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 highly distinctive anatomical features with varying internal structures among different clades: in ray-finned fish, 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 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 and jawless fish, 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 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 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. In some clades, additional unpaired fins were acquired during evolution. In some Acanthodii, one or more pairs of "intermediate" or "prepelvic" spines are present between the pectoral and pelvic fins, but these are not associated with fins.

Pectoral 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 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 * 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		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. 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 the minnows; the thoracic position in sunfish; and the jugular position, when the pelvics are anterior to the pectoral fins, as seen in the burbot
Dorsal fin	1=Dorsal fin of a shark	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 The bones that support the dorsal fin are called pterygiophores. There are two to three of them: "proximal", "middle", and "distal". In rock-hard, spinous fins the distal pterygiophores are often fused to the middle ones, or not present at all 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
Anal/cloacal fin		The anal/cloacal fin is located on the ventral surface behind the anus/cloaca. The bones that support the anal fin are called pterygiophores. There are up to two series, a proximal series and a distal series Most fish use their anal fin to stabilize while swimming Knifefish use their anal fins for thrust Boxfish, pufferfish and ocean sunfish use their anal fin in combination with their dorsal fin for propulsion
Adipose fin	1=Adipose fin of a trout	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. 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	1=Heterocercal caudal fin 1=Homocercal caudal fin	The caudal fin is the tail fin, located at the end of the caudal peduncle. It is used for propulsion in most taxa. The tail fin is supported by the vertebrae of the axial skeleton and pterygiophores. Depending on the relationship with the axial skeleton, four types of caudal fins are distinguished: - Heterocercal means the vertebrae extend into the upper lobe of the tail, often making it longer than the lower lobe. However, the external shape of heterocercal tail fins can also appear symmetric. Heterocercal is the opposite of hypocercal - Protocercal means the vertebrae extend to the tip of the tail and the tail is symmetrical but not expanded - 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. Most modern fishes have a homocercal tail. These come in a variety of shapes, and can appear: - Diphycercal means the vertebrae extend to the tip of the tail and the tail is symmetrical and expanded. Most Palaeozoic fishes had a diphycercal heterocercal tail. Finlets are small fins, generally behind the dorsal and anal fins. 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

es form a taxonomic group called Osteichthyes ; they have skeletons made of bone mostly, and can be contrasted with cartilaginous fishes, which have skeletons made mainly of cartilage.
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". 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.