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s of the male s anal fin are formed into a tube like structure in which the sperm of the fish is ejected 59 when ready for mating the gonopodium becomes erect and points forward towards the female the male shortly inserts the organ into the sex opening of the female with hook like adaptations that allow the fish to grip onto the female to ensure impregnation if a female remains stationary and her partner contacts her vent with his gonopodium she is fertilised the sperm is preserved in the female s oviduct this allows females to fertilise themselves at any time without further assistance from males in some species the gonopodium may be half the total body length occasionally the fin is too long to be used as in the lyretail breeds of xiphophorus helleri hormone treated females may develop gonopodia these are useless for breeding similar organs with similar characteristics are found in other fishes for example the andropodium in the hemirhamphodon or in the goodeidae 60 or the gonopodium in the middle triassic saurichthys the oldest known example of viviparity in a ray finned fish 61 claspers are found on the males of cartilaginous fishes they are the posterior part of the pelvic fins that have also been modified to function as intromittent organs and are used to channel semen into the female s cloaca during copulation the act of mating in sharks usually includes raising one of the claspers to allow water into a siphon through a specific orifice the clasper is then inserted into the cloaca where it opens like an umbrella to anchor its position the siphon then begins to contract expelling water and sperm 62 63 other functions edit 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 64 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 64 65 other uses of fins frogfish use their pectoral and pelvic fins to walk along the ocean bottom 66 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 67 68 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 69 70 other uses of fins the oriental flying gurnard has large pectoral fins with eye spots which it displays to scare predators during courtship the female cichlid pelvicachromis taeniatus displays her visually arresting purple pelvic fin triggerfish squeeze into coral crevices to avoid predators and lock themselves in place with the first spine of their dorsal fin 71 the first spine of the dorsal fin of the anglerfish is modified so it functions like a fishing rod with a lure to attract prey remoras have modified first dorsal fins which take the form of an oval sucker like organ with which they attach themselves to other marine organisms lionfish have venomous spines fin rays on their dorsal pelvic and anal fins which they use for defense evolution edit see also evolution of fish evolution of paired fins edit 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 72 this fell out of popularity in favour 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 73 there is weak support for both hypotheses in the fossil record and in embryology 74 however recent insights from developmental patterning have prompted reconsideration of both theories in order to better elucidate the origins of paired fins classical theories edit carl gegenbaur s concept of the archipterygium was introduced in 1876 75 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 74 in addition there was little to no evidence of an anterior posterior migration of pelvic fins 76 such shortcomings of the gill arch theory led to its early demise in favour of the lateral fin fold theory proposed by st george jackson mivart francis balfour and james kingsley thacher the lateral fin fold theory hypothesised that paired fins developed from lateral folds along the body wall of the fish 73 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 77 in addition it was later demonstrated phylogenically that pectoral and pelvic fins arise from distinct evolutionary and mechanistic origins 74 evolutionary developmental biology edit see also evolutionary developmental biology recent studies in the ontogeny and evolution of paired appendages have compared finless vertebrates such as lampreys with chondrichthyes the most basal living vertebrate with paired fins 78 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 79 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 80 findings such as these have prompted reconsideration of the once debunked gill arch theory 77 from fins to limbs edit fish are the ancestors of all mammals reptiles birds and amphibians 81 in particular terrestrial tetrapods four legged animals evolved from fish and made their first forays onto land about 390 million years ago 82 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 83 much of the genetic machinery that builds a walking limb in a tetrapod is already present in the swimming fin of a fish 84 85 aristotle recognised the distinction between analogous and 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 86 comparison between a the swimming fin of a lobe finned fish and b the walking leg of a tetrapod bones considered to correspond with each other have the same colour in a parallel but independent evolution the ancient reptile ichthyosaurus communis developed fins or flippers very similar to fish or dolphins 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 87 88 further research at the university of chicago found bottom walking lungfishes had already evolved characteristics of the walking gaits of terrestrial tetrapods 89 90 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 91 92 the first mammals appeared during the triassic period between 251 9 and 201 4 million years ago several groups of these mammals started returning to the sea including the cetaceans whales dolphins and porpoises recent dna analysis suggests that cetaceans evolved from within the even toed ungulates and that they share a common ancestor with the hippopotamus 93 94 about 23 million years ago another group of bearlike land mammals started returning to the sea these were the seals 95 what had become walking limbs in cetaceans and seals evolved independently into new forms of swimming fins the forelimbs became flippers while the hindlimbs were either lost cetaceans or also modified into flipper pinnipeds in cetaceans the tail gained two fins at the end called a fluke 96 fish tails are usually vertical and move from side to side cetacean flukes are horizontal and move up and down because cetacean spines bend the same way as in other mammals 97 98 similar adaptations for fully aquatic lifestyle are found both in dolphins and ichthyosaurs ichthyosaurs are ancient reptiles that resembled dolphins they first appeared about 245 million years ago and disappeared about 90 million years ago 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 99 the biologist stephen jay gould said the ichthyosaur was his favourite example of convergent evolution 100 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 edit see also robot fish in the 1990s the cia built a robotic catfish called charlie designed to collect underwater intelligence undetected 101 external videos aquapenguin festo youtube aquaray festo youtube aquajelly festo youtube airacuda festo youtube the use of fins for the propulsion of aquatic animals can be remarkably effective it has been calculated that some fish can achieve a propulsive efficiency greater than 90 43 fish can accelerate and manoeuvre much more effectively than boats or submarine and produce less water disturbance and noise this has led to biomimetic studies of underwater robots which attempt to emulate the locomotion of aquatic animals 102 an example is the robot tuna built by the institute of field robotics to analyse and mathematically model thunniform motion 103 in 2005 the sea life london aquarium displayed three robotic fish created by the computer science department at the university of essex the fish were designed to be autonomous swimming around and avoiding obstacles like real fish their creator claimed that he was trying to combine the speed of tuna acceleration of a pike and the navigating skills of an eel 104 105 106 the aquapenguin developed by festo of germany copies the streamlined shape and propulsion by front flippers of penguins 107 108 festo also developed aquaray 109 aquajelly 110 and airacuda 111 respectively emulating the locomotion of manta rays jellyfish and barracuda in 2004 hugh herr at mit prototyped a biomechatronic robotic fish with a living actuator by surgically transplanting muscles from frog legs to the robot and then making the robot swim by pulsing the muscle fibres with electricity 112 113 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 114 see also edit 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 notes edit in common parlance fin ray may refer to both spiny rays and soft rays but is sometimes used to specifically signify soft rays 26 references edit citations edit standen em 2009 muscle activity and hydrodynamic function of pelvic fins in trout oncorhynchus mykiss the journal of experimental biology 213 5 831 841 doi 10 1242 jeb 033084 pmid 20154199 gene helfman bruce collette douglas facey brian bowen 2009 the diversity of fishes biology evolution and ecology john wiley sons glossary search for pterygiophores fishbase retrieved 9 april 2026 pterygiophore merriam webster com dictionary merriam webster oclc 1032680871 retrieved 9 april 2026 pteryg...