unit 3b.docx

Dorsal and ventral surfaces would have become differentiated, a ventral mouth would have been developed, and the beginning of cephalisation (concentration of neurons and sensory structures in an anterior area) would have occurred. These adaptations would have led to the occurrence of bilateral symme...

Dorsal and ventral surfaces would have become differentiated, a ventral mouth would have been developed, and the beginning of cephalisation (concentration of neurons and sensory structures in an anterior area) would have occurred. These adaptations would have led to the occurrence of bilateral symmetry, and a form like that of flatworms. 3. They evolved from several different groups of unicellular animals. Some zoologists have suggested that sponges, cnidarians, and ctenophores evolved independently, so, no single scheme can account for the development of multicellularity. There is now evidence from similarities in complex biochemical pathways and ribosomal RNA sequences that supports the colonial flagellate hypothesis. Phylum Mesozoa This group received its name, as it was thought by an earlier investigator that it represented a 'missing link' between protozoa and metazoa. They are minute and worm-like ciliated animals, which represent an extremely simple level of organisation. The entire group lives as parasites in marine invertebrates, with the majority between 0.5um --7um in length. Most consist of only 20-30 cells, arranged in two layers, which are not homologous to the germ layers of higher metazoans. It is not certain whether these organisms are a genuine link between unicellular and multicellular animal life, as it has been proposed that they are degenerated flatworms, with supporting molecular evidence. Phylum Porifera (sponges) Sponges bear many tiny pores and canals (hence, phylum Porifera 'porus meaning pore and fera - meaning bearing). These constitute a filter feeding system, which is sufficient for their stationary (sessile) habits. The currents passing through the canals bring food and oxygen, and carry away waste. Their bodies consist of masses of cells embedded in a gelatinous matrix, stiffened by a skeleton of tiny spicules of calcium carbonate, or silica and collagen. There are no organs or true tissues, even though the cells show a certain amount of interdependence as sessile animals with little body movement that have not evolved a nervous system or sense organs. Overall, sponges, although multicellular, have few characteristics of other metazoan groups. Thus, they are sometimes referred to as Parazoa rather than being considered true metazoa. Sponges vary in size from 2mm to 2m and may be brightly coloured (red, yellow, blue, green, orange). Species can be encrusting, erect, tubal branched and lobed. The majority of the 5000 species are marine species, although around 150 species live in freshwater, and a few live in brackish water. Marine sponges are abundant in all seas and at all depths. Adult forms are sessile and may attach to rocks, shells, corals, sand/mud or other submerged objects. Embryos are free swimming. Growth patterns often depend on the shape of the substratum, the local water currents and the availability of space, thus members of the same species may vary wildly in appearance. Many other animals (fish, nudibranchs, mites, bryozoans, crabs) live on or in communally and parasitically with sponges. Larger sponges usually have a variety of invertebrates living within them whilst many sponges live on other animals such as molluscs, barnacles, brachiopods and corals. Some crabs attach pieces of sponge to their carapace for camouflage and protection since most predators find sponges unpalatable, although some reef fish graze on shallow water sponges. Sponges are an ancient group, with an abundant fossil record dating back to the early Cambrian period. Three classes are usually described; calcarea (with calcareous spicules), Hexactinellida (six rayed spicules), and Demospongiae (with siliceous and or spongin, a specialised form of collagen spicules. The success of the sponges as a group may be due to their lack of predators, because of their sharp spicules and foul odour/taste. Given a choice, most predators will opt for more palatable prey. The only body openings of these animals are pores (called ostia) for small incoming water channels and/ or oscula, singular osculum, for large outgoing water canals). Canals lined with flagellated collar cells, called Choanocytes, whose flagella maintain the current through the system, connect these channels. These choanocytes also trap and phagocytise food particles. Amoebocytes (or archaeocytes) are motile cells, which move about, and can differentiate into other types of cell such as choanocytes or spicule forming cells. Reproduction Sponges reproduce both sexually and asexually. Asexual reproduction involves budding and regeneration following fragmentation of an adult form. In sexual reproduction sponges are monoeious (have both sexes in one individual). Sperm and oocytes arise from transformed choanocytes. Most sponges are viviparous, the fertilised zygote is retained within, and derives nourishment from its parent, and a ciliated larva is released. Other species are oviparous with both sperm and eggs being