Animal Diversity: ZOO 102

• Animal Diversity - Course Code: ZOO 102, Credit Units: 2

Introduction

• An animal is a living organism that feeds on organic matter, has sense organs and nervous system, and can respond rapidly to stimuli.
• Characteristics of animals include being multicellular, breathing oxygen, having myocytes (muscle cells), reproducing sexually, growing from a hollow sphere of cells during embryonic development, being unable to produce their own food, and having specialized sensory organs for foraging, hunting, and searching for food.
• Diversity refers to the variety of animals in kind, form, character, structure, functions, lifestyles, experience, and interests.
• The Kingdom Animalia is one of the five major kingdoms of biodiversity.
• Members of Kingdom Animalia are multicellular, eukaryotes, and heterotrophs.
• Most members of Kingdom Animalia can move independently and are regarded as motile.
• An estimated 10 million living species of animals have been identified.
• Scientists are continually discovering more species, with the number of extant species estimated to be up to 30 million.
• Almost all animals (99%) are invertebrates.
• Only 42,500 of the estimated 10 million living species are vertebrates (have a backbone).
• The animal kingdom includes about 35 phyla, most of which occur in the sea, with fewer in freshwater and even fewer on land.
• The phyla Arthropoda, Mollusca, and Chordata dominate animal life on land.
• Animals may lack symmetry or have radial or bilateral symmetry.
• Bilaterally symmetrical animals have dorsal and ventral sides, as well as anterior and posterior ends.
• There are three major clades of bilaterian animals: Deuterostomia, Lophotrochozoa, and Ecdysozoa.
• With one exception, the phyla in these clades consist entirely of invertebrates: Chordata is the only phylum that includes vertebrates.
• Animals vary in complexity from sea sponges to crickets to chimpanzees and are classified based on their anatomy, morphology, evolutionary history, embryological development, and genetic makeup.
• The animal classification system is constantly evolving as new information arises, which helps to understand and conserve the diversity of life on earth.

Evolutionary Origin and History of Animal Diversity

• Geological time is divided into eons, eras, periods, and epochs.
• The span of earth's history (4.6 billion years to the present) is divided into four eons: Hadean, Archean, Proterozoic, and Phanerozoic.
• Hadean Eon (4600-3800 MYA): The earliest eon when water existed in gaseous form, the earth was largely molten, leaving no rock record.
• Archean Eon (3800-2500 MYA): Fossils of microorganisms and stromatolites, layered mats of trapped cyanobacteria, bacteria, and algae, were formed.
• Proterozoic Eon (2500-542 MYA): Also known as early life, eukaryotes with a nucleus and sexual reproduction appeared late in the Proterozoic.
• Phanerozoic Eon (542 MYA): Referred to as the visible life, complicated multicellular organisms made a sudden appearance, divided into Paleozoic, Mesozoic, and Cenozoic eras.
• The time between the Proterozoic and the Paleozoic is called the pre-cambrian or neoproterozoic (Ediacaran period from 635–542 MYA), where early animal life (Ediacaran biota) evolved from protists.
• Some protist species, choanoflagellates, closely resemble the choanocyte cells in sponges (Phylum Porifera).
• Molecular analyses revealed sequence homologies in their DNA.
• A recent fossil from South Australia dating back 650 million years showed small, one-centimeter long, sponge-like creatures, placing the animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period.
• The sponge lineage preserves many features of the animal ancestor on the branch leading to all other animals.
• The other candidate for earliest animal lineage is the group of comb jellies (Phylum Ctenophora).
• The Cambrian period marks the most rapid evolution of new animal phyla and animal diversification in Earth's history.
• Most of the animal phyla in existence today had their origins during the Cambrian explosion where Echinoderms, mollusks, worms, arthropods and chordates arose during this period.
• The history of animals spans more than half a billion years.
• Fossil biochemical evidence and molecular clock analyses indicate that animals arose over 600 million years ago.
• Genomic analyses suggest that key steps in the origin of animals involved new ways of using proteins encoded by genes found in choanoflagellates.

