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 highly 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.
Types of fins 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 (median) of the body. The fin origin is the anterior-most location (the "leading edge") where a particular fin inserts into the body, which allows it to articulate. During evolution, certain fin types may be modified to adapt to different niches: certain species have "lost" certain fins during their evolution (e.g. pelvic fins in swordfish and deep-sea anglerfish, the caudal fin in Molidae), and conversely, additional unpaired fins may have been acquired in some clades (e.g. additional dorsal fins in Gadidae, the 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
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). Bony fishes are divided into ray-finned and lobe-finned fish. Most living fish are ray-finned, an extremely diverse and abundant group consisting of over 30,000 species. It is the largest class of vertebrates in existence today, making up more than 50% of species. In the distant past, lobe-finned fish were abundant; however, there are currently only eight species. Bony fish have fin spines called lepidotrichia or "rays" (due to how the spines spread open). They typically have swim bladders, which allow the fish to alter the relative density of its body and thus the buoyancy, so it can sink or float without having to use the fins to swim up and down. However, swim bladders are absent in many fish, most notably in lungfishes, who have evolved their swim bladders into primitive lungs, which may have a shared evolutionary origin with those of their terrestrial relatives, the tetrapods. Bony fishes also have a pair of opercula that function to draw water across the gills, which help them breathe without needing to swim forward to force the water into the mouth across the gills.
Lobe-fins
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