released into the water current. The free swimming larva (parenchymula) is a solid bodied flagellated form. After the larva settles, the flagellated cells migrate from the outside of the body to the inside to form choanocytes. Radiate animals Phylum Cnidaria (Knide - nettle, aria -connected to) is a very interesting group of animals of over 9000 species. Its name is taken from cells called cnidocytes, which contain stinging organelles (nematocysts), which are found only in this group. It is generally considered that they have evolved early on in the history of Metazoan life. They are an ancient group with the longest fossil history of any Metazoan, going back around 700 million years. They are widespread in marine habitats, although a few are found in fresh water. Mostly they are sessile, or very slow moving, but are efficient predators of organisms, which are far more complex, intelligent and swift. The phylum contains some of the most beautiful and peculiar living animals, from branching plants like hydroids, flower-like sea anemones, jellyfish, horny corals (sea whips, sea fans) and stony corals. They are found in great abundance in shallow marine habitats, particularly in warm, temperate and tropical regions. There are no terrestrial species. Colonial hydroids are found attached to mollusc shells, rocks, manmade structures, and other shallow water animals, although some are found at greater depths. Floating and free-swimming medusae are found in open seas and lakes. Floating colonies such as the Portuguese man-of-war have floats (or sails) to utilise the power of the wind for locomotion. Some other organisms (e.g. ctenophores, molluscs, flatworms) eat hydroids bearing nematocysts, and incorporate the stinging cells, using them for their own defence. Cnidarians sometimes live symbiotically with other animals, often as commensuals on shells or other surfaces of the host. Hydroid species and anemones commonly live on snail shells inhabited by hermit crabs, providing the crab with protection from predators, and a food source for the anemone via debris from the crab's meals. Algal cells frequently live within the tissues of cnidarians, particularly in reef building corals and freshwater hydra. Planktonic medusa may be of some importance as food for other marine organisms, particularly fish that are of economic importance, although the young of these species may also prey on the adult cnidarians forms. Although many cnidarians have limited economic importance, reef-building corals are an exception. Fish and other animals associated with reefs provide substantial amounts of food for humans, and are used to generate income from tourists, produce jewellery, ornaments, and building rock and animals for the aquarium industry. Four classes of cnidarians are usually recognised: Hydrozoa is the most variable class, incorporating hydroids, fire corals, and Portuguese man-of-war. Scyphozoa is the true jellyfish. Cubozoa is the cube jellyfish. Anthozoa is the largest class, and includes sea anemones, stony and soft corals. Form and function form), which is adapted for a free-living floating (sometimes swimming) existence. Probably the most interesting aspect of this phylum is the dimorphism (often polymorphism), displayed by many species within their life cycles. All Cnidarians can be classed as one of two morphological types, a Polyp (or hydroid), which is a sedentary, sessile form, and a medusa (jellyfish Most polyps have tubular bodies, with a mouth at one end, which is surrounded by tentacles. The posterior end is usually attached to a substratum by a pedal disc. Polyps may live singularly or in colonies. Some species have colonies of morphologically differing individuals (polymorphism), each specialised for a certain function, such as feeding, reproduction or defence. Medusae are usually free-swimming, and have bell or umbrella shaped bodies and tetrahedron symmetry (parts arranged in fours). The mouth is usually centred on the concave underside, and tentacles arise from the rim of the bell. Sea anemones and corals (class Anthozoa) are all polyps and are not dimorphic. Members of Class Scyphozoa (true jellyfish) have a conspicuous medusae form, but often have a polyp larval stage. Colonial members of class Hydrozoa sometimes have both a polyp and a free-swimming medusae stage. The advantage of this system is that the species can take advantage of both the feeding and distribution advantages of both open water (pelagic) and bottom dwelling (benthic) environments. Many hydrozoa are also polymorphs, with several distinct types of polyps within a colony. Nematocysts Over 20 different types of nematocysts have been described, and are important in classification of species. Nematocysts are tiny capsules composed of material similar to chitin and contain a coiled thread or filament, which is a continuation of the narrow end of the capsule. The end of the capsule is covered by a lid (operculum). This inside of the undercharged thread may bear tiny barbs, or spines. A nematocyst is contained within the cell, which created it, the cnidocyte, (termed a cnidoblast during development). Except in Anthozoa, cnidocytes are equipped with a trigger like cnidocil, which is a modified cilium. Tactile stimulation causes the