Fish fin

Fins are moving

("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.

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 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 (from Latin venter 'belly') 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[1] 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 burbot[2]
Dorsal fin (Spinal fins)	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: " 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 pufferfish and ocean sunfish use their dorsal fin in combination with their anal fin for propulsion (tetraodontiform 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 distal series (baseosts) Most fish use their anal fin to stabilize while swimming gymnotiform propulsion) pufferfish and ocean sunfish use their anal fin in combination with their dorsal fin for propulsion (tetraodontiform propulsion)
Adipose fin	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 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.[4][5] 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.[6][7] 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.[3] Research published in 2014 indicates that the adipose fin has evolved repeatedly in separate lineages.[8]
Caudal fin (Tail fin)	Heterocercal caudal fin (A) Homocercal caudal fin (C)	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 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 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.[9] Hemiheterocercal refers to a condition that is intermediate between heterocercal and homocercal (see below), where the vertebrae do not extend to the tip the upper lobe (e.g. in stem Neopterygii)[10] (B) - Protocercal means the vertebrae extend to the tip of the tail and the tail is symmetrical but not expanded (as 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 Palaeozoic fishes had a diphycercal heterocercal tail.[11] Abbreviate diphycercal is a diphycercal fin with a short base (†Saurichthyiformes)[10][12]
Caudal keel Finlets		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 sauries , they are rayless, non-retractable, and found between the last dorsal and/or anal fin and the caudal fin.

Bony fishes

ray-finned fish

denticles

).

Bony fishes are divided into

The coelacanth is one type of living lobe-finned fish. Both extant members of this group, the West Indian Ocean coelacanth (Latimeria chalumnae) and the Indonesian coelacanth (Latimeria menadoensis), are found in the genus Latimeria. Coelacanths are thought to have evolved roughly into their current form about 408 million years ago, during the early Devonian.^[18]

Locomotion of the coelacanths is unique to their kind. To move around, coelacanths most commonly take advantage of up or downwellings of the current and drift. They use their paired fins to stabilize their movement through the water. While on the ocean floor their paired fins are not used for any kind of movement. Coelacanths can create thrust for quick starts by using their caudal fins. Due to the high number of fins they possess, coelacanths have high maneuverability and can orient their bodies in almost any direction in the water. They have been seen doing headstands and swimming belly up. It is thought that their rostral organ helps give the coelacanth electroperception, which aids in their movement around obstacles.^[19]

Ray-fins

Spines have a variety of uses. In catfish, they are used as a form of defense; many catfish have the ability to lock their spines outwards. Triggerfish also use spines to lock themselves in crevices to prevent them being pulled out.

Lepidotrichia are usually composed of

Cartilaginous fishes

Shark fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.^[23] Originally the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and puboischiadic bars evolved. In rays, the pectoral fins have connected to the head and are very flexible. One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion.^[24] Most sharks have eight fins. Sharks can only drift away from objects directly in front of them because their fins do not allow them to move in the tail-first direction.^[25]

Unlike modern cartilaginous fish, members of

Tiger sharks have a large upper lobe, which allows for slow cruising and sudden bursts of speed. The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, whereas the porbeagle shark, which hunts schooling fish such as mackerel and herring, has a large lower lobe to help it keep pace with its fast-swimming prey.^[13] Other tail adaptations help sharks catch prey more directly, such as the thresher shark's usage of its powerful, elongated upper lobe to stun fish and squid.

On the other hand, rays rely on their enlarged pectoral fins for propulsion. Similarly enlarged pectoral fins can be found in the

Shark finning

According to the Humane Society International, approximately 100 million sharks are killed each year for their fins, in an act known as shark finning.^[31] After the fins are cut off, the mutilated sharks are thrown back in the water and left to die.

In some countries of Asia, shark fins are a culinary delicacy, such as shark fin soup.^[32] Currently, international concerns over the sustainability and welfare of sharks have impacted consumption and availability of shark fin soup worldwide.^[33] Shark finning is prohibited in many countries.

Fin functions

Generating thrust

Moving fins can provide thrust

Fish get thrust moving vertical tail fins from side to side.

Stingrays get thrust from large pectoral fins.

Drawing by Dr Tony Ayling

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Finlets may influence the way a vortex

develops around the tail fin.

