Marine Vertebrates and Evolutionary Biology

• Marine Vertebrates and Evolutionary Biology is about the classification of life, domains of life, symmetry in organisms, and the phyla of chordates

The Classification of Life

• Life classification follows a hierarchy: Kingdom → Phylum → Class → Order → Family → Genus → Species
• Trend: As you go down the classification, similarity increases and as you go up, the number of organisms increases
• Mnemonic for hierarchy: Kids Prefer Cheese Over Fried Green Spinach

Domains of Life

• Prokaryotes are cells without a nucleus
• Eukaryotes are cells with a nucleus
• The three domains include Bacteria, Archaea and Eukarya

Symmetry in Organisms

• Radial Symmetry is is found in jellyfish and anemones
  • Have a top (dorsal) and bottom (ventral) side, but no left or right sides
  • Often sessile or planktonic (floating)
  • Interact with the environment from all sides
• Bilateral Symmetry is found in turtles, lobsters, and humans
  • Have anterior (head), posterior (tail), left and right sides
  • Associated with cephalization (development of a head)
  • Typically actively mobile

Phylum Hemichordata

• Considered a link between Echinodermata and Chordata
• Embryonic similarities with Chordates
  • Dorsal hollow nerve cord is present
  • Pharyngeal pouches/gill slits are present
• Acorn Worms are included in this group (deposit feeders)

Phylum Chordata

• Key characteristics are present at some stage in life
  • Notochord, a flexible rod for support between the nerve cord and gut
  • Dorsal, hollow nerve cord develops into the nervous system (brain & spinal cord)
  • Pharyngeal slits filter-feed (invertebrates) or gas exchange (vertebrates)
  • Post-anal tail is important for locomotion in aquatic species

Subphyla of Chordata

• Subphylum Tunicata (Urochordata)
  • Larvae have all 4 key chordate characteristics
  • Adults lose the notochord and dorsal nerve cord
  • Adults are sessile, filter feeders
  • Sea squirts, salps, larvaceans are included in this group
• Classes of Tunicata:
  • Ascidiacea: Sessile adults, solitary/colonial forms
  • Appendicularia: Free-swimming, retains larval tail
  • Thaliacea: Transparent, barrel-shaped salps, form colonial chains
• Subphylum Cephalochordata (Lancelets) is composed of small fish-like organisms
  • a notochord acts as a spine
  • They are filter-feeders (planktivores)
  • They burrow in sand, leaving only the head exposed
  • Gas exchange occurs through skin
• Subphylum Vertebrata
  • First vertebrates were oceanic (~500 million years ago)
  • A vertebral column protects the nerve cord
    • The skull protects the brain & sensory organs
    • The endoskeleton supports the body and includes jointed appendages
    • Internal organs include a complete digestive tract, closed circulatory system, gills/lungs for breathing, paired kidneys for excretion/osmoregulation, and mostly separate sexes with sexual reproduction

Evolution of Vertebrates

• Craniates: Chordates with a head
• Advantages of Vertebrae & Skull
  • Provide more protection for the brain & spinal cord
  • Offer an advanced sensory system
• Evolutionary Adaptations
  • Head development is better for feeding
  • Paired appendages (fins/limbs) improved locomotion
  • Increased skeletal support offers more stability
• Physiological upgrades
  • A closed circulatory system
  • Efficient blood flow to appendages for movement

Vertebrate Phylogeny

• Superclass: Agnatha (Jawless Fishes)
  • Cartilaginous skeleton
  • Examples include Hagfish & Lampreys (only surviving species)
  • Cylindrical, eel-like body
  • Lacks paired fins
  • The Notochord remains throughout life
  • Use suction-feeding
  • Gill slits are present
  • Secrete slime for defense

Evolution and Classification of Fish

• There are over 33,000 species worldwide, the largest vertebrate group
• Aquatic vertebrates include
  • Gills for breathing
  • Ectothermic (cold-blooded) metabolism
  • Scales of dermal origin
  • Fins for stability and movement
• Jaws Evolved from anterior gill support arches
• Advantages of jaws:
  • Allowed feeding on larger prey
  • Paired fins developed for stability & controlled movement
• Major Fish Groups
  • Superclass Agnatha (Jawless Fish) – e.g., Lampreys & Hagfish
  • Superclass Gnathostomata (Jawed Fish)
    • Class Chondrichthyes (Cartilaginous Fish)
    • Class Osteichthyes (Bony Fish)

Class Chondrichthyes (Cartilaginous Fish)

