Pelvic fins or ventral fins are paired fins located on the ventral (belly) surface of

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

, and are the lower of the only two sets of paired fins (the other being the laterally positioned

pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

s). The pelvic fins are homologous to the hindlimbs of

tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...

s, which evolved from

lobe-finned fish Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates ar ...

during the

Middle Devonian In the geological timescale, the Middle Devonian epoch (from 393.3 ± 1.2 million years ago to 382.7 ± 1.6 million years ago) occurred during the Devonian period, after the end of the Emsian age. The Middle Devonian epoch is subdivided into two ...

Structure and function

Structure

In actinopterygians, the pelvic fin consists of two endochondrally-derived bony girdles attached to bony radials. Dermal fin rays ( lepidotrichia) are positioned distally from the radials. There are three pairs of muscles each on the dorsal and ventral side of the pelvic fin girdle that abduct and adduct the fin from the body. Pelvic fin structures can be extremely specialized in actinopterygians. Gobiids and lumpsuckers modify their pelvic fins into a sucker disk that allow them to adhere to the substrate or climb structures, such as waterfalls. In

priapiumfish Phallostethidae, also known as priapium fish, is a family of Atheriniformes, atheriniform fish native to freshwater and brackish habitats in southeast Asia. They are small, no more than in length, with partially translucent bodies. They are fou ...

, males have modified their pelvic structures into a spiny copulatory device that grasps the female during mating. File:Pelvic fin skeleton.png, Pelvic fin skeleton for ''Danio rerio'', zebrafish. File:Zuignap waarmee de zwartbekgrondel zich vastzuigt.JPG, Gobiids have modified their pelvic fins into adhesive suckers. File:Spiny lumpsucker (Eumicrotremus orbis).jpg, Lumpsuckers use their modified pelvic fins to adhere to the substrate.

Function

In actinopterygian steady state swimming, the pelvic fins are actively controlled and used to provide powered corrective forces. Careful timing of the pelvic fin movement during whole-body movements allows the pelvic fins to generate forces that dampen the forces from the entire body, therefore stabilizing the fish. For maneuvers,

electromyogram Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...

data shows that pelvic fin muscles are activated after the start of the maneuver, indicating that the fins are used more for stabilization instead of generating the maneuver. In rays and skates, pelvic fins can be used for "punting," where they asynchronously or synchronously push off the substrate to propel the animal forwards.

Development

Unlike limb development in tetrapods, where the forelimb and hindlimb buds emerge at roughly the same timepoint, the pelvic fin bud emerges much later than the pectoral fin. While the pectoral fin bud is apparent at 36 hours post fertilization (hpf) in zebrafish, the pelvic fin bud is only clear at around 21 days post fertilization (dpf), roughly when the animal is 8 mm in length.

In zebrafish, the pelvic fin bud starts as a mesenchymal condensation that forms an apical ectodermal thickening. A fin fold forms from this thickening, which is then invaded by migratory

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...

, separating the fin bud into the proximal mesenchyme (which will give rise to the endoskeletal girdle and radials) and the distal mesenchyme (which will give rise to dermal fin rays).

References

{{diversity of fish Fish anatomy