Zoology Fundamentals of Zoology PDF

1 4 FUNDAMENTALS OF ZOOLOGY UNIT IV: CLASSIFICATION OF ANIMALS 4.0 Intended Learning Outcomes: At the end of this learning packet, the students are expected to: a. I...

1 4 FUNDAMENTALS OF ZOOLOGY UNIT IV: CLASSIFICATION OF ANIMALS 4.0 Intended Learning Outcomes: At the end of this learning packet, the students are expected to: a. Identify and describe the major groups of animals, including both invertebrates and vertebrates. b. Understand the principles of taxonomy and modern classification systems, including the hierarchal structure from Kingdom to Species 4.1 Introduction This unit is designed to provide students with a thorough understanding of animal classification systems, encompassing both invertebrates and vertebrates. It will explore the principles of taxonomy, the hierarchical structure of classification from Kingdom to Species, and the major phyla and classes relevant to fisheries. By the end of this unit, students will be able to apply taxonomy principles, recognize the major groups and classes of animals, and understand their significance in various ecosystems, especially in aquatic environments. 4.2 The Linnaean System The evolution of life on Earth over the past 4 billion years has resulted in a huge variety of species. For more than 2,000 years, humans have been trying to classify the great diversity of life. The science of classifying organisms is called taxonomy. Classification is an important step in understanding the present diversity and past evolutionary history of life on Earth. All modern classification systems have their roots in the Linnaean classification system. It was developed by Swedish botanist Carolus Linnaeus in the 1700s. He tried to classify all living things that were known at his time. He grouped together organisms that shared obvious physical traits, such as number of legs or shape of leaves. For his contribution, Linnaeus is known as the “father of taxonomy.” The Linnaean system of classification consists of a hierarchy of groupings, called taxa (singular, taxon). Taxa range from the kingdom to the species. The kingdom is the largest and most inclusive grouping. It consists of organisms that share just a few basic similarities. Examples are the plant and animal kingdoms. The species is the smallest and most exclusive grouping. C. M. D. -ay  2 4 FUNDAMENTALS OF ZOOLOGY 4.2.1. Binomial Nomenclature The system of binomial nomenclature was introduced by Carl Linnaeus. Multiple local names make it extremely difficult to identify an organism globally and keep a track of the number of species. Thus, it creates a lot of confusion. To get rid of this confusion, a standard protocol came up. According to it, each and every organism would have one scientific name which would be used by everyone to identify an organism. This process of standardized naming is called as Binomial Nomenclature. The naming follows certain conventions. Each scientific name has two parts: Generic name Specific epithet The rest of the binomial nomenclature rules for writing the scientific names of organisms include the following: 1. All the scientific names of organisms are usually Latin. Hence, they are written in italics. 2. There exist two parts of a name. The first word identifies the genus and the second word identifies the species. 3. When the names are handwritten, they are underlined or italicized if typed. This is done to specify its Latin origin. 4. The name of the genus starts with a capital letter and the name of the species starts with a small letter. Figure 1. The Taxonomical Hierarchy 4.3 Classification of Animals The classification of animals largely can be divided into two groups: invertebrates and vertebrates. The basic difference between the two is the presence of a backbone or a spinal column. Animals like birds, snakes, and human beings are C. M. D. -ay  3 4 FUNDAMENTALS OF ZOOLOGY vertebrates due to the presence of backbone, and flatworms and insects are examples of invertebrates. It is worthy to be mentioned here that 97 percent of the animals on the earth are invertebrates and only 3 percent create vertebrates. The vertebrates can be classified into five groups mammals, birds, reptiles, amphibians, and fish; whereas the vertebrates are classified as 30 phyla. 