• First appeared ~450 million years ago
• Skeleton made of cartilage, not bone
• Over 1,000 species worldwide, ~204 in South Africa
• Subclasses include
  • Elasmobranchii (Sharks, Skates, Rays) with 5-7 pairs of gill openings, movable upper jaw and separate teeth
  • Holocephali (Chimaeras, Elephantfish) with 1 pair of gill slits, upper jaw fused to the cranium, teeth fused into solid dental plates, and example includes the St Joseph Shark (Callorhinchus capensis)
• General Characteristics
  • Tough skin covered in placoid scales (or naked in Chimaeras)
    • Paired pectoral and pelvic fins
    • Males have claspers for internal fertilisation
    • Ventral mouth with jaws and polyphydont teeth
    • Spiral valve intestine for efficient digestion
    • No swim bladder, buoyancy maintained via oil-filled liver
    • Heterocercal tail (asymmetrical, larger upper lobe in sharks & rays)
    • Diphycercal tail in chimaeras
• Sensory Systems: lateral line detects vibrations in water; Ampullae of Lorenzini detect electric fields (electroreception); and highly developed smell, vision, and hearing
• Circulatory & Excretory System: Two-chambered heart; mesonephric kidneys excrete urea and a rectal gland for osmoregulation

Reproduction in Chondrichthyes

• Separate sexes and internal fertilisation (via claspers in males)
• Reproductive Strategies:
  • Oviparous: Egg-laying (e.g. skates)
  • Ovoviviparous: Eggs hatch inside the mother (e.g. some sharks)
  • Viviparous: Live birth (e.g. hammerhead sharks)

Adaptations of Sharks & Rays

• Shark Skin (Placoid Scales)
  • Reduces swimming turbulence
  • Modified into rows of teeth
  • Used for leather & sandpaper production
• Countershading
  • Dark dorsal (top) side, light ventral (bottom) side
  • Camouflage against predators & prey
• Teeth & Feeding
  • Polyphydont teeth (continuously replaced)
  • Teeth shape depends on diet:
    • Sharp, serrated teeth for cutting (e.g. Great White Shark)
    • Flat teeth for crushing (e.g. Rays)

Rays vs. Skates

• Rays are live-bearing, have an absent or reduced dorsal fin, a whip-like tail with a stinging spine, flat plate-like teeth for crushing, a kite-shaped body, and are generally larger than skates
• Skates are egg-laying, have a prominent dorsal fin, a fleshy tail with no spine, small teeth, a more rounded body, and are smaller than rays

Structural Differences from Agnatha (Jawless Fish)

• Presence of scales (denticles) covers the body
• Movable jaws with enamel-covered teeth are present
• Paired lateral fins (pectoral & pelvic fins) for stability and movement are present
• Three semicircular canals (balance and orientation) are present
• Paired reproductive organs & ducts are present
• Skeletal System: Cartilaginous skeleton (Chondocranium):
  • Cranium: Houses the brain , and Paired capsules for olfactory, optic, and auditory organs are present
  • Visceral skeleton includes jaws, hyoid arch, and 5 pairs of branchial arches which support the gills, and upper jaw is not fused to the braincase
  • Appendicular skeleton:
    • U-shaped pectoral girdle supports pectoral fins
    • Flatter pelvic girdle supports pelvic fins
    • Fins are supported by small jointed cartilages

Sensory Systems

• Nostrils & Olfactory Sacs detect dissolved chemicals (smell)
• Taste Buds are located on basihyal (tongue-like structure)
• Eyes are controlled by 3 pairs of muscles, has no eyelids, and contains a retina with only rods (adapted for dim light)
• Ears contain an organ of balance (3 semicircular canals)
• The lateral line is a groove running along the body and contains sensory hairs that detect low-frequency vibrations
• Ampullae of Lorenzini serve as an electroreception system that detects electric fields of prey
• No swim bladder

Respiration

• Gills for oxygen exchange
• Ram ventilation: Water enters the mouth and passes over gills while swimming
• Buccal pumping: Some species actively pump water over gills
• Counter-current exchange: Maximises oxygen absorption
• Spiracles are a modified gill slit used for water intake

Circulatory & Excretory Systems

• Two-chambered heart
• Main arteries include aortic arches, sinus venosus, atrium & ventricle, and the conus arteriosus
• Excretory System
  • Mesonephric kidneys are located along the dorsal aorta
  • Urine formation occurs as urine is collected → urinary duct → ureter → cloaca
  • The rectal gland removes excess salt from blood