Cavitation occurs when negative pressure causes bubbles (cavities) to form in a liquid, which then promptly and violently collapse. It can cause significant damage and wear.^[35] Cavitation damage can occur to the tail fins of powerful swimming marine animals, such as dolphins and tuna. Cavitation is more likely to occur near the surface of the ocean, where the ambient water pressure is relatively low. Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful.^[36] Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.^[36]

Fish use multiple fins, so it is possible that a given fin can have a hydrodynamic interaction with another fin. In particular, the fins immediately upstream of the caudal (tail) fin may be proximate fins that can directly affect the flow dynamics at the caudal fin. In 2011, researchers using volumetric imaging techniques were able to generate "the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes". They found that "continuous tail beats resulted in the formation of a linked chain of vortex rings" and that "the dorsal and anal fin wakes are rapidly entrained by the caudal fin wake, approximately within the timeframe of a subsequent tail beat".^[40]

Controlling motion

Once motion has been established, the motion itself can be controlled with the use of other fins.^[34][41]

This male mosquitofish has a gonopodium, an anal fin which functions as an intromittent organ.^[48][49]

This young male spinner shark has claspers, a modification to the pelvic fins which also function as intromittent organs.

Male

Similar organs with similar characteristics are found in other fishes, for example the andropodium in the Hemirhamphodon or in the Goodeidae^[51] or the gonopodium in the Middle Triassic †Saurichthys, the oldest known example of viviparity in a ray-finned fish.^[52]

Other functions

Other uses of fins include walking and perching on the sea floor, gliding over water, cooling of body temperature, stunning of prey, display (scaring of predators, courtship), defence (venomous fin spines, locking between corals), luring of prey, and attachment structures.

The Indo-Pacific sailfish has a prominent dorsal fin. Like scombroids and other billfish, they streamline themselves by retracting their dorsal fins into a groove in their body when they swim.^[55] The huge dorsal fin, or sail, of the sailfish is kept retracted most of the time. Sailfish raise them if they want to herd a school of small fish, and also after periods of high activity, presumably to cool down.^[55][56]

Other uses of fins

Frogfish use their pectoral and pelvic fins to walk along the ocean bottom.^[57]

Flying fish achieve sufficient lift to glide above the surface of the water thanks to their enlarged pectoral fins.

Large retractable dorsal fin of the Indo-Pacific sailfish, possibly used for cooling (thermoregulation).

The thresher shark uses its very elongated caudal fin to stun prey.

Species of tripod fish Bathypterois have elongated pectoral and pelvic fins, and an elongated caudal fin, which allow them to move and perch on the ocean floor.

The oriental flying gurnard has large pectoral fins which it normally holds against its body, and expands when threatened to scare predators. Despite its name, it is a demersal fish, not a flying fish, and uses its pelvic fins to walk along the bottom of the ocean.^[58][59]

Fins can have an adaptive significance as sexual ornaments. During courtship, the female cichlid, Pelvicachromis taeniatus, displays a large and visually arresting purple pelvic fin. "The researchers found that males clearly preferred females with a larger pelvic fin and that pelvic fins grew in a more disproportionate way than other fins on female fish."^[60][61]

Other uses of fins

venomous

spines (fin rays) on their dorsal, pelvic and anal fins, which they use for defense.

Evolution

See also: Evolution of fish

Evolution of paired fins

There are two prevailing hypotheses that have been historically debated as models for the evolution of paired fins in fish: the gill arch theory and the lateral fin-fold theory. The former, commonly referred to as the “Gegenbaur hypothesis,” was posited in 1870 and proposes that the “paired fins are derived from gill structures”.^[63] This fell out of popularity in favor of the lateral fin-fold theory, first suggested in 1877, which proposes that paired fins budded from longitudinal, lateral folds along the epidermis just behind the gills.^[64] There is weak support for both hypotheses in the fossil record and in embryology.^[65] However, recent insights from developmental patterning have prompted reconsideration of both theories in order to better elucidate the origins of paired fins.

Classical theories

Carl Gegenbaur's concept of the “Archipterygium” was introduced in 1876.^[66] It was described as a gill ray, or “joined cartilaginous stem,” that extended from the gill arch. Additional rays arose from along the arch and from the central gill ray. Gegenbaur suggested a model of transformative homology – that all vertebrate paired fins and limbs were transformations of the Archipterygium. Based on this theory, paired appendages such as pectoral and pelvic fins would have differentiated from the branchial arches and migrated posteriorly. However, there has been limited support for this hypothesis in the fossil record both morphologically and phylogenically.^[65] In addition, there was little to no evidence of an anterior-posterior migration of pelvic fins.^[67] Such shortcomings of the gill-arch theory led to its early demise in favor of the lateral fin-fold theory proposed by St. George Jackson Mivart, Francis Balfour, and James Kingsley Thacher.