4.3.1. Invertebrates Invertebrates can be simply identified as animals that do not have a backbone. They are found almost everywhere, from the hottest deserts and the deepest seabeds to the darkest caves and the tallest mountains. As stated before, invertebrates are the animals which mainly lack a skeletal system. This means most of them do not possess a rigid body structure and as a result, cannot grow very large. Anatomically, most invertebrates have an open circulatory system where blood flows in an open cavity. Most invertebrates also possess a simple respiratory system, with the most common form being gills and trachea. To compensate for the lack of an internal skeleton, most invertebrates have an external skeleton that protects their soft, inner body. This material is usually made from chitin, a derivative of glucose. Invertebrates constitute more than 97 per cent of all species in the animal kingdom. The number of invertebrates (out of 1.37 Million Animals species) exceeds more than 1.3 Million species, with more being found almost every other day. 4.3.2. Vertebrates Vertebrates are the most advanced of species in the animal kingdom. Members possess a well-defined internal skeleton system, which includes a backbone. In humans, the spinal cord runs along the body between the caudal and cranial regions connecting to the nerve tissues. Vertebrates also have more complex and specialized organ systems when compared to invertebrates. Organ systems like the respiratory systems are quite complex, with many additional functions. Even the sensory organs are advanced, which helps vertebrates adapt to their respective environment. On average, the overall count of vertebrates existing on planet earth is around 62,000. Examples of vertebrates include all chordates such as mammals, birds, fish, reptiles, and amphibians etc. The main characteristics of the vertebrates are that members have bilaterally symmetrical bodies and a brain enclosed by a skull. Phylum Chordata belongs to the Kingdom Animalia and includes all the vertebrates, i.e., animals with a backbone. C. M. D. -ay  4 4 FUNDAMENTALS OF ZOOLOGY ❖ Characteristics and Classification of Phylum Chordata: 1. Notochord - It is a longitudinal, cartilaginous rod running between the nerve cord and the digestive tract. It acts as a support for the nerve cord and is replaced by the vertebral column after the embryonic stage in all vertebrates. 2. Dorsal Nerve Cord - It is a bundle of nerves running along the “back” and splits into the brain and the spinal cord. It is hollow and lies dorsal to the notochord. 3. Pharyngeal Slits - They are the openings which allow the entry of water through the mouth without entering the digestive system viz. they connect mouth and throat. All Chordates have these openings on the lateral sides of the pharynx at some stage of their life. 4. Post-anal Tail - It is an extension of the body to the anus. In chordates, the tail is composed of skeletal muscles which help in locomotion in fish-like species. It is absent in most of the adult Chordates. Figure 2. Diagram of Phylum Chordata 4.4 Major Phyla of Invertebrates Relevant to Fisheries 4.4.1. Mollusca The animals belonging to the phylum Mollusca have soft-bodies, triploblastic and bilaterally symmetrical and coelomate. They are sluggish invertebrates, with a thin fleshy envelope or mantle covering the visceral organs. The term Mollusca was derived from the term given by Aristotle to cuttlefish. Mollusc means soft. Class Gastropoda Gastropods are the second largest class of animals (after the Insecta)—with 40,000–90,000 living species and at least 13,000 extant and fossil genera (Ponder and Lindberg, 2020)—and are also one of the most evolutionarily successful groups in the variety of ecosystems and habitats that they occupy. They live in the world’s oceans, freshwater lakes and streams, and terrestrial ecosystems, C. M. D. -ay  5 4 FUNDAMENTALS OF ZOOLOGY including deserts, mountains, backyards, and beaches. Their shells range in size from almost a meter to less than a millimeter. The name “Gastropoda” comes from the Greek word “gastro” (stomach) and “pod” (foot). Snails were given this name because many have the appearance of crawling around on their stomachs. Most gastropods have bodies that consist of several basic parts. Snails crawl upon a fleshy foot that spans the length of their body; movement occurs via muscle action and hydrostatic pressure. The soft-tissue mantle covers the internal organs and is used to build the shell (when present). All gastropods have a head, which has a mouth, sensory structures such as tentacles or siphons, and eyes, which are sometimes at the ends of stalks. The opening of the shell (aperture) defines the ventral surface; the opposite side (or, top) is the dorsal surface. Because gastropod shells grow by coiling about an axis, parts of the shell that were once “ventral” eventually become oriented dorsally as new shell material is deposited at