Fishes and Aquatic Vertebrates

Fishes are the most diverse vertebrates and the first to evolve, spanning jawless lampreys and hagfishes, cartilaginous sharks and rays, and the dominant ray-finned bony fishes.

Definition

Fishes and aquatic vertebrates are the cold-blooded, gill-breathing vertebrates that live primarily in water, comprising the jawless fishes, the cartilaginous fishes, and the bony fishes, and representing the earliest and most diverse vertebrate radiations.

Scope

This topic surveys the aquatic vertebrates and the key innovations that arose among them: the origin of vertebrae and the head, the evolution of jaws, paired fins, and bony or cartilaginous skeletons, and the gill-based respiration and buoyancy control suited to water. It covers the major groups, the jawless fishes, the cartilaginous chondrichthyans, and the bony fishes including the ray-finned and lobe-finned lineages, the last of which gave rise to land vertebrates.

Core questions

• What innovations distinguish the early vertebrates and the origin of jaws?
• How do fishes achieve respiration, buoyancy, and locomotion in water?
• How do jawless, cartilaginous, and bony fishes differ?
• How did lobe-finned fishes set the stage for the move onto land?

Key theories

Origin and significance of jaws

Jaws, thought to derive from anterior gill-supporting arches, transformed early vertebrates from filter feeders into active predators, a key innovation underlying the diversification of nearly all later fishes and tetrapods.

Lobe-finned ancestry of tetrapods

The lobe-finned, or sarcopterygian, fishes have fleshy, bone-supported fins and gave rise to the tetrapods, making them more closely related to land vertebrates than to the ray-finned fishes.

Mechanisms

Fishes extract oxygen from water by pumping it over gills, where countercurrent flow between water and blood maximises oxygen uptake. Many bony fishes regulate buoyancy with a gas-filled swim bladder, while sharks rely on a large oil-rich liver and continuous swimming. Locomotion is produced by waves of muscular contraction passing along the body and tail, with paired and median fins providing steering and stability. Reproduction ranges from external fertilisation and vast numbers of eggs in many bony fishes to internal fertilisation and live birth in some sharks.

Clinical relevance

Fishes are a primary global food source and the basis of fisheries and aquaculture, indicators of aquatic ecosystem health, and important model organisms, with the zebrafish widely used in developmental and genetic research. This is educational context, not clinical advice.

History

Fossil fishes were studied systematically by Louis Agassiz in the nineteenth century, and early-twentieth-century paleontologists such as Stensiö reconstructed the anatomy of jawless and early jawed fishes. Romer and later workers traced the transition from lobe-finned fishes to the first tetrapods, a sequence now richly documented by transitional fossils.

Key figures

• Louis Agassiz
• Erik Stensiö
• Alfred Romer

Seminal works

• pough2018
• kardong2019

Frequently asked questions

Are sharks fish?

Yes. Sharks are cartilaginous fishes, the chondrichthyans, whose skeletons are made of cartilage rather than bone; they are true fishes distinct from the bony fishes.

Which fishes are most closely related to land animals?

The lobe-finned fishes, or sarcopterygians, are the closest fish relatives of land vertebrates, since tetrapods evolved from within this lineage.