Diversity of Animal Forms, Structure, and Functions

• Phylum Porifera (Sponges): Simplest sponges take the shape of a cylinder with a large central cavity, the spongocoel.
• Water enters the spongocoel from numerous pores and is extruded via a large common opening called the osculum.
• Lack true tissues.
• Body is supported by a skeleton made up of spicules or sponging fibers.
• Sexes are not separate (hermaphrodite).
• Reproduce asexually by fragmentation and sexually by formation of gametes.
• Examples: Sycon(Scypha), Spongilla(Fresh water sponge) and Euspongia(Bath sponges).
• Phylum Ctenophora (Comb jellies): Delicate, transparent animals with eight meridional rows of cilia, two main body cavities: the gastrovascular system (gvs) and the mesoglea.
• Eight rows of ciliated "combs" or ctenes.
• Bioluminescence is well-marked in ctenophores.
• Examples: Pleurobrachia and Ctenoplana.
• Phylum Coelenterata (Cnidaria): Cnidarians have two distinct body plans: Medusa and Polyp.
• All cnidarians have two membrane layers, with a jelly-like mesoglea between them.
• Cnidarians have specialized cells known as cnidocytes ("stinging cells") containing organelles called nematocysts (stingers).
• These cells are present around the mouth and tentacles and serve to immobilize prey with toxins.
• Unique stinging structures (nematocysts) housed in specialized cells (cnidocytes).
• Corals have a skeleton composed of calcium carbonate.
• Cnidarians exhibit two basic body forms called polyp and medusa.
• Cnidarians exhibit alternation of generation (metagenesis)
• Examples: Physalia(Portuguese man-of-war), Adamsia(Seaanemone), Pennatula(Sea-pen), Gorgonia(Sea-fan) and Meandrina(Brain coral).
• Phylum Platyhelminthes (flatworms): Bilaterally symmetrical, lacking specialized respiratory, skeletal, and circulatory systems; no body cavity (coelom) is present; The body is not segmented and has spongy connective tissue (mesenchyme).
• A central nervous system that processes information from sensory structures.
• Specialized cells called flame cells help in osmoregulation and excretion.
• Development is through many larval stages.
• Some members possess high regeneration capacity.
• Examples: Taenia(Tapeworm), Fasciola(Liver fluke).
• Phylum Nematoda (roundworms): Cylindrical pseudocoelomates with tapered ends, undergo ecdysis, have a complete alimentary canal and an excretory tube.
• An excretory tube removes body wastes from the body cavity through the excretory pore.
• Examples: Ascaris (Round Worm), Wuchereria (Filaria worm), Ancylostoma and Necator (Hookworm).
• Phylum Entoprocta (Endoprocta): Small, almost microscopic animals below 5mm in length
• The body is divided into a rounded or oval mass (calyx) containing all the viscera, stalk, and basal attachments.
• The body wall is typically pseudocoelomate.
• E.g. Pedicellina.
• Phylum Nemertinea: Elongate, vermiform, flattened acoelomate Bilateria without appendages.
  • E.g. Nemertine.
• Phylum Acanthocephala: Endoparasitic pseudocoelomate, vermiform Bilateria without a digestive tract.
  • The body is divided into short slender forebody/presoma and longer stouter trunk.
  • E.g. Echinorhynchus.
• Phylum Rotifera: Minute pseudocoel animals (0.04 to 2 mm).
  • The body is divided into broad, narrowed, or lobed anterior trunk and slender terminal region, tail, or foot.
  • E.g. Brachionus, Pedalia.
• Phylum Ectoprocta (Bryozoa): Microscopic, sessile, colonial, unsegmented coelomate animals in exoskeletal cases or gelatinous material.
  • They form colonies known as Sea-mats or Corallines.
  • A complete ectoproct colony (zoarium) is composed of zooids.
  • E. g. Bugula.
• Phylum Brachiopoda: Coelomate Bilateria enclosed in a bilaterally symmetrical bivalve shell.
• The body is situated inside the posterior end of the shell.
• E.g. Magellania.
