BIO121- VERTEBRATE ZOOLOGY

What are chordates and what are their defining characteristics? Never Dive Past Extreme Pressure.

Chordates = animals of Phylum Chordata, defined by five hallmarks:

• Notochord

  • Flexible rod running along the body axis.

  • Provides skeletal support and a base for muscle attachment.

  • In vertebrates, usually replaced by a vertebral column.

• Dorsal Tubular Nerve Cord

  • Hollow nerve cord located above the notochord.

  • Enlarges at the anterior end to form the brain in vertebrates.

  • Protected by vertebrae and cranium.

• Pharyngeal Pouches/Slits

  • Openings in the pharynx.

  • In aquatic chordates → function in filter-feeding or respiration (gills).

  • In terrestrial vertebrates → develop into structures like the Eustachian tube, tonsils, and parathyroid glands.

• Endostyle / Thyroid Gland

  • Endostyle secretes mucus for trapping food in protochordates.

  • In vertebrates, it evolves into the thyroid gland, regulating metabolism.

• Postanal Tail

  • Extends beyond the anus.

  • Provides locomotion in aquatic species (e.g., tunicate larvae, fish).

  • Reduced in humans to the coccyx.

What are tunicates and how do they express chordate traits?

• General: Marine animals (~3,000 spp), body covered in tunic.

• Habitat: Found from shallow shores to deep waters.

• Lifestyle: Adults mostly sessile filter‑feeders; some free‑living.

• Chordate hallmarks: Present only in larval stage (notochord, dorsal nerve cord, pharyngeal slits, endostyle, postanal tail).

• Metamorphosis:

  • Notochord + tail disappear.

  • Dorsal nerve cord reduces to a simple ganglion.

• Significance: Start life with full chordate features, but lose most as adults → example of retrogressive metamorphosis.

Describe Ascidiacea (sea squirts) in terms of metamorphosis, feeding, circulation, and nervous system.

• Metamorphosis: Larvae show all chordate hallmarks; settle head‑first; tail, notochord, muscle segments, and nerve cord disappear → sessile adult.

• Feeding: Oral siphon in, atrial siphon out; pharynx with slits; endostyle secretes mucus; cilia trap food → oesophagus → stomach → absorption; wastes via anus.

• Circulation: Ventral heart + two vessels; diffuse network over organs; heart reverses pumping direction.

• Nervous system: Reduced; only dorsal nerve ganglion + subneural gland connected to pharynx.

How do Ascidiacea (sea squirts) reproduce?

• Sexual reproduction: Hermaphroditic (ovary + testis). Gametes released into atrial cavity → water.

  • Broadcast spawning (solitary species, external fertilization).

  • Internal fertilization/brooding (colonial species, viviparous, embryos brooded in atrium).

  • Self‑incompatibility prevents selfing, but possible at low densities.

  • Eggs have vitelline coat + follicle cells producing sperm attractants.

• Asexual reproduction: Budding in colonial species.

  • Stolonial budding (rootlike stolons).

  • Epicardial/parietal budding (from digestive tract or epidermis).

  • Fragmentation → colonies regenerate after division/damage.

What are the main characteristics of Class Thaliacea?

• Lifestyle: Free‑swimming, pelagic (live in open water).

• Body: Transparent test (outer covering).

• Openings: Branchial (oral) and atrial apertures at opposite ends.

• Pharynx: Small compared to Ascidiacea.

• Chordate hallmarks: Absent in adults (no notochord, nerve cord, or tail).

• Reproduction:

  • Bisexual (hermaphroditic).

  • Asexual reproduction by budding.

  • Alternation of generations common (switch between sexual and asexual stages).

  • Tailed larva may be present or absent depending on species.

What are the main characteristics of Class Appendicularia?

• Lifestyle: Free‑swimming, pelagic, very small tunicates.

• Body covering: Lack a true tunic; instead, they secrete a gelatinous “house” that surrounds them.

• Feeding: The house acts as a filter‑feeding apparatus, trapping food particles from water.

• Chordate hallmarks: Retain larval features throughout life (notochord and tail persist).

• Reproduction: Sexual, usually hermaphroditic.

• Significance: Show neoteny (larval traits retained in adult stage).

Cephalochordata; feeding,circulation,nervous system, reproduction and purpose of study.

Feeding

• Filter-feeding suspension feeder.

• Oral hood + cilia drive water into pharynx.

• Endostyle secretes mucus → traps food.