nematocyst to discharge. Nematocysts may inject poison into a prey item, or, if adhesive, recoil again instantly holding the prey item. High osmotic pressure produced as the nematocyst is formed is responsible for the rapid firing mechanism of this effective weapon. Nematocysts of most cnidarians are not harmful to humans, although the sting of the Portuguese man-of-war can be dangerous. Neural net The neural net of cnidarians is possibly the most primitive of all animal nervous systems, and, as such, it has been studied extensively to give insight into the more complex systems of 'higher' animals. There is no concentrated grouping of nerve cells to suggest a 'central nervous system', as there is little adaptive value for a radially symmetrical animal to have a central nervous system with a brain. The environment approaches from all sides equally, and there is no control over the direction of approach to a prey organism. Nerve cells in cnidarians differ from those of more complex animals by having vesicles, which release neurotransmitter chemicals into the synaptic space on both sides of a synapse. There is also an absence of any insulating substance (myelin) on the axons (nerve fibres). Neurology, the study of nervous systems, will be dealt with in detail in a later module. Class Hydrozoa Obelia is a typical member of class Hydrozoa, and is a colonial marine form with a distinct medusae stage, which is involved in sexual reproduction. Obelia captures and ingests prey such as tiny crustaceans, worms and larvae, via the feeding polyps. After partial extra cellular digestion in the feeding polyp, the digestive 'broth' passes along the common gastro vascular cavity, where it is taken up by gastrodermal cells, and intracellular digestion occurs. Obelia can reproduce by budding off the colonial form, to produce new hydroids. However, a reproductive polyp (gonangium) buds off free-swimming medusae which, when mature, produce gametes. In some species, the medusae remain attached to the hydroid and release gametes from there. The Portuguese man-of-war (Physalia) is also a member of this class, as it consists of a colony with a bladder held above the water in its tropical marine habitat. Long tentacles laden with nematocysts are capable of inflicting painful stings on humans. The float (pneumatophore) is believed to have expanded from the original larval polyp. It contains a sac arising from the body wall and is filled with a gas similar to air. Physalia has an interesting relationship with a small fish species (Nomeus), which would normally constitute a food item. This particular fish manages to swim amongst the tentacles without harm, due to skin mucus, which does not stimulate the nematocysts, rather in the same way that clown fish live with anemones. Class Scyphozoa (true jellyfish) This group is considered to be more evolutionarily advanced than hydrozoans, due to the more advanced nervous system and locomotary skills. Most of these 'cup' animals (skyphos - cup) vary between 2cm 40cm, but some individuals (e.g. Cyanea) reach a bell diameter of up to 2m, and have tentacles 60m-70m long. Most of this group inhabit open ocean areas, some up to depths of 3000m. Their colouring ranges from colourless to vivid orange, blue or pink. The bell is made up of 95-96% water, and contains amoeboid cells and fibres. Movement is by rhythmic pulsations of the umbrella. The mouth is on the underside of the umbrella, surrounded by oral arms that are used in capturing and ingesting prey. The nervous system is a neural net, with a sub umbrella net controlling bell pulsation and a second more diffuse net controlling local reactions such as feeding. Sexes are separate, with gonads located under the bell, behind the oral arms. Fertilisation is internal, with sperm being carried by cilliary currents into the gastric pouch of the female. Zygotes maybe released into the water to develop, or may remain in the folds of the oral arms for protection. The ciliated planula larva becomes attached to a substratum and develops into a hydra-like form (scyphistoma). By a process called strobilation, the scyphistoma forms a series of saucer shaped buds (ephyrae), which, when they break loose, develop into medusae. Class Cubozoa: (box jellyfish) Until recently this class was considered to be part of the Scyphozoa. The medusa is the prominent form, with an inconspicuous polyphonic stage. Some of the medusae may be up to 25cm tall, although most are in the 2cm-3cm range. In transverse section, the bell appears virtually square, with a tentacle (or group of tentacles) at each corner of the umbrella margin. The base of each tentacle is differentiated into a flattened, tough blade (pendulum). Cubozoa are strong swimmers and voracious predators, feeding mostly on fish. Stings of some species may be fatal to humans. The complete life cycle is known for only one species, Tripedalia cystophora. The polyp is very small (1mm tall) and is solitary and sessile. Polyps do not produce ephyrae, but metamorphose directly into medusae. Class Anthozoa Anthozoa, 'flower animals', are polyps with a flower-like appearance. There is no medusae stage. They are always marine species, and are found in both deep and shallow water. They may be solitary or colonial and