Actinopterygii — the ray-finned fishes

Anatomy and distinguishing features

Ray-finned fishes owe their name to the fin rays, which are keratinous or bony structures that support webs of skin. These rays may be bony or horny and can form flexible spines and segmented supports often called spines. This arrangement contrasts with the fleshy, lobed fins of the Sarcopterygii, which include coelacanths and lungfishes.

Distribution and diversity

Actinopterygians are the largest class of vertebrates, comprising roughly the majority of modern fish diversity — about 25,000 species by common estimates. They occupy virtually every aquatic environment, from marine seas and estuaries to fresh water rivers and lakes. Members range from deep ocean specialists adapted to the deep sea to small fishes found in high-mountain and alpine streams.

Evolution and classification

The group appears in the fossil record by the Devonian and has undergone major radiations since. Living actinopterygians are commonly organized into early-branching lineages and the more derived groups; most modern species are teleosts, which display an enormous range of body plans and life histories.

Uses, roles and notable facts

• Ecological: key components of aquatic food webs as predators, prey, and ecosystem engineers.
• Human importance: central to fisheries, aquaculture, and the aquarium trade, and important in scientific research.
• Size and form: includes some of the smallest vertebrates and some very large species; adaptations include swim bladders, varied jaw mechanics, and diverse reproductive strategies.

Actinopterygii illustrate how a basic anatomical innovation — fin rays supporting a webbed fin — can diversify into an enormous array of shapes and lifestyles, making them essential to both natural ecosystems and human economies.