The lateral fin-fold theory hypothesized that paired fins developed from lateral folds along the body wall of the fish.^[64] Just as segmentation and budding of the median fin fold gave rise to the median fins, a similar mechanism of fin bud segmentation and elongation from a lateral fin fold was proposed to have given rise to the paired pectoral and pelvic fins. However, there was little evidence of a lateral fold-to-fin transition in the fossil record.^[68] In addition, it was later demonstrated phylogenically that pectoral and pelvic fins arise from distinct evolutionary and mechanistic origins.^[65]

Evolutionary developmental biology

See also: Evolutionary developmental biology

Recent studies in the ontogeny and evolution of paired appendages have compared finless vertebrates – such as lampreys – with chondricthyes, the most basal living vertebrate with paired fins.^[69] In 2006, researchers found that the same genetic programming involved in the segmentation and development of median fins was found in the development of paired appendages in catsharks.^[70] Although these findings do not directly support the lateral fin-fold hypothesis, the original concept of a shared median-paired fin evolutionary developmental mechanism remains relevant.

A similar renovation of an old theory may be found in the developmental programming of chondricthyan gill arches and paired appendages. In 2009, researchers at the University of Chicago demonstrated that there are shared molecular patterning mechanisms in the early development of the chondricthyan gill arch and paired fins.^[71] Findings such as these have prompted reconsideration of the once-debunked gill-arch theory.^[68]

From fins to limbs

Fish are the ancestors of all mammals, reptiles, birds and amphibians.^[72] In particular, terrestrial tetrapods (four-legged animals) evolved from fish and made their first forays onto land 400 million years ago.^[73] They used paired pectoral and pelvic fins for locomotion. The pectoral fins developed into forelegs (arms in the case of humans) and the pelvic fins developed into hind legs.^[74] Much of the genetic machinery that builds a walking limb in a tetrapod is already present in the swimming fin of a fish.^[75][76]

Aristotle recognised the distinction between

homologous structures

, and made the following prophetic comparison: "Birds in a way resemble fishes. For birds have their wings in the upper part of their bodies and fishes have two fins in the front part of their bodies. Birds have feet on their underpart and most fishes have a second pair of fins in their under-part and near their front fins."

– Aristotle, De incessu animalium^[77]

In 2011, researchers at Monash University in Australia used primitive but still living lungfish "to trace the evolution of pelvic fin muscles to find out how the load-bearing hind limbs of the tetrapods evolved."^[78][79] Further research at the University of Chicago found bottom-walking lungfishes had already evolved characteristics of the walking gaits of terrestrial tetrapods.^[80][81]

In a classic example of convergent evolution, the pectoral limbs of pterosaurs, birds and bats further evolved along independent paths into flying wings. Even with flying wings there are many similarities with walking legs, and core aspects of the genetic blueprint of the pectoral fin have been retained.^[82][83]

The first mammals appeared during the

"This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail fin for improved aquatic locomotion. These structures are all the more remarkable because they evolved from nothing — the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor."^[90]

The biologist Stephen Jay Gould said the ichthyosaur was his favorite example of convergent evolution.^[91]

Fins or flippers of varying forms and at varying locations (limbs, body, tail) have also evolved in a number of other tetrapod groups, including diving birds such as penguins (modified from wings), sea turtles (forelimbs modified into flippers), mosasaurs (limbs modified into flippers), and sea snakes (vertically expanded, flattened tail fin).

Robotic fins

CIA built a robotic catfish called Charlie, designed to collect underwater intelligence undetected.^[92]

External videos
AquaPenguin - Festo, YouTube
AquaRay - Festo, YouTube
AquaJelly - Festo, YouTube
AiraCuda - Festo, YouTube

The use of fins for

The AquaPenguin, developed by Festo of Germany, copies the streamlined shape and propulsion by front flippers of penguins.^[98][99] Festo also developed AquaRay,^[100] AquaJelly^[101] and AiraCuda,^[102] respectively emulating the locomotion of manta rays, jellyfish and barracuda.

In 2004,

Robotic fish offer some research advantages, such as the ability to examine an individual part of a fish design in isolation from the rest of the fish. However, this risks oversimplifying the biology so key aspects of the animal design are overlooked. Robotic fish also allow researchers to vary a single parameter, such as flexibility or a specific motion control. Researchers can directly measure forces, which is not easy to do in live fish. "Robotic devices also facilitate three-dimensional kinematic studies and correlated hydrodynamic analyses, as the location of the locomotor surface can be known accurately. And, individual components of a natural motion (such as outstroke vs. instroke of a flapping appendage) can be programmed separately, which is certainly difficult to achieve when working with a live animal."[105]

See also

• Cephalopod fin
• Fin and flipper locomotion
• Fish locomotion
• Polydactyly in early tetrapods
• RoboTuna
• Shark fin soup
• Tradeoffs for locomotion in air and water
• Undulatory locomotion

References

Citations