• Food → intestine → hepatic caecum (digestion).

• Waste → anus; water → atriopore.

Circulation

• No heart, no blood pigments.

• Ventral aorta contracts peristaltically → pumps blood forward.

• Blood → branchial arteries → dorsal aorta → tissues.

• Function: nutrient distribution only; gas exchange via body surface.

Nervous System

• Hollow dorsal nerve cord with paired roots.

• Simple sensory organs: ocellus (light/dark detection).

• No image-forming eyes.

• Anterior nerve cord homologous to vertebrate brain.

Reproduction

• Separate sexes.

• Gametes released into atrium → external fertilization in water.

• Larvae hatch soon after → gradual metamorphosis into adult.

Purpose of Study

• Retains all five chordate hallmarks (notochord, dorsal nerve cord, pharyngeal slits, endostyle, post-anal tail).

• Body plan similar to vertebrate ancestor.

• Primitive vertebrate-like features: hepatic caecum, segmented musculature, basic circulation.

• Key model for studying vertebrate evolution.

Adaptations in Early Vertebrate Evolution

1. Musculoskeletal Modifications

• Endoskeleton → allowed unlimited body size, structural support, protection.

• Neural spines → increased surface area for muscle attachment.

• Myomeres: shifted from V‑shaped → W‑shaped → stronger, more complex folding for powerful swimming.

• Fin rays (dermal origin) → improved locomotion.

• Bone tissue → stronger than cartilage, ideal for muscle attachment; also stored/regulate Ca & P (needed for high metabolism).

• Keratinized structures → protective coverings (scales, feathers, hair, claws, horns).

2. Physiological Upgrades

• Digestive, respiratory, circulatory, excretory systems adapted for higher metabolic demand.

• Muscular pharynx → powerful pump for water movement.

• Highly vascularized gills → efficient gas exchange.

• Digestive glands (liver, pancreas, gastric glands) → managed larger food intake.

• Three‑chambered heart (sinus venosus, atrium, ventricle) + hemoglobin → efficient transport of gases/nutrients.

• Paired kidneys → waste removal + fluid/ion regulation.

3. Head, Brain & Sensory Systems

• Anterior nerve cord enlarged → protected by cranium.

• Evolution of paired sense organs: eyes, pressure receptors, chemical receptors, lateral line, electroreceptors.

• Neural crest → contributed to cranium, pharyngeal skeleton, tooth dentine, nerves, ganglia, endocrine glands.

• Ectodermal placodes → gave rise to sensory structures (lens, inner ear, olfactory epithelium, mechanoreceptors).

• Hox genes → controlled body plan along head‑tail axis.

Evolutionary Impact

• These adaptations supported larger body size, higher activity, and metabolic rates.

• Enabled vertebrates to diversify into modern fishes and tetrapods.

Early Jawed Vertebrates (Gnathostomes)

Origin of Jaws

• Derived from 1st & 2nd gill arches

• Expansion + new muscles → hinged biting structure

• Function: capture larger prey, manipulate food

Paired Appendages

• Pectoral & pelvic fins/limbs

• Began as stabilizers → evolved into locomotory structures

• Improved speed & maneuverability

Evolutionary Impact

• Jaws + paired fins = key innovations

• Enabled vertebrate diversification → modern fishes & tetrapods (including humans)

Class Myxini (Hagfishes)

Key Traits

• Marine, ~78 species

• Slender, eel‑like body; naked skin with slime glands

• No paired appendages, no dorsal fin

• Cartilaginous skeleton; mouth with two rows of eversible teeth

• 3 accessory hearts + main heart

• 5–16 pairs of gills; no stomach

Biology

• Scavengers/predators (annelids, molluscs, crustaceans, dead fish)

• Knot‑tying for leverage & cleaning

• Reproduction poorly known: few large yolky eggs, no larval stage

• Separate sexes, external fertilization

Class Petromyzontida (Lampreys)

General Traits

• ~41 species; slender, eel‑like body with naked skin

• 1–2 dorsal fins; no paired appendages

• Cartilaginous skeleton; sucker‑like oral disc with keratinized teeth

• 7 pairs of gills with external openings

• Intestine with spiral fold; no distinct stomach

• Well‑developed eyes, smell, taste, hearing; 2 semicircular canals

Feeding

• Parasitic species: attach to fish with oral disc, rasp flesh, suck fluids, inject anticoagulants to keep blood flowing

