In-Depth Notes on Fishes

Chapter 16: Fishes

What Is a Fish?

  • Definition:

    • Fish refer specifically to aquatic vertebrates, excluding non-vertebrate names like jellyfish and starfish.

  • Key Characteristics:

    1. Gills: Extract oxygen from water with 1/20 the oxygen concentration of air.

    2. Fins: Appendages usually in the form of fins.

    3. Scales: Skin typically covered in scales of dermal origin.

    4. Lateral line system: Detects water currents and vibrations.

  • Evolutionary Link:

    • Fishes are vital in the evolutionary history connecting aquatic vertebrates to tetrapods (land vertebrates).

Ancestry and Relationships of Major Groups of Fishes

  • Diversity: Fishes constitute approximately half of the known 64,000 vertebrate species.

  • Major Groups:

    1. Jawless Fishes:

    • Examples: Hagfishes, Lampreys.

    1. Cartilaginous Fishes.

    2. Bony Fishes:

    • Divided into Ray-finned and Lobe-finned fishes.

Geological Era: Age of Fishes

  • Devonian Period (419.2-358.9 MYA): Known as the “Age of Fishes,” marking the prevalence of various fish groups.

  • Placoderms: An ancient class of giant armored fish that became extinct without descendants.

Osteichthyes - Bony Fish

  • Clades:

    • Composed of:

    1. Actinopterygii:

      • Known as ray-finned fishes.

      • Most observable bony fishes.

    2. Sarcopterygii:

      • Include lobe-finned fishes and tetrapods.

      • Includes few modern species: Lungfish, Coelacanths.

Living Jawless Fishes - Cyclostomata

  • Characteristics:

    • Approximately 119 species, divided into Hagfishes (Myxini) and Lampreys (Petromyzontida).

    • Lack jaws, scales, and paired fins, and have pore-like gill openings.

    • Controversial grouping in taxonomy based on molecular data.

Hagfishes - Myxini

  • Habitat: Entirely marine; scavengers and predators.

  • Adaptations:

    • Acute sense of smell and touch, and keratinized plates on the tongue.

    • Unique behavior: ties knots in tail to leverage against prey.

    • Produces a slimy substance for defense.

Lampreys - Petromyzontida

  • Species: 41 species globally, includes parasitic and non-parasitic types.

  • Life Cycle:

    • Ascend freshwater streams for breeding; undergo metamorphosis from larvae to adults.

    • Poor life span post-metamorphosis with most species dying after spawning.

Parasitic Behavior

  • Marine parasitic lampreys attach to fish using sucker-like mouths.

  • They inflict wounds and use anticoagulant substances to feed on fish blood.

Cartilaginous Fishes: Class Chondrichthyes

  • Diversity: Around 1200 living species.

  • Characteristics:

    • Lack true bones; instead possess mineralized tissues in scales and teeth.

    • Divided into two clades:

    1. Elasmobranchii – Sharks, skates, rays.

    2. Holocephali – Chimaeras.

Sharks and Their Physiology

  • General Traits: Predaceous with well-developed sensory organs.

  • Shark Anatomy:

    • Have 5-7 pairs of gill slits; streamlined bodies.

    • Asymmetrical heterocercal tail for thrust and lift.

  • Reproduction: Varies; includes oviparous, ovoviviparous, and viviparous methods.

Rays

  • More than half of elasmobranchs; characterized by flat bodies and enlarged pectoral fins.

  • Feeding Adaptations: Flat teeth suitable for crushing prey like mollusks and crustaceans.

Bony Fishes and Tetrapods: Class Osteichthyes

  • Unified Features:

    1. Bone replaces cartilage during development.

    2. Presence of a lung or swim bladder.

    3. Shared cranial and dental characteristics.

Diversification of Bony Fishes

  • Two major clades evolved by the Devonian:

    1. Ray-finned Fishes (Actinopterygii).

    2. Lobe-finned Fishes (Sarcopterygii).

Swim Bladder Function

  • Enabling neutral buoyancy through gas regulation, crucial for depth control in various aquatic ecosystems.

Respiratory Adaptations

  • Fish gills extract oxygen through a countercurrent exchange mechanism, maximizing gas exchange efficiency.

Osmoregulation Strategies Freshwater Fish: Hyperosmotic regulators; actively excrete excess water while absorbing salts via specialized cells.

  • Marine Fish: Hypoosmotic regulators; drink seawater and use specialized cells to expel excess salt from their bodies.

Migration Patterns

  • Catadromous - eels migrate to the sea to spawn.

  • Anadromous - such as salmon, migrate upstream to spawn after spending adult life at sea.

Reproductive Strategies

  • Most fish are dioecious with external fertilization and many are oviparous, releasing large numbers of eggs during spawning season.

Aging Fish

  • Fish growth is determined by environment; age can be estimated by counting annuli (distinct opaque zones in otolith structures). Otoliths provide both sense of balance and hearing.