Fish evolution and diversity

Assessment 1: Feeding Ecology Report (40%)

  • Based on lab practical (fish dissection)

  • Practical:

    • Collection of feeding morphology data

    • Data pooled into a large class dataset

  • Analysis focuses on:

    • Mouth size vs gut length

    • Diet type

    • Trophic level

  • Attendance optional if uncomfortable with dissection:

    • Video alternatives available

    • Must notify staff in advance

Assessment 2: Online Exam (60%)

  • 24-hour online exam

  • Four questions, answer two essays

  • Emphasis on independent thinking

  • Generative AI use = academic misconduct

    • AI may be used as a tool, not to generate answers

    • Scientific careers require understanding, not just output

3. Why Teleost Fish Matter

Teleost dominance

  • Teleosts = >95% of all fish species

  • Also the most diverse vertebrate group

  • ~37,000 species described

  • 300–400 new species described annually

Evolutionary context

  • Earlier fish groups:

    • Jawless fishes

    • Cartilaginous fishes

    • Primitive bony fishes (gars, sturgeons, bowfin)

  • Many groups declined or went extinct

  • Teleosts diversified rapidly, especially after mass extinction events

4. Evolutionary Origin of Teleosts

  • Appeared ~200–215 million years ago

  • Likely evolved from holostean ancestors

  • Underwent major diversification during:

    • Late Jurassic–Cretaceous

  • Multiple evolutionary radiations

  • Originally up to ~7 teleost lineages

  • Four extant groups remain

Major teleost groupings

(from most primitive → most derived)

  1. Osteoglossomorpha

  2. Elopomorpha

  3. Otocephala

  4. Euteleostei (most advanced)

You are not expected to memorise all taxonomic names, but should understand primitive vs derived trends

5. Why Are Teleosts So Successful? (Core Question)

Central answer:

Teleost success is driven by refinements in feeding and locomotion

Key evolutionary trends:

  1. Reduction in bone

  2. Fin repositioning and specialisation

  3. Caudal fin modification

  4. Improved buoyancy control

  5. Advanced feeding mechanisms

  6. Diet diversification

6. Reduction in Bone: Buoyancy & Efficiency

Why reduce bone?

  • Bone is heavy → promotes sinking

  • Reducing bone improves:

    • Neutral buoyancy

    • Swimming efficiency

How bone reduction occurs

  • Fewer vertebrae:

    • Primitive teleosts: 60–80 vertebrae

    • Advanced teleosts: 20–30 vertebrae

  • Vertebrae become:

    • More rigid

    • Partially fused

  • Reduction in:

    • Skull bones

    • Vertebral spines

    • Fin rays

    • Scale ossification

Functional consequences

  • Stiffer vertebral column:

    • Less lateral flex

    • More muscle force converted into forward thrust

  • Less energy wasted maintaining position in water column

  • Improved swimming efficiency

7. Caudal Fin Evolution & Locomotion

Tail types through evolution

  1. Heterocercal tail

    • Upper lobe larger

    • Found in sharks, sturgeons, primitive bony fish

  2. Homocercal tail

    • Externally symmetrical

    • Characteristic of teleosts

Hypural plate

  • In advanced teleosts:

    • Hypural bones fused into a hypural plate

  • Function:

    • Stiffens tail base

    • Enhances thrust transmission

    • Focuses propulsive force into caudal fin

8. Vertebral Spines & Skeletal Simplification

  • Primitive teleosts:

    • Numerous fine vertebral spines

  • Advanced teleosts:

    • Fewer spines

    • Spines remain attached to vertebrae

  • Consequence:

    • Cleaner muscle blocks

    • More efficient force transfer

  • Familiar example:

    • Small pin bones in salmon vs sardines

9. Scale Reduction

Primitive teleost scales

  • Large

  • Thick

  • Heavily ossified (apatite)

  • Very rigid (e.g. Arapaima)

Advanced teleost scales

  • Smaller

  • Thinner

  • Reduced mineralisation

  • Increased flexibility

Benefit

  • Reduced body mass

  • Greater manoeuvrability

10. Median Fins (Dorsal, Anal, Caudal)

Primitive condition

  • Single dorsal fin

  • Mid-body position

  • Function:

    • Prevents rolling

Advanced condition

  • Multiple dorsal fins

  • First dorsal often spiny

  • Functions:

    • Stability

    • Manoeuvrability

    • Defence

Additional functions

  • Locomotion (e.g. sunfish sculling)

  • Social signalling (dominance/subordination)

  • Feeding (anglerfish lure)

  • Attachment (remoras)

  • Venom delivery (scorpionfish, weaverfish)

  • Camouflage (Sargassum fish)

  • Sexual selection (ornamental fins)

11. Paired Fins (Pectoral & Pelvic)

Primitive arrangement

  • Pectoral fins: thoracic

  • Pelvic fins: abdominal

  • Orientation: horizontal

  • Primary function: stability

Derived arrangement

  • Pectoral fins:

    • Move up body side

    • Rotate to vertical orientation

  • Pelvic fins:

    • Move forward (thoracic or jugular)

Functional improvements

  • Fine-scale manoeuvrability

  • Braking

  • Slow controlled swimming

  • Stability in complex habitats

  • Defensive spines

12. Swimming Modes & Efficiency

Evolution of swimming styles

  1. Anguilliform

    • Whole-body undulation

    • Multiple contraction waves

    • Inefficient (eels)

  2. Sub-carangiform

    • Rear half of body undulates

  3. Carangiform

    • Rear third moves

  4. Thunniform

    • Minimal body movement

    • Tail-driven propulsion (tuna)

Key trend

  • Increasing rigidity

  • Power focused into caudal fin

  • Faster, more efficient swimming

13. Swim Bladder Evolution

Two types

  • Physostomous (open)

    • Pneumatic duct to gut

    • Primitive condition

  • Physoclistous (closed)

    • No duct

    • Advanced teleosts

Benefits of closed swim bladder

  • Precise buoyancy control

  • Small positional adjustments

  • Greater ecological versatility

14. Feeding Apparatus Innovation

Primitive feeding

  • Upper jaw fused to skull

  • Simple open-close motion

Teleost innovation: protrusible jaw

  • Maxilla loosely attached

  • Premaxilla mobile

  • Mouth opens:

    • Downwards

    • Outwards

  • Produces suction feeding

Advantages

  • Increased gape size

  • Efficient prey capture

  • Enables feeding without direct contact

Limitations

  • Large gape reduces suction efficiency

  • Large predators rely more on:

    • Active pursuit

    • Grasp-and-swallow feeding

15. Diet Diversification

Teleosts exhibit extreme dietary diversity:

  • Herbivores

  • Molluscivores

  • Piscivores

  • Corallivores

  • Planktivores

Problem: hard-to-digest prey

  • Shells

  • Exoskeletons

  • Cellulose

Solution: pharyngeal teeth

  • Located in pharynx

  • Used to:

    • Crush

    • Grind

    • Process prey

  • Examples:

    • Sheephead (separate teeth)

    • Freshwater drum (tooth plates)

16. Summary: Why Teleosts Dominate

  • Teleosts = ~95% of fish species

  • Evolutionary success driven by:

    • Reduced skeletal mass

    • Efficient propulsion

    • Fin specialisation

    • Advanced buoyancy control

    • Highly effective feeding mechanisms

    • Extreme diet diversification

Teleosts are fast, manoeuvrable, spiny fish with highly efficient feeding systems — making them the most successful vertebrate radiation on Earth.