The Biology of Fish Flashcards
What does it mean to say fish are a ‘grade’ rather than a ‘clade’
Fish are a paraphyletic group, meaning they include some but not all descendants of a common ancestor. They exclude tetrapods, so they form a grade, not a clade (which would include all descendants). This makes “fish” a term of convenience, not strict evolutionary lineage
What are the three major extant groups of fish and their characteristics
Cyclostomes (Agnatha): Jawless, e.g. lampreys, hagfish
Chondrichthyes: Cartilaginous skeleton (sharks, rays, chimaeras)
Osteichthyes: Bony fish; includes
- Actinopterygians: Ray-finned
- Sarcopterygians: Lobe-finned, includes ancestors of tetrapods
Describe the major evolutionary timeline of fish
Ordovician (~485–443 MYA): First chordates → Craniata → Agnatha
Devonian (~419–359 MYA): “Age of Fishes”; evolution of jaws → Gnathostomata
Post-Devonian: Bony fish diversify, Sarcopterygians give rise to tetrapods, Chondrichthyes and teleosts flourish
What features define Agnathans
No jaws
No paired fins
Notochord persists
7+ gill pouches
2-chambered heart
Single gonad
Long, cylindrical body
What makes Hagfish unique among vertebrates
No vertebrae (not technically vertebrates)
Produce slime via mucous glands
Feed on carrion using rasping plates
Tie body in knots to remove slime
Isosmotic to seawater (unusual for vertebrates
What are distinctive features of lampreys
Parasitic as adults (oral disc with teeth)
Tidal ventilation for respiration while feeding
Have rudimentary vertebrae
No stomach, paired fins, or jaw
2 semicircular canals
What major traits define Gnathostomes
Jaws (evolved from mandibular gill arches)
Paired fins
Duplication of Hox genes (added complexity)
Third semicircular canal
Advanced vertebrae, ribs, two nostrils
Lateral line system
How do jaws help vertebrate evolution
Enable active predation
Greater food variety
Support for new respiratory and sensory structures
Derived from 1st branchial (gill) arch
What is the lateral line system and neuromasts
A mechanosensory system detecting water vibrations
Neuromasts: sensory hair cells that detect movement
- Superficial neuromasts: on skin
- Canal neuromasts: within subdermal canals
Used to detect predators, prey, water currents
Describe the role of semicircular canals in fish
Detect angular movement (pitch, roll, yaw)
Hagfish: 1 canal
Lamprey: 2 canals
Jawed vertebrates: 3 canals
What is the vestibular system responsible for
Detects body rotation and orientation
Works with vision and lateral line for balance
Involved in controlling eye and fin movements
How did paired fins evolve and what are their roles
Evolved from spiny projections (as in Euthacanthus)
Became discrete pectoral and pelvic fins
Functions:
Locomotor: Steering, braking, hovering
Non-locomotor: Defense, sexual display, communication
How do fish generate thrust
Produce traveling waves from head to tail
Propulsion by reaction forces (Newton’s Third Law)
Shed vortices, creating vortex trails
What types of drag do swimming fish encounter
Skin friction drag – reduced by mucus/scales
Pressure drag – streamlined shape reduces resistance
Vortex (form) drag – minimised with proper movement
How do fish reduce drag
Streamlined shape
Slime
Scale Orientation
Compare Chondrichthyes and Osteichthyes
Chondrichthyes (cartilaginous):
No swim bladder, use oil-rich liver
Placoid scales
Internal fertilisation
Osteichthyes (bony):
Swim bladder
Bony, overlapping scales
External fertilisation common
What are placoid scales and their functions
Tooth-like structures in sharks
Made of vitrodentine, dentine, pulp
Reduce drag and turbulence via microturbulence
How do teleosts ventilate their gills
Use a dual pump: buccal and opercular cavities
Water flows unidirectionally over gills
Efficient counter-current gas exchange
Shorter diffusion distance in active species (e.g. tuna)
What alternative breathing methods exist in fish
Evolved in hypoxic environments
Structures:
Labyrinth organs (e.g., bettas)
Vascularised gill chambers
Skin respiration (mudskippers)
Air can be swallowed or bubbled for respiration
How do physostomous and physoclistous fish differ
Physostomes:
Swim bladder connected to gut
Control gas by gulping/burping
Physoclists:
No gut connection
Use gas gland, rete mirabile, and Root effect to regulate gas
How do Chondrichthyes maintain bouyancy without a swim bladder
Oil-rich liver (contains squalene)
Generate dynamic lift with pectoral fins
Some (e.g. skates) rest on the bottom
How do fish regulate salt and water in different environments
Teleosts:
Marine: Drink seawater, excrete salt, low urine
Freshwater: Don’t drink, dilute urine, actively uptake salts
Chondrichthyes (marine):
Maintain high urea and TMAO
Use rectal gland to excrete NaCl
Freshwater species have low urea/ions
What is the significance of the duplication of Hox genes in jawed fish
Gene duplication allowed for greater morphological complexity, especially in the development of jaws and paired appendages - a key innovation for vertebrate diversification.
How do Hagfish feed and what adaptations support this
Scavengers: feed on dead/dying animals
Use keratinous tooth plates to rasp flesh
Tie body in knots for leverage and to clean slime
Can survive months without food