• Phylum Phoronida: Tubicolous, hermaphrodite, vermiform, coelomate Bilateria.
  • Terminal tentaculated horse-shoe-shaped lophophore, dorsal anus, and one pair of metanephridia that serve as gonoducts.
  • E.g. Phoronis.
• Phylum Annelids (segmented worms): Segmented body wall (metameres) and internal organs (except digestive tract).
  • Aquatic annelids possess lateral appendages (parapodia) for swimming.
  • Nephridia help in osmoregulation and excretion.
  • Reproduction is sexual.
  • Examples: Lumbricus terrestris, Nereis, Pheretima earthworm, and Hirudinaria blood-sucking leech.
• Phylum Arthropoda: Largest phylum of Animalia, with segmented body, jointed appendages, and exoskeleton (protein and chitin).
  • The body has a head, thorax, and abdomen.
  • Respiratory organs are gills, book gills, book lungs, or tracheal system.
  • Open circulatory system (hemolymph).
  • Sensory organs include antennae, compound and simple eyes, statocysts for balance.
  • Excretion through malpighian tubules.
  • Examples: Economically important insects (Apis, Bombyx, Laccifer), Vectors (Anopheles, Culex, Aedes), Gregarious pest (Locusta), Living fossil (Limulus).
• Phylum Mollusca: Second largest animal phylum, with body parts that include muscular foot, visceral mass, and mantle.
  • Coelom reduced.
  • Most mollusks have a hard shell (calcium carbonate).
  • Mantle cavity in which feather-like gills are present.
  • The mouth contains a radula.
  • Examples: Pila apple snail, Pinctada pearl oyster, Sepia cuttlefish, Loligo squid, Octopus devil fish, Aplysia seahare, Dentalium tusk shell, and Chaetopleura chiton.
• Phylum Echinodermata: Adult echinoderms are radially symmetrical but larvae are bilaterally symmetrical.
  • The most distinctive feature: water vascular system helps in locomotion, capture, and transport of food and respiration.
  • An excretory system is absent.
  • Examples: Asterias star fish, Echinus sea urchin, Antedon sea lily, Cucumaria sea cucumber, and Ophiura brittlestar.
• Phylum Chordata is characterized by a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits.
• Subphylum Urochordata and Cephalochordata: Often referred to as protochordates.
  • In Urochordata: notochord is present only in the larval tail.
  • In Cephalochordata: it extends from head to tail region and is persistent throughout life.
  • Examples: Urochordata (Ascidia, Salpa, Doliolum), Cephalochordata (Branchiostoma).
• Subphylum Vertebrata: Members have a notochord during the embryonic period, later replaced by a cartilaginous or bony vertebral column.
  • Vertebrates have a ventral muscular heart (two, three, or four chambers), kidneys for excretion/osmoregulation, and paired appendages (fins or limbs).
• Class Pisces (Fishes):
  • Agnatha Myxini Cyclostomata: Elongated body bearing 6-15 pairs of gill slits for respiration.
    • Only vertebrates with pronephric and mesonephric kidneys in the adult.
    • Cyclostomes have a sucking and circular mouth without jaws.
    • Example: Myxine hagfish.
  • Cephalaspidomorphi: Have elongated body bearing 7 pairs of gill pouches.
    • Example: Petromyzons lampreys
  • Gnathostomata Chondrichthyes: Notochord is persistent throughout life, gill slits are separate, skin contains placoid scales, and lack air bladder.
    • Heart is two-chambered.
    • Cold-blooded (poikilothermous) animals that cannot regulate their body temperature.
    • Examples: Scoliodon dog fish, Pristis saw fish, Carcharodon great white shark, Trygon sting ray.
  • Osteichthyes: Bony endoskeleton, four pairs of gills covered by an operculum, air bladder is present, and mostly oviparous.
    • Examples: Marine (Exocoetus flying fish, Hippocampus seahorse) and Freshwater (Labeo Rohu, Catla Katla, Clarias Magur), also Aquarium (Betta fighting fish, Pterophyllum angel fish).