the edge of older material. The anterior side of the shell is the “front end” (i.e., is located near the head, so the snail crawls in that direction); the posterior side of the shell usually bears the spire (if present). Figure 3. A Gastropod Class Bivalvia Bivalves are members of the phylum Mollusca, at first glance, bivalves do not appear to have much in common with these other groups besides having a shell made of calcium carbonate, which most possess. In fact, the only major feature that all mollusks possess is a sheet of tissue covering the body called the mantle, which (in most) covers the viscera and gills, and secretes the shell. Unlike all other mollusks, bivalves lack a distinct head with all of its associated organs (tentacles, radula, etc.), and have a pair of shells (called valves), which enclose a laterally compressed soft body, and are usually connected by a hinge and a flexible ligament. All bivalves live in or very close C. M. D. -ay  6 4 FUNDAMENTALS OF ZOOLOGY to the water, and have lamellar gills for respiration (which are also used for feeding in many species). As their name implies, bivalves have two shells that are called valves. The valves are made of calcium carbonate (CaCO3) in the form of the minerals aragonite and/or calcite. In most species the valves are approximately the same size, but in some they are unequal, a condition called inequivalve. The meeting of the two valves is sometimes called the commissure or shell margin. The knob-like, sometimes-pointed (and earliest-formed) part of a valve is the umbo (or “beak”, pl. umbones). Figure 4. External shell features of a bivalve The lunule and escutcheon are impressed external features anterior or posterior, respectively, to the umbo. Bivalves use muscles to close their shells. The areas where the muscles connected with the inner surfaces of the shell are represented by muscle scars. These scars indicate the position of the muscles in the bivalve's body, as well as the overall sizes of the muscles C. M. D. -ay  7 4 FUNDAMENTALS OF ZOOLOGY Figure 5. External shell features of a bivalve (lunule, escutcheon, and umbo) Most bivalves have a large foot, which is a muscular and expandable structure in the middle of the mantle cavity. The foot primarily functions as a burrowing organ in infaunal species. It can also function as an escape organ for some epifaunal forms. Digging happens by pumping blood into the foot which expands the end and acts as an anchor; pedal retractor muscles then pull the bivalve into the substrate. The byssus is a set of elastic or calcified fibers secreted by a gland in the foot, used to anchor the bivalve to a hard substrate. Most bivalves have a byssus as larvae, and many lose it as adults. The mantle covers the body and secretes the shell. The mantle is a complex sheet of tissue consisting of soft exterior membranes that secrete the shell and usually form a cavity – the mantle cavity – that encloses the gills and other organs, and which produces and enlarges the shell by secreting new shell material at the free edge. The mantle contains pallial muscles, which retract its edge and control the flow of water into and out of the mantle cavity. The mantle is comprised of two or three layers or folds. Figure 6. Bivalve showing its mantle The gut or digestive system in bivalves runs from mouth to anus, including labial palps surrounding the mouth for initial particle sorting. One or two adductor muscles hold the shell tightly closed when they contract. Species with two approximately equal-sized adductor muscles are called isomyarian; C. M. D. -ay  8 4 FUNDAMENTALS OF ZOOLOGY those with two unequal muscles are anisomyarian; those with only one muscle, including oysters and scallops, are monomyarian. Class Cephalopoda Cephalopoda is the most morphologically and behaviorally complex class in phylum Mollusca. Cephalopoda means "head foot" and this group has the most complex brain of any invertebrate. Cephalopods are characterized by a completely merged head and foot, with a ring of arms and/or tentacles surrounding the head. The arms, tentacles, and funnel are all derivatives of the foot. Members of the order Nautiloidea have more than 90 tentacles; members of orders Sepioidea and Teuthoidea have eight arms and two tentacles; and members of orders Ocotopoidea and Vampyromorpha have eight arms. All cephalopods are carnivorous. The strong beak is at the entrance to the buccal cavity, on the floor of which lies the radula. There are two pairs of salivary glands, one of which may be poisonous. The digestive tract consists of three parts: esophagus, which may contain a