FISHES

Phylum Chordata Overview

• Phylum Chordata: Contains animals possessing a notochord at some stage of life.

• Sub-phylum Vertebrates (Craniata): Characterized by vertebral columns and cranium.

Classification and Groups

• Grade of Fishes: All vertebrates excluding tetrapods.

• Major groups within Chordata include:

  • Vertebrata: Chordates with vertebrae and cranium.

  • Gnathostomata: Vertebrates with jaws.

  • Osteichthyes: Bony fish and tetrapods.

  • Tetrapoda: Four-limbed vertebrates.

  • Amniota: Tetrapods whose embryos have extraembryonic membranes.

Key Subgroups

Protochordata

• Cephalochordata: Example is lancelets.

Agnatha

• Includes hagfish and lampreys (jawless fishes).

Cyclostomata

• Includes jawless fishes like lampreys and hagfishes.

Chondrichthyes

• Cartilaginous fishes including sharks and rays.

Actinopterygii

• Ray-finned fishes.

Sarcopterygii

• Lobe-finned fishes, includes tetrapods.

Characteristics of Fishes

• General Features:

  • Aquatic habitat, gills for gas exchange.

  • Fins for appendages when present.

  • Skin usually covered with bony scales.

• Approximately 28,000 species known, comparable to tetrapods.

Jawless Fishes (Agnatha)

General Characteristics

• Lack jaws; have rasping teeth made of keratin.

• Cartilaginous skeleton, notochord retained in adults.

• Absence of paired appendages and bony scales.

Feeding and Lifestyle

• Use pharyngeal gill slits for gas exchange (except in filter-feeding larval stage).

• Adults are scavengers or parasites.

Jawed Vertebrates (Gnathostomata)

Key Features

• Presence of articulated jaws allowing for diverse feeding habits.

• Paired appendages (fins/limbs), improved sensory systems.

• Skeleton may be bony or cartilaginous.

Evolutionary Innovations

• Inner ear with three semicircular canals vital for balance.

• More elaborate skeletal structure with paired appendages.

Cartilaginous Fishes (Chondrichthyes)

Anatomy and Physiology

• Exposed gill slits and a ventral mouth; skeleton primarily cartilaginous.

• Various specialized features to reduce water turbulence and assist in gas exchange.

Unique Adaptations

• Continuous swimming aids in gas exchange (ram ventilation).

• Heavy skeletal structure compensated by large oily liver aiding in buoyancy.

Excretion and Osmoregulation

• Adapted to coastal and oceanic environments, strategies for maintaining osmolarity include drinking seawater and pumping out excess ions.

Reproductive Strategies

• Internal fertilization is common; may exhibit oviparity, ovoviviparity, or viviparity.

Bony Fish (Osteichthyes)

Key Features

• Possess a bony skeleton and covered gill slits (operculum).

• Presence of lungs or swim bladder enhancing buoyancy and gas exchange.

Classification

• Includes Ray-finned fishes (Actinopterygii) with >28K species and Lobe-finned fishes with fewer species.

Synapomorphies of Osteichthyes

• Endoskeleton made primarily of bone, development of swim bladder, aiding buoyancy and going deeper into water.

Gas Exchange Mechanism

• Operculum facilitates active ventilation of gills; counter-current gas exchange maximizes efficiency.

Environmental Adaptations

• Adaptations to various osmotic conditions: strategies for excreting excess salts or reabsorbing water as needed in different environments.