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Concept
Fish fin
Summary
Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of Actinopterygii and Chondrichthyes fishes. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as seen in sharks. Apart from the tail or caudal fin, fish fins have no direct connection with the spine and are supported only by muscles. Their principal function is to help the fish swim.
Fins located in different places on the animal serve different purposes such as moving forward, turning, keeping an upright position or stopping. Most of them use fins when swimming, flying fish use pectoral fins for gliding, and frogfish use them for crawling. Fins can also be used for other purposes; male sharks and mosquitofish use a modified fin to deliver sperm, thresher sharks use their caudal fin to stun prey, reef stonefish have spines in their dorsal fins that inject venom, anglerfish use the first spine of their dorsal fin like a fishing rod to lure prey, and triggerfish avoid predators by squeezing into coral crevices and using spines in their fins to lock themselves in place.
TOC
Fins can either be paired or unpaired. The pectoral and pelvic fins are paired, whereas the dorsal, anal and caudal fins are unpaired and situated along the midline of the body. For every type of fin, there are a number of fish species in which this particular fin has been lost during evolution (e.g. pelvic fins in Bobasatrania, caudal fin in ocean sunfish). In some clades, additional unpaired fins were acquired during evolution (e.g. additional dorsal fins, adipose fin). In some Acanthodii ("spiny sharks"), one or more pairs of "intermediate" or "prepelvic" spines are present between the pectoral and pelvic fins, but these are not associated with fins.
Bony fishes (Actinopterygii and Sarcopterygii) form a taxonomic group called Osteichthyes (or Euteleostomi, which includes also land vertebrates); they have skeletons made of bone mostly, and can be contrasted with cartilaginous fishes (see below), which have skeletons made mainly of cartilage (except for their teeth, fin spines, and denticles).
Official source
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