crop; stomach, which mashes food; and caecum, where most digestion and absorption occur. The posterior portion of the caecum contains a diverticulum that serves as an ink gland, producing a suspension of melanin that can be expelled through the mantle cavity. Figure 7. Digestive Tract of a Squid C. M. D. -ay  9 4 FUNDAMENTALS OF ZOOLOGY Cephalopods are gonochoric. A female typically possesses a single oviduct. A male produces spermatophores that it transfer to the female's genital pore by means of a specialized arm or tentacle. In some species, the specialized arm tip may be pinched off and left in the female's mantle cavity: this is known as the hectocotylus arm. Mating in some cephalopods includes courtship rituals that may consist of color changes, body movements, or combinations of both. Cephalopods exhibit spiral cleavage and are protostomous, but they have no larval stage: their development is direct. Octopods typically tend their eggs until hatching. 4.4.2. Arthropoda Arthropods are organisms belonging to the kingdom Animalia and the phylum Arthropoda. They are a very diverse group of animals that includes but is far from limited to insects, crustaceans, spiders, scorpions, and centipedes. Arthropods make up the largest phylum in the world, with more numbers and species diversity than most other phyla. With more than 800,000 known species of arthropods, it is no wonder that they dominate the land and sea. ❖ Characteristics of Arthropods 1. Jointed legs - Jointed legs allow arthropods to travel quickly regardless of their method of transportation. Whether swimming or scurrying across the ground, arthropods are speedy because of their jointed legs. 2. A segmented body - An arthropod's body can be divided into one, two, or three main sections. If they have one section, it is called a trunk. If they have two sections, these are called the cephalothorax and abdomen. If they have three sections, the third section is the head. 3. A hard exoskeleton - The exoskeleton of an arthropod is made of a strong polysaccharide called chitin. This hard shell protects the animal, retains moisture, and sometimes even plays a role in reproduction. 4. Compound eyes - Compound eyes allow arthropods to take in their environment in a variety of ways. Arthropods can see through a very wide lens and use their compound eyes to detect the slightest of motions and perceive any depth. Arthropods usually reproduce sexually through external fertilization or, more uncommonly, asexually in cases where both male and female reproductive organs are present in one organism. External fertilization occurs when a male arthropod encases its sperm in a pouch that is deposited directly into a female arthropod or sent free to be taken up by a female. The offspring of most species of arthropods begin as eggs, then hatch from these and enter a larval stage. In many arthropods, such as crabs, you C. M. D. -ay  10 4 FUNDAMENTALS OF ZOOLOGY can see these eggs attached to the hard abdomen. The larvae undergo metamorphosis, sometimes emerging from a cocoon during the pupal stage, to progress into adulthood. Some of the examples of aquatic arthropods are lobsters, crabs, horseshoe crabs, sea spiders, Barnacles, Copepods, Isopods, Krill, etc. Figure 8. Common Types of Arthropods 4.4.3. Echinodermata Class Echinodermata is a phylum of marine animals known for their radial symmetry, a water vascular system, and a unique ability to regenerate lost body parts. This group includes starfish, sea urchins, sand dollars, and sea cucumbers. Echinoderms possess a calcareous endoskeleton composed of ossicles, providing both structural support and protection. Class Holothuridae Holothurians generally look long and worm-like, but retain the pentaradial symmetry characteristic of the Echinodermata. Some may be spherical in body shape. The mouth and anus are located on opposite poles, and five rows of tube feet run from the mouth to the anus along the cylindrical body. Ten to 30 branching tentacles surround the mouth. The tentacles are actually part of the water vascular system. Holothurians have a single gonad, and most are dioecious. Although most spawn and are fertilized externally, there are approximately thirty brooding species.. C. M. D. -ay  11 4 FUNDAMENTALS OF ZOOLOGY Figure 9. Internal Anatomy of a Sea cucumber Generally, holothuridians are sedentary and/or slow moving, usually burrowing into soft sediments or are lodged in cracks or crevices under rocks. Holothurians crawl using podia or by using body wall muscles. Some deep sea species have elongate podia used for walking. As suspension or deposit feeders holothurians