• Non‑parasitic species: do not feed as adults; digestive tract degenerates into a non‑functional strand

Reproduction

• Marine species are anadromous: migrate upstream to spawn

• Males build nests by moving stones with oral discs and body vibrations

• Females attach to rocks, spawn; males attach to female’s head dorsally and fertilize eggs externally

• Sticky eggs adhere to pebbles; lightly covered with debris

• Adults die soon after spawning (semelparous)

Life Cycle

• Eggs hatch in ~2 weeks → larvae called ammocoetes

• Ammocoetes: suspension feeders, burrow in sandy low‑current areas, live 3–7 years

• Metamorphosis: eruption of eyes, oral hood replaced by oral disc with teeth, fins enlarge, gonads mature

• Adults migrate to sea (marine spp.) or remain in freshwater

• Adult stage lasts 12–20 months (parasitic feeding or non‑feeding depending on species)

Class Chondrichthyes (General)

Traits

• ~1200 extant species (28 freshwater)

• Appeared in Devonian (419–358 mya)

• Cartilaginous endoskeleton

• Large fusiform body or dorsoventrally depressed

• Paired pectoral fins, ventral mouth

• Skin naked or with placoid scales

• Respiration via pairs of gills → exposed gill slits

• No swim bladder or lung

• Sexes separate, gonads paired, internal fertilization

Sharks (Subclass Elasmobranchii)

Traits

• Large fusiform body

• Heterocercal tail: vertebral column extends into dorsal lobe → thrust + lift

• Mouth ventral

• Skin naked or with placoid scales

• 5 gill slits; spiracle behind eye

• Paired nostrils (olfaction)

Reproduction

• Internal fertilization

• Oviparous: yolky eggs in keratinized “mermaid’s purse”

• Ovoviviparous: embryos retained in uterus, nourished by yolk

• Viviparous: embryos nourished via placenta

• Males: pelvic fin modified into claspers for copulation

Prey Detection

• Olfactory organs → detect prey chemicals at distance

• Lateral line → mechanoreceptors sense vibrations

• Vision at close range

• Ampullae of Lorenzini → detect bioelectric fields, guide to prey, electroreception for buried prey

Rays (Subclass Elasmobranchii)

Traits

• Bottom dwellers

• Dorso‑ventrally flattened body, enlarged pectoral fins

• Spiracle dorsal; gill openings ventral

• Fins move in wavelike fashion for propulsion

Special Types

• Stingrays: slender whiplike tail with serrated venomous spine → painful wounds, slow healing

• Electric rays: sluggish; large electric organs on each side of head → ~50 volts, up to ~1 kW power → stun prey or deter predators

Chimaeras (Subclass Holocephali)

• ~48 extant species

• Upper jaw fused to cranium

• No teeth → large flat crushing plates

• Feed on molluscs, echinoderms, crustaceans, fishes

Class Actinopterygii (Ray‑Finned Fishes)

General

• ~29,600 spp (97% of all living fishes)

• Occupy nearly all habitats: high altitude lakes, deep sea, hot springs, Antarctic ice, caves, swamps, even land excursions (mudskippers)

Morphological Adaptations

• Light, thin cycloid & ctenoid scales (overlapping); some spp (eels, catfish) lack scales

• Homocercal tail → greater speed

• Dorsal fin flexible → camouflage, braking, complex movements, streamlining, social communication

• Swim bladder → buoyancy control

• Jaw modifications → improved suction feeding

• Gill arches → pharyngeal jaws for chewing, grinding, crushing

Characteristics

• Bony skeleton

• Caudal fin heterocercal or homocercal

• Skin with mucous glands + dermal scales

• Paired & median fins supported by dermal rays

• Jaws with teeth

• Respiration by gills

• Swim bladder present

• Single circulation, nucleated erythrocytes

• Sexes separate, external fertilisation

Class Sarcopterygii (Lobe‑Finned Fishes)

General

• Only 8 spp: 6 lungfishes, 2 coelacanths

• Early forms: lungs + gills, heterocercal tails, powerful jaws, heavy enamelled scales, strong fleshy lobed fins

Lungfishes

• South American & African spp can survive long dry periods

• Burrow into mud, secrete slime → cocoon → estivation until rains return

Coelacanths

• Origin: Devonian period

• Thought extinct ~70 mya; rediscovered 1938 (Comoro Islands), second spp found 1998 (Indonesia)

Locomotion in Water

Traits

• Swimming = most economical locomotion

• Myomers with connective tissue partitions → undulatory movement (waves of contraction)

