Fish fin

Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and lift, which help the fish swim. Apart from the tail or caudal fin, fish fins have no direct articulations with the axial skeleton and are attached to the core only via muscles and ligaments.

Fish fins are distinctive anatomical features with varying internal structures among different clades: in ray-finned fish (Actinopterygii), fins are mainly composed of spreading bony spines or "rays" covered by a thin stretch of scaleless skin, resembling a folding fan; in lobe-finned fish (Sarcopterygii) such as coelacanths and lungfish, fins are short rays based around a muscular central bud internally supported by a jointed appendicular skeleton; in cartilaginous fish (Chondrichthyes) and jawless fish (Agnatha), fins are fleshy "flippers" supported by a cartilaginous skeleton. The limbs of tetrapods, a mostly terrestrial clade evolved from freshwater lobe-finned fish, are homologous to the pectoral and pelvic fins of all jawed fish.

Fins at different locations of the fish body serve different functions, and are divided into two groups: the midsagittal unpaired fins and the more laterally located paired fins. Unpaired fins are predominantly associated with generating linear acceleration via oscillating propulsion, as well as providing directional stability; while paired fins are used for generating paddling acceleration, deceleration, and differential thrust or lift for turning, surfacing or diving and rolling. Fins can also be used for other locomotions other than swimming, for example, flying fish use pectoral fins for gliding flight above water surface, and frogfish and many amphibious fishes (e.g. mudskippers) use pectoral and/or pelvic fins for crawling. Fins can also be used for other purposes: remoras and gobies have evolved sucker-like dorsal and pelvic fins for attaching to surfaces and "hitchhiking"; male sharks and mosquitofish use modified pelvic fins known as claspers to deliver semen during mating; thresher sharks use their caudal fin to whip and stun prey; reef stonefish have spines in their dorsal fins that inject venom as an anti-predator defense; 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 anchor themselves in place.