trap particles and plankton on mucus-covered tentacles. The tentacles are pushed into the mouth to ingest food. Secretory cells from papillae of the tentacles and gland cells of the foregut secrete mucus. Class Echinoidea Echinoidea comes from the Greek word ‘echinos’ meaning “spiny” and includes the sea urchins, sand dollars, and heart urchins. Echinoids are the sister taxa to holothuroidians. In regular echinoids, the mouth (or, peristome) is on the bottom, substrate-facing side of individuals and the anus (or, periproct) is located on top. Inside the mouth is a five-part jaw apparatus, called Aristotle’s lantern, which is used to break apart food and to scrape surfaces for algae during grazing. Given that they commonly feed by excavating sediment and ingesting food particles, irregular echinoids do not possess a lantern and, in many species within this group, the mouth is on the bottom, but the anus is shifted to either the posterior side or bottom of the test. Thus, irregular echinoids are secondarily bilaterially symmetrical in form. C. M. D. -ay  12 4 FUNDAMENTALS OF ZOOLOGY Figure 10. Internal and External Anatomy of Sea Urchin Class Asteroidea Sea star, any marine invertebrate of the class Asteroidea (phylum Echinodermata) having rays, or arms, surrounding an indistinct central disk. Despite their older common name, they are not fishes. Sea star arms—typically five in number—are hollow and, like the disk, covered with short spines and pedicellariae (pincerlike organs); on the lower side are grooves with rows of tube, which may be sucker-tipped or pointed. A sea star can lose one or more arms and grow new ones. Its tube feet enable it to creep in any direction and cling to steep surfaces. At the end of each arm (or, ray) is an eye spot that can sense changes in light (photoreception). A sea star’s tube feet can also sense or 'taste' chemicals (chemoreception), water currents, and feel objects around them (mechanoreception). Their tube feet are the most sensitive parts of their bodies. Sea star reproduction typically is heterosexual, but hermaphroditism (reproductive organs of both sexes in one animal) occurs, and a few sea stars reproduce asexually by division of the body (fragmentation). Some brood their eggs and young; non-brooders may release into the water as many as 2.5 million eggs at a time C. M. D. -ay  13 4 FUNDAMENTALS OF ZOOLOGY Figure 11. Sea Star External Morphology 4.4.4. Porifera Phylum Porifera, also known as sponges, is a group of simple aquatic animals that lack true tissues and organs. Animals belonging to the Porifera phylum are the smallest multicellular creatures in the animal kingdom. ‘Porifera’ word is taken from Latin which means “pore-bearing species”. All adult sponges are sessile, meaning they live permanently attached to rocks or other submerged objects and do not move about on their own. Some sponges grow in thin encrusting layers over surfaces The skeletal elements of the sponge are produced by the amoebocytes. The amoebocytes produce spongin, the soft fiber that forms natural bath sponges. These sponges feel soft and springy to the touch because they have soft skeletons made of flexible fibrous spongin. Other sponges have a stiff skeleton that feels prickly because it is made of hard, sliver-like spicules, which are also built by the amoebocytes. Some sponges have both spicules and spongin and feel both prickly and flexible. Sponges can reproduce either asexually or sexually. Asexually reproduction (without eggs and sperm) often occurs by budding, similar to growing a new branch on a tree. Cells on the side or base of the parent begin to bulge out and form a new organism. The buds may remain attached to the parent, or they may detach and settle down nearby to form a separate organism. Sponges also reproduce sexually when specialized gametocyte cells produce sperm and eggs. Sponges undergo synchronous spawning and eject sperm and egg cells into the water. If gametes (sex cells; either sperm or egg) from the same species meet, they form a larval sponge. C. M. D. -ay  14 4 FUNDAMENTALS OF ZOOLOGY Figure 12. Anatomy of a Sponge 4.4.5. Cnidaria Cnidaria is derived from a Greek word “cnidos” meaning stinging thread. These entities are characterized by the presence of cnidae. Cnidaria is a phylum under Animalia Kingdom, a classification comprising 10,000 described species, some of them are corals, sea anemones, jellyfishes and hydras. These aquatic entities are seen in both marine and freshwater habitats. Class Scyphozoa Class Scyphozoa, commonly known as true jellyfish, are a fascinating group of marine cnidarians characterized