skeletons support many forms. There are three subclasses, the Hexacorallia/Zoantharia (sea anemones, hard corals), Ceriantipatharia (tube anemones and thorny corals) and Octocorallia/Alcyonaria (soft and horny corals, e.g. sea fans, sea pens). Hexacorallians and ceriantipatharians have a hexahedrons plan (of six or multiples of six) or polymerous symmetry and have simple tubular tentacles arranged in one or two circles on the oral disc. Octocorallians are, alternatively, based on a plan of eight, and always have eight pinnate (feather-like) tentacles arranged around the margin on the oral disc. The most important of these groups are the Hexacorallia. Sea anemones (order Actinaria) polyps are larger and heavier than hydrozoan polyps, and range from 5mm to 100mm in diameter, and from 5mm to 200 mm long. Sea anemones are usually colourful and are found in coastal regions worldwide. They are attached by a pedal disc to shells, timber, rocks or other substrata. They are cylindrical in shape, with a crown of tentacles arranged in one or more circles surrounding the oral disc. The slit shaped mouth leads to a pharynx, which leads into a large gastro vascular cavity, divided into six radial chambers by mesenteries. The free edges of the mesenteries contain nematocysts and digestive secretary glands. Sea anemones are carnivorous, feeding on fish, or any live animals of suitable size, although some feed on planktonic animals. Feeding behaviour is triggered by chemical stimulation. Muscles are well developed, and most anemones can move slowly along the substrata. They can contract and recoil from threatening stimuli. Anemones form mutuality relationships with other organisms such as dinoflagellates, profiting by the products of their photosynthesis, whilst providing protection for the protozoa. Other animals, which benefit from the protection of anemones, are certain species of crab and damselfish. The fish may be beneficial to the anemone by providing ventilation through its movements, keeping the anemone free of sediment, or luring another species to seek shelter, thus providing food. Hexacorallian corals (true/stony corals) have been described as sea anemones living in calcareous cup, that they have secreted themselves. They have a similar internal structure. Instead of a pedal disc, the epidermis at the base of the animal secretes a calcareous substance to form the exoskeleton into which the animal retreats if threatened or not feeding. In many colonial corals, the skeletons may become massive, building up over many years, with living coral forming the top layer. Thorny corals, which are members of the ceriantipatharia, are colonial and attach to a firm substratum. The exoskeleton is made of a horny substance and has protective spines, and there are no internal. Septa is in this group. Octocorallian corals have strict octomerous symmetry. They are all colonial, with a skeleton of calcareous spicules, and/or a horny protein, and, thus, they have an endoskeleton. Coral reefs Coral reefs are amongst the most productive ecosystems on the planet. Their diversity of life forms is only rivalled by tropical rainforests. Hexacorallians are large formations of calcareous (limestone) rock in shallow warm clear waters, formed over thousands of years. The requirements of undiluted salinity, little sediment and shallow warm water confines these structures to 30 degrees north and south latitude and excludes areas of upwelling cold currents. Living plants and animals are confined to the top layers, where they add calcium carbonate to the structure as they grow. Shallow water is essential, as these corals live mutualisitically as photosynthesising dinoflagellates in their tissues. The occurrence of coral deposits at greater depths was used by Darwin as further evidence that life was not created in a single instant, but had built up over a long period of time, as once shallow islands sank beneath the seas. Several types of reefs are commonly recognised; fringing reefs occur close to a landmass, with either a shallow lagoon or no lagoon separating it from land. A barrier reef runs roughly parallel to the shore and there is a wider, deeper lagoon between it and land. Atolls encircle a lagoon but not an island. Their formation is shown below. They typically shelve steeply on their outer fringe. Platform and bank reefs can occur between the outer fringe of a barrier reef or inside an atoll. Different animals colonise different areas of reefs according to water depth, and current. Octocorallians tend to favour more sheltered areas away from wave action. Enormous numbers of animals populate coral reefs, e.g. there are approximately 300 common fish species on Caribbean reefs, and more than 1200 populating the great barrier reef of Australia. This is the only other phyla exhibiting radial symmetry. There are less than 100 known species, which are all marine species and include 'sea walnuts' and 'comb jellies'. They are named for their eight rows of comb like plates used for locomotion. Only one species has nematocysts, and this species gains them by ingesting cnidarians. Most of them are free swimming, although they are feeble swimmers, but may occasionally be found at depth, especially during rough weather. Many species are luminous.