• Eels: efficient at low speed but high drag at rapid movement

Adaptations

• Streamlined body, scales, slimy surface → reduce resistance

• Fins maintain stability:

  • Dorsal/ventral → reduce roll & yaw

  • Pectoral/pelvic → control pitch

  • Tail fin → forward thrust

Hearing in Fishes

Traits

• Sound vibrations detected in inner ear

• Ostariophysi: Weberian ossicles transmit vibrations from swim bladder → inner ear

• Herrings/anchovies: swim bladder directly connected to inner ear

Buoyancy Adaptations

Sharks

• Slightly heavier than water → must keep moving

• Heterocercal tail provides lift

• Large liver with squalene (0.86 g/ml) aids buoyancy

Bony Fishes

• Swim bladder → neutral buoyancy without muscular effort

• Origin: paired lungs of Devonian bony fishes

• Absent in tunas, abyssal fishes, bottom dwellers

Respiration in Fishes

Gills

• Located in pharyngeal cavity, covered by operculum

• Gill filaments + lamellae → gas exchange

• Countercurrent flow: water vs blood → maximizes O₂ uptake (up to 85%)

• Some spp (mackerel, herring) use ram ventilation

Alternative Respiration

• Lungs (lungfishes)

• Skin (freshwater eels)

• Mouth lining (electric eels)

• Special air chambers (Indian climbing perch)

Osmotic Regulation

Freshwater Fishes (Hyperosmotic regulators)

• Body fluids saltier than surroundings

• Water enters osmotically, salts diffuse out

• Do not drink water; salts absorbed via gills + food

• Kidneys remove excess water → dilute urine

Marine Fishes (Hypoosmotic regulators)

• Body fluids less salty than seawater

• Lose water, gain salts

• Drink seawater; salts pumped out via gills

• Small volume of concentrated urine

• Excess salts excreted by kidneys

Feeding Adaptations

Carnivorous Fishes

• Feed on zooplankton, insect larvae, vertebrates

• Swallow prey whole with sharp teeth

• Short digestive tract

Herbivorous Fishes

• Eat plants, macroalgae

• Longer digestive tract

Suspension Feeders

• Mostly pelagic

• Use sieve‑like gill rakers to filter plankton

Evolutionary Adaptations

• Jaws evolved from filter feeders → predators

• Stronger muscles → better movement & prey capture

• Improved sense organs → enhanced predation

General Gas & Water Exchange in Amphibia

• Amphibians rely on moist skin for gas exchange → must remain in damp environments

• Cutaneous respiration: skin richly supplied with blood vessels, thin epidermis → direct O₂ & CO₂ exchange

• Gas exchange also via lungs, buccal cavity, and gills (in larval stages)

• Water balance closely tied to environment → skin permeable to water

Order Gymnophiona (Apoda – Caecilians)

General Features

• Limbless, burrowing amphibians; worm‑ or snake‑like body

• Small eyes, often covered → poor vision

• Live in moist soil environments

• About 160 species

Reproduction

• Internal fertilisation

• Some species viviparous (young develop inside female)

• Others oviparous (eggs laid in moist soil)

• Parental care present in some species

Order Urodela (Caudata – Salamanders)

General Features

• Elongated body, tail retained throughout life

• Limbs short, similar size front & back

• About 500 species

Respiration

• Cutaneous respiration (skin)

• Buccal cavity respiration

• Some species retain gills; others develop lungs

• Moist environments essential for gas exchange

Order Anura (Frogs & Toads)

Habits

• Solitary, silent except breeding season

• Jump into water when disturbed; hibernate in temperate climates

• Males establish territories during breeding

Respiration

• Adults: lungs, buccal breathing, cutaneous respiration

• Tadpoles: internal gills + skin

Feeding & Digestion

• Adults: carnivorous, short digestive tract, sticky protrusible tongue, teeth prevent escape (not mastication)

• Larvae: herbivorous, long digestive tract

Sensory Organs

• Tripartite brain (fore, mid, hind)

• Olfactory epithelium (smell), ears (airborne sounds), eyes (vision)

• Lateral line in aquatic larvae/adults

• Taste buds on tongue & palate

Reproduction

• Breed in warm season; external fertilisation during amplexus

• Eggs laid in water → jelly surrounds them

• Other strategies: foam masses, eggs on leaves/stems overhanging water, damp burrows, tree cavities