by their dominant medusa stage, which is typically free-swimming and bell-shaped. These jellyfish are primarily recognized for their thick, gelatinous mesoglea layer that provides buoyancy, and their tentacles, which are equipped with cnidocytes—specialized stinging cells used for capturing prey and defense. Scyphozoans undergo a life cycle that includes both sexual and asexual reproduction, featuring a prominent medusa stage and a less conspicuous polyp stage. They inhabit various marine environments around the world, predominantly found in open waters, but also in coastal and deep-sea areas. The class includes several notable orders such as Semaeostomeae, which encompasses well-known species like the moon jellyfish (Aurelia aurita) and the lion’s mane jellyfish (Cyanea capillata), both characterized by their large, translucent bells and long tentacles. Another order, Rhizostomeae, includes species like the upside-down jellyfish (Cassiopea) and the cannonball jellyfish (Stomolophus meleagris), which often have a more rigid structure and lack marginal C. M. D. -ay  15 4 FUNDAMENTALS OF ZOOLOGY tentacles. Additionally, the order Coronatae includes deep-sea jellyfish that possess a distinct crown-like structure on their bells and are often bioluminescent. Ecologically, scyphozoans play significant roles in marine ecosystems as both predators and prey. They help control plankton populations and serve as a crucial food source for larger marine animals such as sea turtles and certain fish species. Some species are also indicators of environmental changes, with their populations often blooming in response to factors like overfishing and climate change. While most scyphozoans are harmless to humans, a few species can deliver painful stings, which can be medically significant. Their unique biology and ecological importance make them a subject of interest in scientific research, offering insights into the health of marine ecosystems and the impacts of human activities on the ocean. Figure 13. The Cnidaria Class Anthozoa Class Anthozoa comprises marine invertebrates such as corals, sea anemones, and sea pens. Unlike other cnidarians, anthozoans do not have a medusa stage in their life cycle; they exist exclusively in the polyp form. These polyps can be solitary or colonial, forming intricate and diverse structures, particularly in coral reefs. Anthozoans are distinguished by their radial symmetry, tubular bodies, and a central mouth surrounded by tentacles. These tentacles are equipped with cnidocytes, which contain stinging cells used for capturing prey and defense. The body structure includes a gastrovascular cavity that functions in both digestion and circulation, divided by mesenteries or septa that increase surface area for digestion and nutrient absorption. C. M. D. -ay  16 4 FUNDAMENTALS OF ZOOLOGY Corals, a significant subgroup within Anthozoa, secrete calcium carbonate skeletons, contributing to the formation of coral reefs. These reefs are vital marine ecosystems that provide habitat and protection for numerous marine species. Sea anemones, another subgroup, are typically solitary polyps that attach to surfaces like rocks or the ocean floor, capturing prey with their tentacles. Reproduction in anthozoans can be sexual or asexual. Sexual reproduction involves the release of gametes into the water, resulting in a free-swimming planula larva that eventually settles and develops into a new polyp. Asexual reproduction occurs through processes like budding, fission, or pedal laceration, allowing polyps to clone themselves and form colonies. Anthozoans play crucial ecological roles in marine environments. Coral reefs, formed by colonial anthozoans, are biodiversity hotspots that protect coastlines from erosion and support fisheries and tourism. Sea anemones often engage in mutualistic relationships with clownfish, offering protection in exchange for food scraps and cleaning. Understanding the biology and ecology of Class Anthozoa is essential for marine conservation efforts, especially in the face of threats like climate change, ocean acidification, and pollution, which significantly impact coral reefs and other anthozoan habitats. Figure 14. Parts of a Coral Poly 4.5 Major Classes of Vertebrates Relevant to Fisheries 4.5.1. Fish Fish are a diverse group of aquatic vertebrates that inhabit a wide range of environments, from deep oceanic waters to freshwater rivers and lakes. They are characterized by their gills, fins, and typically a streamlined body that allows them to move efficiently through water. Class Agnatha Class Agnatha, known as jawless fish, represents