Tetrapoda

• Class Amphibian: Body is divisible into head and trunk, skin is moist (without scales), eyes have eyelids, tympanum represents the ear, and alimentary, urinary and reproductive tracts open into the cloaca.
  • Respiration via gills, lungs, and skin.
  • Heart is three-chambered (two auricles and one ventricle).
  • Cold-blooded animals.
  • Examples: Bufo toad, Rana frog, Hyla tree frog, Salamandra salamander, Ichthyophis limblessamphibia.
• Class Reptilia: Body covered by dry, cornified skin, epidermal scales or scutes, and they do not have external ear openings.
  • Tympanum represents ear.
  • Heart is usually three-chambered (four-chambered in crocodiles). Reptiles are poikilotherms.
  • Snakes and lizards shed scales.
  • They are oviparous. Examples: Chelone turtle, Testudo tortoise, Chameleon treelizard, Calotes garden lizard, Crocodilus crocodile, Alligator alligator, Hemidactylus wall lizard, Naja cobra, Bangarus krait, Vipera viper.
• Class Aves: Presence of feathers, most can fly except flightless birds.
  • Forelimbs modified into wings, hind limbs modified for walking, swimming, or clasping.
  • Dry skin without glands (except the oil gland).
  • Fully ossified endoskeleton, hollow bones with air cavities.
  • Digestive tract with crop and gizzard.
  • Heart is completely four-chambered.
  • Warm-blooded (homoiothermous) animals i.e., able to maintain constant body temperature.
  • Oviparous.
  • Examples: Corvus crow, Columba pigeon, Psittacula parrot, Struthio ostrich, Pavo peacock, Aptenodytes penguin, Neophron vulture.
• Class Mammalia: Presence of milk-producing glands (mammary glands).
  • Unique skin possessing hair.
  • External ears or pinnae. Different types of teeth.
  • Four-chambered heart. Homeothermous.
  • Examples: Ornithorhynchus platypus (oviparous) Macropus kangaroo, Pteropus flying fox, Camelus camel, Macaca monkey, Rattus rat, Canis dog, Felis cat, Elephas elephant, Equus horse, Delphinus common dolphin, Balaenoptera blue whale, Panthera tigris tiger, Panthera leo lion (viviparous)

Bases of Categorization of the Diversity of Animals

• Animal classification based on symmetry:
• Asymmetry: Sponges are mostly asymmetrical.
• Radial symmetry: Plane passing through the central axis divides the organism into two identical halves (ex. Cnidarians, ctenophores and echinoderms).
• Bilateral symmetry: Body can be divided into identical left and right halves in only one plane (ex. annelids, arthropods).
• Animal classification based on level of organization:
  • Cellular level: Protozoa (single-celled eukaryotic organisms) and Metazoa (multicellular eukaryotic organisms). Parazoa exhibit cellular level with loose cell aggregates (ex. Porifera), while Eumetazoa has tissue or organ system level.
  • Tissue level: Cells performing the same function are arranged into tissues (ex. Cnidaria and Ctenophora).
  • Organ level: Tissues are grouped together to form organs, each specialized for a particular function (ex. Platyhelminthes). Animal classification based on body cavity:
  • Coelomates: Animals with a fluid- or air-filled space between the digestive tract and body wall, the coelom (ex. annelids, molluscs, arthropods, echinoderms, and chordates).
  • Pseudocoelomates: Body cavity is not lined by mesoderm (ex. aschelminths/nematodes).
  • Acoelomates: Body cavity is absent (ex. Platyhelminthes). Animal classification based on the number of germ layers:
  • Diploblastic: Cells arranged in two embryonic layers: ectoderm and endoderm with some times an undifferentiated layer, mesoglea or mesophyl, in between the ectoderm and endoderm (ex. Cnidarians and Ctenophores).
  • Triploblastic: Developing embryo has three germinal layers (ectoderm, mesoderm, and endoderm) (ex. platyhelminths to chordates). Animal classification based on embryonic development & segmentation:
  • Embryonic development: Zygote undergoes cleavage to form morula then blastula. Blastula folds to form a gastrula with blastopore.
  • Protostomes: Spiral, determinate cleavage, coelom forms from splits in the mesoderm, mouth forms from the blastopore. (ex. Mollusca, Annelida, and Arthropoda).
  • Deuterostomes: radial, indeterminate cleavage, coelom forms from mesodermal outpocketings of the archenteron, mouth forms secondarily, and anus develops from blastopore. Ex. Echinodermata and Chordata. Animal classification based on presence or absence of notochord:
  • Non-chordates: Animals which do not form a notochord (ex. porifera to echinoderms).
  • Chordates: Presence notochord, dorsal hollow nerve cord, pharyngeal slits or clefts as well as a muscular, post-anal tail.