unavailable for the plants. This absorption of light by water is the reason that vast majority of the water masses of the ocean is dark below a certain level. Because of this absorption of light by water, photosynthesis is automatically restricted to the thin, upper most lighted layer. Water, however, does not absorb all wavelengths of light equally. Sunlight spectrum includes all the visible colours ranging from violet to red or wavelengths from about 400 - 700 mm. As these wavelengths enter into the seawater, the violet and red components are very quickly absorbed by the water. The green and blue components are absorbed less rapidly and hence penetrate most deeply. Even though blue and green light penetrate deeper into the water column, the intensity decreases with depth, and it is intensity that is needed by plants. Intensity is measured by the extinction co-efficient, which is the ratio between the intensity at a given depth and intensity at surface. For pure water, it is 0.035. 25 Where there are large numbers of particles in the water, such as in coastal waters, the depth of light penetration may be greatly reduced and hence the amount of light insufficient for photosynthesis below a few meters. The rate of photosynthesis is high in high light levels and decreases as the light intensity decreases. On the other hand, the rate of respiration of the phytoplankton cells is essentially constant at all depths. That is, as the algal cells go deeper in the water column, the rate of photosynthesis declines as the light intensity decreases, until at some point, the photosynthetic rate equals the respiration rate. At this point, there is net production of organic material and this depth is called Compensation depth and it is the depth to which 1% of the incident radiation penetrates. The compensation depth also changes with season, due to the change in the position of the sun and it may be absent during the winter months in high latitudes. This depth marks the lower limit of the euphotic zone and varies geographically from a few meters in very turbid in shore waters to depths of 120 m or more in the open waters of tropical oceans. 2.Temperature Temperature acts along with other factors in influencing the variation of photosynthetic production. Generally, the rate of photosynthesis increases with an increase in temperature but diminishes sharply after a point is reached. Each species of phytoplankton is adapted to a particular temperature. In addition to the direct influence, temperature has same important indirect effects on production, particularly in relation to its role in the establishment of a thermocline and also in the mixing of water, resulting in the supply of nutrients to the euphotic zone 3. Salinity Besides light and temperature, salinity also is known to influence primary production. For example, Skeletonema shows an optimum rate of photosynthesis at salinities ranging from 15 and 20%, although the process could go on in a much wider range of 11 - 40 %. Further, many species of dinoflagellates such as Ceratium, Peridinium and Prorocentrum reproduce actively at lower salinities. However, generally salinity has not much influence on the overall productivity of ocean, mainly because there is no significant salinity variation in the open ocean. 4. Nutirents The concentration of phosphates and nitrates, which are the two major plant nutrients, has been recoginsed as one of the major factors limiting primary production in aquatic systems. It is known that during photosynthesis phytoplankton absorb these nutrients for the formation of particulate organic matter. Due to this absorption, the concentration of the nutrients in the euphotic zone decreases and this naturally limits further organic production. A certain amount of nutrients utilized by phytoplankton are however, regenerated by bacterial activity within the euphotic zone itself. But a good amount is lost from the euphotic zone as a result of the sinking of phytoplankton as well as through consumption by zooplankton inhabiting deeper levels during day time. Thus, much of the nutrients absorb from the euphotic layer are transferred to the deeper zones of the aquatic systems (marine or lakes), where they are regenerated. The nutrients that accumulate in the deeper levels, particularly in the oceans, are mostly returned to the surface waters by vertical mixing process such as upwelling, eddy diffusion, vertical convection and wind mixing. In addition to this, land drainage and river influx also contribute to the replenishment of nutrients of the surface waters, at least of the coastal areas. Seasonal and regional variations of primary production are often attributed to the influence of these plant nutrients. The cycle of phytoplankton in temperate latitudes, with marked peak in spring and decrease in production during summer, has been correlated with the changes in nutrient levels. During winter, owing to vertical mixing, the euphotic zone is enriched with nutrients which result in high productivity during spring. On the other hand, in summer, owing to the increase in temperature and the consequent formation of the thermocline, the nutrient replenishment in the euphotic zone by vertical mixing is prevented and this results in the decrease of production. 26 In the tropical waters, a permanent thermocline is present throughout the year and consequently, replenishment of nutrients by normal mixing process does not take place. Thus, short supply of nutrients in the tropical waters seems to impose a restriction on the rate of primary production. The high productivity recorded in the waters of the south-west coast of India, especially during the south-west monsoon period, has been correlated with the upwellings in this region, which bring about high nutrient concentration in the surface waters. However, the individual species requirements for phosphates and nitrates are not fully known. Grazing by zooplankton The grazing rate of zooplankton is one of the major factors influencing the size of the standing crop of phytoplankton and thereby rate of production. The grazing rate of