one of the oldest and most primitive groups of vertebrates, encompassing lampreys and hagfish. These fish lack C. M. D. -ay  17 4 FUNDAMENTALS OF ZOOLOGY jaws and paired fins, possessing an elongated, eel-like body structure with a cartilaginous skeleton. Agnathans breathe through multiple gill pouches, with lampreys typically having seven pairs and hagfish having between five and fifteen pairs. Lampreys are characterized by their parasitic behavior, using their circular, sucker-like mouths and sharp, keratinized teeth to latch onto and feed on the blood and tissues of other fish. In contrast, hagfish are scavengers, feeding on dead or dying fish and marine invertebrates, and are known for their ability to produce copious amounts of slime as a defense mechanism. Lampreys undergo a larval stage called an ammocoete, which is a filter feeder living in freshwater sediments before transforming into the adult form. An example of a lamprey is the Sea Lamprey (Petromyzon marinus), an invasive species in the Great Lakes. Hagfish, such as the Atlantic Hagfish (Myxine glutinosa), have poor eyesight but use sensitive barbels around their mouths to detect food. Agnathans play unique roles in their ecosystems, with lampreys impacting fish populations as parasites and hagfish contributing to nutrient recycling by consuming decaying matter. Figure 15. Common types of Agnatha Class Chondrichthyes Class Chondrichthyes, comprising cartilaginous fish, is distinguished by its members' skeleton composed of cartilage rather than bone. This diverse group includes sharks, rays, and skates, renowned for their streamlined bodies, tough skin with dermal denticles, and formidable jaws equipped with rows of sharp teeth. Sharks, such as the Great White Shark and Hammerhead Shark, are apex predators with varied sizes and habitats, known for their efficient hunting strategies and continuous tooth replacement. Rays and skates, like the Manta Ray and Stingrays, have flattened bodies suited for life on the ocean floor, utilizing venomous spines for defense and feeding primarily on bottom-dwelling organisms. C. M. D. -ay  18 4 FUNDAMENTALS OF ZOOLOGY Reproductively, Chondrichthyes exhibit diverse strategies; many are ovoviviparous, with embryos developing inside the female's body until live birth, while others lay eggs externally (oviparous) or give birth to fully developed young (viviparous). Ecologically, these fish play crucial roles as top predators, regulating prey populations and contributing to ecosystem balance. They are economically significant through fisheries and ecotourism, with activities like shark diving and ray-watching attracting enthusiasts worldwide. Figure 16. Common types of Chondrichthyes Class Osteichthyes Class Osteichthyes, encompassing bony fish, comprises a vast and diverse group of vertebrates found in both freshwater and marine environments worldwide. These fish are characterized by their skeletons primarily composed of bone, providing structural support and housing their swim bladders, which aid in buoyancy control. Bony fish exhibit a wide array of body shapes and sizes, ranging from the tiny seahorse to the massive ocean sunfish. Adaptations such as streamlined bodies, paired fins for stability and maneuverability, and protective scales made of enamel or ganoin contribute to their success in aquatic habitats. Within Osteichthyes, two major groups are prominent: ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). Ray-finned fish, the largest subgroup, are characterized by fins supported by bony rays and include species like salmon, trout, and the iconic Clownfish known for its symbiotic relationship with sea anemones. Lobe-finned fish have fleshy, lobed fins supported by a central axis of bones and include the coelacanths and lungfish, which exhibit adaptations that hint at their evolutionary connection to terrestrial vertebrates. C. M. D. -ay  19 4 FUNDAMENTALS OF ZOOLOGY Reproductively, bony fish display diverse strategies, with many species laying eggs externally (oviparous), others giving birth to live young (viviparous), and some hatching eggs internally with live birth (ovoviviparous). Ecologically, bony fish play crucial roles as both prey and predators, contributing to ecosystem dynamics and food webs. They are economically significant, supporting global fisheries and aquaculture industries that provide food and livelihoods for millions of people. Figure 17. Common Groups of bony fishes (Actinopterygii and Sarcopterygii) 4.5.2 Aquatic Reptiles Aquatic reptiles encompass a diverse group of reptiles that have adapted to living predominantly in water, whether