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. __TOC__
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 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.
+ Paired fins Pectoral fins(Arm fins) The paired pectoral fins are located on each side, usually kept folded just behind the operculum, and are homologous to the forelimbs of quadrupedal tetrapods or the upper limbs of bipedal tetrapods. A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish, such as sharks, in maintaining depth and also enables the "flight" for flying fish In many fish, the pectoral fins aid in walking, especially in the lobe-like fins of some anglerfish and in the mudskipper (see also walking fish) Certain rays of the pectoral fins may be adapted into finger-like projections, such as in sea robins and flying gurnards In skates and rays, the pectoral fins are used for propulsion (rajiform propulsion) The "horns" of manta rays and their relatives are called cephalic fins; this is actually a modification of the anterior portion of the pectoral fin Pelvic / Ventral fins(Belly fins) The paired pelvic or ventral fins are the belly fins () are typically located ventrally below and behind the pectoral fins, although in many fish families they may be positioned in front of the pectoral fins (e.g. cods). They are homologous to the hindlimbs of quadrupedal tetrapods or the lower limbs of bipedal tetrapods. The pelvic fin assists the fish in going up or down through the water, turning sharply, and stopping quickly. In gobies, the pelvic fins are often fused into a single sucker disk. This can be used to attach to objects Pelvic fins can take many positions along the ventral surface of the fish. The ancestral abdominal position is seen in (for example) the minnows; the thoracic position in sunfish; and the jugular position, when the pelvics are anterior to the pectoral fins, as seen in the burbotGene Helfman, Bruce Collette, Douglas Facey, & Brian Bowen. (2009) The Diversity of Fishes: biology, evolution, and ecology. John Wiley & Sons.
+ Median/Unpaired fins Dorsal fin(Spinal fins) The dorsal fins are located on the back. A fish can have up to three dorsal fins. The dorsal fins serve to protect the fish against rolling, and assist it in sudden turns and stops. In anglerfish, the anterior of the dorsal fin is modified into an illicium and esca, a biological equivalent to a fishing rod and lure Gymnarchus uses only its dorsal fin for propulsion Boxfish, pufferfish and ocean sunfish use their dorsal fin in combination with their anal fin for propulsion (tetraodontiform propulsion) s are the basal elements of the dorsal and anal fins which allows for their articulation; they connect the median fins to the body. These elements may either be cartilagenous or ossified. There are up to three series of pterygiophores; from farthest to closest to the fin, they are the proximal, median, and distal pterygiophores. In certain species there are only the proximal and distal pterygiophore series. The proximal series may alternatively be called basals (especially if they contact the vertebrae), interneurals (for the dorsal series) or interhaemals (for the anal series). Anal/cloacal fin The anal/cloacal fin is located on the ventral surface behind the anus/cloaca. Most fish use their anal fin to stabilize while swimming Knifefish use their anal fins for thrust (gymnotiform propulsion) Boxfish, pufferfish and ocean sunfish use their anal fin in combination with their dorsal fin for propulsion (tetraodontiform propulsion) Adipose fin The adipose fin is a soft, fleshy fin found on the back behind the dorsal fin and just forward of the caudal fin. It is absent in many fish families, but found in nine of the 31 euteleostean orders (Percopsiformes, Myctophiformes, Aulopiformes, Stomiiformes, Salmoniformes, Osmeriformes, Characiformes, Siluriformes and Argentiniformes). Famous representatives of these orders are salmon, characids and catfish.
The function of the adipose fin is something of a mystery. It is frequently clipped off to mark hatchery-raised fish, though data from 2005 showed that trout with their adipose fin removed have an 8% higher tailbeat frequency. Additional information released in 2011 has suggested that the fin may be vital for the detection of, and response to, stimuli such as touch, sound and changes in pressure. Canadian researchers identified a neural network in the fin, indicating that it likely has a sensory function, but are still not sure exactly what the consequences of removing it are.
A comparative study in 2013 indicates the adipose fin can develop in two different ways. One is the salmoniform-type way, where the adipose fin develops from the larval-fin fold at the same time and in the same direct manner as the other median fins. The other is the characiform-type way, where the adipose fin develops late after the larval-fin fold has diminished and the other median fins have developed. They claim the existence of the characiform-type of development suggests the adipose fin is not "just a larval fin fold remainder" and is inconsistent with the view that the adipose fin lacks function.
Research published in 2014 indicates that the adipose fin has evolved repeatedly in separate lineages. Caudal fin(Tail fin) The caudal fin is the tail fin (from the Latin cauda meaning tail), located at the end of the caudal peduncle. It is used for propulsion in most taxa (see also body-caudal fin locomotion). The tail fin is supported by the vertebrae of the axial skeleton and pterygiophores (radials). Depending on the relationship with the axial skeleton, four types of caudal fins (A-D) are distinguished:
(A) - Heterocercal means the vertebrae extend into the upper lobe of the tail, often making it longer than the lower lobe (as in sharks, Placodermi, most stem Actinopterygii, and sturgeons and paddlefish). However, the external shape of heterocercal tail fins can also appear symmetric (e.g. Birgeria, Bobasatrania). Heterocercal is the opposite of hypocercal Hypocercal, also known as reversed heterocercal, means that the vertebrae extend into the lower lobe of the tail, making it longer than the upper lobe (as in the Anaspida). It is the opposite of heterocercal. Hemiheterocercal refers to a condition that is intermediate between heterocercal and homocercal (see below), where the vertebrae do not extend to the tip of the upper lobe (e.g. in stem Neopterygii)
(B) - Protocercal means the vertebrae extend to the tip of the tail and the tail is symmetrical but not expanded (as in the first fishes and the cyclostomes, and a more primitive precursor in lancelets)
(C) - Homocercal where the fin usually appears superficially symmetric but in fact the vertebrae extend for a very short distance into the upper lobe of the fin. Homocercal caudal fins can, however, also appear asymmetric (e.g. blue flying fish). Most modern fishes (teleosts) have a homocercal tail. These come in a variety of shapes, and can appear: rounded or pointed (e.g. round goby) truncated, ending in a more-or-less vertical edge (e.g. trout) emarginate, ending with a slight inward curve (e.g. Eurasian carp) forked, ending in two prongs (e.g. catla) lunate or shaped like a crescent moon (e.g. swordfish)
(D) - Diphycercal means the vertebrae extend to the tip of the tail and the tail is symmetrical and expanded (as in the bichir, lungfish, lamprey, coelacanths and Tarrasiiformes). Most Palaeozoic fishes had a diphycercal heterocercal tail.von Zittel KA, Woodward AS and Schlosser M (1932) Text-book of Paleontology Volume 2, Macmillan and Company. Page 13. Abbreviate diphycercal is a diphycercal fin with a short base (Saurichthyiformes) Caudal keelFinlets Some types of fast-swimming fish have a horizontal caudal keel just forward of the tail fin. Much like the keel of a ship, this is a lateral ridge on the caudal peduncle, usually composed of scutes (see below), that provides stability and support to the caudal fin. There may be a single paired keel, one on each side, or two pairs above and below.
Finlets are small fins, generally behind the dorsal and anal fins (in bichirs, there are only finlets on the dorsal surface and no dorsal fin). In some fish such as tuna or sauries, they are rayless, non-retractable, and found between the last dorsal and/or anal fin and the caudal fin.
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).