Chapter 24- Fishes

Learning Outcomes

  • 24.1: List the five major groups of living fishes and diagram their phylogenetic relationship.

  • 24.2: Compare and contrast the anatomy and ecology of hagfishes and lampreys.

  • 24.3: Describe the functional anatomy of sharks, rays, and chimaeras, focusing on sensory and reproductive systems.

  • 24.4: Compare the two major clades containing bony fishes.

  • 24.5: Describe how fishes swim, maintain buoyancy, and osmoregulate.

24.1 Fishes

  • The term "fish" is used for convenience and is not a cohesive taxonomic term; includes all vertebrates that are not tetrapods (four-legged animals).

  • Five Groups of Living Fishes:

    1. Myxini (Hagfishes)

    2. Petromyzontida (Lampreys)

    3. Chondrichthyes (Sharks, rays, chimaeras)

    4. Actinopterygii (Ray-finned fishes)

    5. Sarcopterygii (Lobe-finned fishes)

24.2 Cyclostomata

  • Extant jawless fishes, part of Agnatha including Myxini (Hagfishes) and Petromyzontida (Lampreys).

  • Vertebrae may be absent or reduced but they are still considered vertebrates due to ancestral lineage.

Myxini (Hagfishes)

  • Marine jawless fish; feed on annelids, crustaceans, and decaying animals.

  • Mostly blind, relying on strong smell and taste for navigation.

  • Notably slimy, they enter carcasses through an orifice and may tear flesh using a unique tongue mechanism with keratinized plates.

Petromyzontida (Lampreys)

  • Can be marine or freshwater; around half are parasitic.

  • All swim upstream to spawn; marine species are anadromous (migrate from sea to freshwater).

  • Larva (ammocoete) are filter feeders, resembling amphioxus; non-parasitic species do not feed after metamorphosis.

  • Parasitic forms attach to fish using sucker-like mouths, consume body fluids, and use anticoagulants to promote blood flow in wounds.

24.3 Chondrichthyes

  • Comprised of cartilaginous fishes including elasmobranchs (sharks, skates, rays) and holocephali (chimaeras).

  • Well-developed sensory organs, powerful jaws, and strong musculature facilitate carnivorous predation.

Body Plan of Sharks

  • Adapted for speed with fusiform body design and heterocercal tail.

  • Pectoral fins aid in maneuvering; spiracles for respiration and multiple external gill slits.

  • Covered in tough skin with placoid scales to reduce water turbulence during swimming.

Senses in Sharks

  • Highly sensitive olfactory and visual systems; can detect chemical cues from significant distances.

  • Lateral-line system used for detecting water movements; electroreception through ampullae of Lorenzini helps in locating electric fields.

Digestion and Reproduction in Sharks

  • Digestion characterized by a spiral valve that increases absorptive surface area.

  • Internal fertilization with varied reproductive strategies: oviparous (egg-laying), ovoviviparous (live birth from yolk), and viviparous (live birth with placental nourishment).

Rays

  • Specialized for bottom dwelling with flattened bodies and wing-like pectoral fins.

  • Gill slits located ventrally; adapted for feeding off the seabed without clogging.

  • Some species like stingrays have venomous spines for defense.

Chimaeras (Holocephali)

  • Distinctive features include large grinding plates instead of teeth.

  • Gills are covered by an operculum, with a single respiratory opening.

24.4 Osteichthyes

  • Encompasses 96% of living fishes and all tetrapods; characterized by endochondral bone, lungs or swim bladders, and a unique cranial structure.

  • Two main groups:

    1. Actinopterygii: Ray-finned fishes, largest group (31,000 species); includes teleosts.

    2. Sarcopterygii: Lobe-finned fishes, include lungfishes and coelacanths.

Actinopterygii vs. Sarcopterygii

  • Actinopterygii: Bony rays support fins.

  • Sarcopterygii: Few species remaining; lungfishes can live out of water for extended periods, while coelacanths were once believed extinct but were rediscovered.

24.5 Fish Function and Form

Swimming

  • Fish movement is facilitated by trunk and tail musculature, with myomeres organized in zigzag patterns allowing efficient propulsion.

  • The shape of fish affects swimming efficiency: stiffer bodies like tuna enhance speed, while flexible bodies like eels improve maneuverability.

Buoyancy

  • All fish are denser than water; Sharks must swim continuously to avoid sinking; other fishes possess swim bladders for buoyancy.

  • Some fish, like lungfish, possess lungs instead of swim bladders.

Osmoregulation

  • Critical for managing salt and water levels in varying environments:

    • Freshwater fish: Hyperosmotic regulators; absorb excess water, pump it out through kidneys, and actively absorb salts via gills.

    • Marine fish: Hypoosmotic regulators; drink seawater to manage dehydration, excrete excess salts through specialized cells in the gills or via the kidneys.