freshwater or marine environments. These reptiles have evolved a range of specialized adaptations that enable them to thrive in aquatic habitats. Crocodilians, such as crocodiles and alligators, are powerful predators with streamlined bodies, webbed feet, and strong tails, allowing them to move swiftly through water and ambush prey. Sea turtles, including species like the loggerhead and green turtle, are adapted for life in the oceans with streamlined shells and flipper-like limbs, facilitating efficient swimming and long-distance migrations for nesting. Marine iguanas found in the Galápagos Islands are unique among reptiles for their ability to dive and forage underwater, feeding on algae and seaweed. Additionally, sea snakes are venomous reptiles specialized for marine life, with paddle-like tails and the ability to spend their entire lives in shallow waters around coral reefs, preying on fish and crustaceans. C. M. D. -ay  20 4 FUNDAMENTALS OF ZOOLOGY These aquatic reptiles play crucial ecological roles in their respective habitats, contributing to nutrient cycling, regulating prey populations, and maintaining biodiversity. However, many face significant conservation challenges due to habitat loss, pollution, climate change, and human activities such as poaching and bycatch. Conservation efforts focus on protecting nesting beaches for sea turtles, establishing marine protected areas, reducing plastic pollution, and promoting sustainable fishing practices to minimize impacts on aquatic reptile populations. Understanding and conserving these unique species not only ensures their survival but also contributes to the health and resilience of aquatic ecosystems worldwide, upon which both wildlife and human populations depend. Figure 18. Types of Aquatic Reptiles 4.5.4. Marine Mammals Marine mammals encompass a remarkable group of mammals uniquely adapted to life in aquatic environments, predominantly in oceans and seas worldwide. This diverse group includes cetaceans such as whales, dolphins, and porpoises, which are characterized by their streamlined bodies, powerful tails, and ability to communicate using echolocation. Cetaceans range from the massive Blue Whale, the largest animal on Earth, to the acrobatic and social Bottlenose Dolphin. Pinnipeds like seals, sea lions, and walruses possess flipper-like limbs and are adept at both swimming and maneuvering on land, often congregating in coastal regions for breeding and molting. Sirenians, represented by manatees and dugongs, are herbivorous mammals with paddle-shaped flippers, feeding on seagrasses in shallow coastal waters. Marine otters, particularly sea otters, play a vital ecological role in kelp forest ecosystems by controlling sea urchin populations through their foraging activities. C. M. D. -ay  21 4 FUNDAMENTALS OF ZOOLOGY These marine mammals exhibit a range of adaptations that facilitate their survival in aquatic habitats, including specialized respiratory systems for prolonged dives, thermoregulatory adaptations such as blubber or dense fur, and complex social behaviors. Despite their adaptations, marine mammals face significant conservation challenges due to habitat degradation, pollution, climate change, and human activities such as fishing bycatch and vessel strikes. Conservation efforts are crucial to safeguarding these species, focusing on habitat protection, reducing human impacts, and promoting sustainable practices to ensure their continued presence and ecological roles in marine ecosystems. Understanding and conserving marine mammals not only preserves biodiversity but also underscores the interconnectedness of ocean health and human well-being. Figure 19. The Marine Mammals. 4.6 References Winsor, M. P. (2015). Reading the Shape of Nature: Comparative Zoology at the Agassiz Museum. University of Chicago Press. Sarkar, S., & Plutynski, A. (Eds.). (2018). A Companion to the Philosophy of Biology. John Wiley & Sons. Carroll, R. L. (2013). Patterns and Processes of Vertebrate Evolution. Cambridge University Press. (Although slightly before 2014, it provides a foundational understanding relevant to modern classifications.) Simpson, G. G. (2018). Principles of Animal Taxonomy. Columbia University Press. Zhang, Z. Q. (Ed.). (2013). Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013). Zootaxa. Kent, G. C., & Carr, R. K. (2001). Comparative Anatomy of the Vertebrates. McGraw- Hill Education. Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2003). Invertebrate Zoology: A Functional Evolutionary Approach. Brooks/Cole. Campbell, N. A., & Reece, J. B. (2013). Biology. Pearson. C. M. D. -ay