Polyandry in fishes is a mating system where females mate with multiple males within one mating season. This type of mating exists in a variety of animal species. Polyandry has been found in both oviparous and viviparous bony fishes and sharks. General examples of polyandry occur in fish species, such as green swordtails and Trinidadian guppies. Specific types of polyandry have also been classified, such as classical polyandry in pipefish cooperative polyandry in cichlids and convenience polyandry in sharks.
Examples Poeciliids are freshwater live-bearing fish and internal fertilizers that are able to store sperm for months, setting the stage for sperm competition and allowing female cryptic sperm choice. These Poeciliid species include green swordtails, Xiphophorus helleri and Trinidadian guppies, Poecilia reticulata. When females mate promiscuously and copulate with multiple males, the interests of the sexes may differ, leading to sexual conflict. These conflicts include mating and fertilization frequency, parental efforts, and power struggles between male and female dominance. The Trinidadian guppy, Poecilia reticulata has a resource-free mating system, meaning males do not provide during mating or defend their territories against other males. Guppies demonstrate one of the highest levels of female multiple mating in the fish species. Females tend to copulate with multiple males to ensure that males with strongly competitive sperm have increased paternity rates. Females who engage in polyandry obtain certain advantages such as shorter gestation times, larger broods, and the production of offspring with better phenotypes and abilities. Females prefer phenotypically bright colored males that are usually orange, red, yellow, or blue. Male offspring from polyandrous mating tend to be more colorful than offspring from monogamous mating, which contain more black spots rather than multiple colors. Brightly colored males tend to display stronger sigmoidal displays, correlating sperm production rate with courtship intensity and body size. Offspring were more phenotypically diverse than their parents, suggesting a diversified selection that allows offspring to cope better with the environment and have variability in mating. However, there are also costs associated with polyandry. Placental fish Heterandria formosa offspring from females who mate with multiple males, have a longer maturation time, leading to potentially higher levels of fatality in slower developing offspring.
Classical polyandry Classical polyandry occurs when the evolution of sex role reversal has occurred and a female copulates with multiple males. These males raise their own progeny without any help from females. This mating system is hypothesized to occur in three steps. The first step, which is an important prerequisite step in classical polyandry, involves the evolution of male care for eggs. In the second step, females have the ability to produce more clutches than a male can handle, leading to an increase in female fecundity because these females need to find other males to mate with for the remaining eggs they produced. The third step occurs as females compete to lay a clutch into a nest for the next male while the original male is caring for the initial clutch. More successful females tend to produce greater amounts of offspring. Male pregnancy is a common feature in the family Syngnathidae, which includes pipefish, seahorses, and sea dragons. This type of polyandry has been demonstrated by analyzing the genetic composition of Gulf pipefish, Sygnathus scovelli and straightnose pipefish, Nerophis ophidion, which shows that males only mate once during their pregnancy, whereas females mate multiple times. This extreme form of polyandry indicates that this species has a much stronger intensity of sexual selection on females than on males, in which females tend to be larger and more adorned than males. Evidence for stronger sexual selection in females in Gulf pipefish, Syngnathus scovelli, include having secondary sexual characteristics, such as longer abdomens and stripes that are not found in males. Males are usually capable of combining uniparental care with defending their territories or nest, attracting females for copulation. Some species of male pipefish have a fully or partially enclosed pouch where females deposit eggs. Males then fertilize and carry the offspring in or on his body until the offspring hatch. The pipefish species, Syngnathus typhle males can only carry approximately half of the brood produced by a larger female. This male limitation allows females to increase their fitness by developing eggs for multiple males. These females can then mate with multiple males, which leads to increased female fecundity and supports the second step of the evolution of classical polyandry.
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