Thermoregulation & Osmoregulation

• Most sharks are ectothermic, but some species (e.g., great white sharks) can maintain a higher body temperature for better hunting performance
• Osmoregulation of sharks
  • Maintain high urea & trimethylamine oxide (TMAO) concentrations in blood- Kidneys conserve urea, making blood osmotically balanced with seawater
• Bull sharks adjust kidney function to survive in both salt & freshwater

Reproduction in Elasmobranchs

• Separate sexes with internal fertilisation (males use claspers)
• Types of Reproduction:
  • Oviparous: egg-laying species (e.g., skates), eggs encased in leathery egg cases ("mermaid's purses")
  • Ovoviviparous: embryos develop inside egg cases within the mother with no placental connection, young feed on yolk sac, intrauterine cannibalism (oophagy & adelphophagy) in some species
  • Viviparous: live birth with placental nourishment (e.g., rays & some sharks)

Osteichthyes (Bony Fishes)

• Superclass: Gnathostomata (Jawed vertebrates)
• Class: Osteichthyes (Bony fishes)
  • Subclass: Actinopterygii (Ray-finned fishes, >27,000 species)
  • Subclass: Sarcopterygii (Lobe-finned fishes, e.g., lungfishes, coelacanths)
• Skin: Mucous glands, embedded dermal scales (cycloid, ctenoid, sometimes ganoid); some scaleless species.
• Fins: Median and paired fins; supported by cartilage or bone rays.

External and Internal Characteristics

• Mouth: Terminal with well-developed jaws and teeth; olfactory sacs separate from mouth cavity.
• Skeleton: Mostly bony; numerous vertebrae; typically homocercal tails.
• Circulatory System: Two-chambered heart (atrium & ventricle), venous blood only.
• Respiration: Gills covered by an operculum and the swim bladder is used for buoyancy regulation.
• Excretion via mesonephric kidneys
  • Larvae excrete ammonia; adults excrete urea.
• Reproduction: Usually oviparous, external fertilization common.
• Drag reduction is achieved via Skin Functions
  • Protection against predators
  • Osmoregulation & respiration
  • Secretion of mucus
  • Camouflage & communication
• Scales are cycloid, ctenoid, and ganoid; some species are scaleless
• Bony structure with numerous vertebrae provide the skeletal system and neural arches protect the spinal cord.
• Segmental muscles (myomeres) are W-shaped
  • Large muscles handle body & tail propulsion, small muscles control fins, gill region, head (fine movement)

Fish Shape and Feeding

• Body shape varies with lifestyle, fusiform for fast swimming, dorsoventrally flattened for bottom dwellers.
• Feeding adaptations include Suction feeding, Ram feeding, and Manipulative feeding

Buoyancy & Swim Bladder

• The swim bladder maintains neutral buoyancy
• The Gas gland & rete mirabile control gas levels
• Boyle's Law (↑ depth → ↑ pressure → compressed swim bladder → fish sinks).

Nervous System & Sensory Organs

• The brain & spinal cord coordinate body activities (CNS) with via 10 pairs of cranial nerves
  • Olfactory sacs facilitate Smell (via nares).
  • Lateral line detects vibrations.
  • Inner ear facilitates Balance & hearing.
  • Otoliths facilitate Balance, aging, and species identification.
  • Eyes are well-developed, focus on objects.

Excretion & Osmoregulation

• Marine fish lose water by osmosis, drink seawater but excrete excess salt via chloride cells in gills, and produce small amounts of concentrated urine.
• Freshwater fish gain water by osmosis, absorb salts in gills & excrete dilute urine.

Reproduction & Development

• Most species have separate sexes, gonads paired, with urogenital openings, sex hormones regulate the timing of spawning & larval growth
• Fertilization types are external and internal
• Breeding strategies -Non-guarders: Large numbers of small eggs; no parental care -Guarders: Eggs laid in nests/burrows & guarded by parents -Bearers: Few large eggs carried inside the body or in mouth/skin pouches
• Hermaphroditism includes simultaneous and sequential hermaphroditism
• Migratory patterns involve moving to spawning grounds in response to seasonal timing affected by temperature and day length
• Courtship displays use color changes, fin movement, “dances”

Evolutionary Adaptations

• Some features evolved for one function but were later adapted for another these include
  • Fins for swimming/walking/tasting/suction
  • The swim bladder for digestion/ respiration/buoyancy/communication
  • Gills/jaws originally for respiration but moved to biting, chewing, and defense
  • Scales were initially for protection and became for minimizing drag during swimming.