Question: How many animals are hermaphrodites?

Hermaphrodite




In biology, a hermaphrodite (/hɜːrˈmæfrədaɪt/) is an organism that has complete or partial reproductive organs and produces gametes normally associated with both male and female sexes.[1] Many taxonomic groups of animals (mostly invertebrates) do not have separate sexes.[2] In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which either partner can act as the "female" or "male." For example, the great majority of tunicates, pulmonate snails, opisthobranch snails, earthworms and slugs are hermaphrodites. Hermaphroditism is also found in some fish species and to a lesser degree in other vertebrates. Most plants are also hermaphrodites.
Historically, the term hermaphrodite has also been used to describe ambiguous genitalia and gonadal mosaicism in individuals of gonochoristic species, especially human beings. The word intersex has come into preferred usage for humans, since the word hermaphrodite is considered to be misleading and stigmatizing,[3][4] as well as "scientifically specious and clinically problematic."[5]
A rough estimate of the number of hermaphroditic animal species is 65,000.[6] Since the estimated total number of animal species is 8.6 million, the percentage of animal species that are hermaphroditic is about 0.7%. Arthropods are the phylum with the largest number of species.  Most hermaphroditic species exhibit some degree of self-fertilization. The distribution of self-fertilization rates among animals is similar to that of plants, suggesting that similar processes are operating to direct the evolution of selfing in animals and plants.[6]
Etymology
The term derives from the Latin: hermaphroditus, from Ancient Greek: ἑρμαφρόδιτος, romanized:hermaphroditos,[7] which derives from Hermaphroditus (Ἑρμαφρόδιτος), the son of Hermes and Aphrodite in Greek mythology. According to Ovid, he fused with the nymph Salmacis resulting in one individual possessing physical traits of male and female sexes;[8] according to the earlier Diodorus Siculus, he was born with a physical body combining male and female sexes.[9] The word hermaphrodite entered the English lexicon as early as the late fourteenth century.[10] Alexander ab Alexandro stated, using the term hermaphrodite, that the people who bore the sexes of both man and woman were regarded by the Athenians and the Romans as monsters, and thrown into the sea at Athens and into the Tiber at Rome.[11]
Zoology
Sequential hermaphrodites


Sequential hermaphrodites (dichogamy) occur in species in which the individual is born as one sex, but can later change into the opposite sex.[12] This contrasts simultaneous hermaphrodites, in which an individual may possess fully functional male and female genitalia. Sequential hermaphroditism is common in fish (particularly teleost fish) and many gastropods (such as the common slipper shell), and some flowering plants. Sequential hermaphrodites can only change sex once.[13] Sequential hermaphroditism can best be understood in terms of behavioral ecology and evolutionary life history theory, as described in the size-advantage mode[14] first proposed by Michael T. Ghiselin[15] which states that if an individual of a certain sex could significantly increase its reproductive success after reaching a certain size, it would be to their advantage to switch to that sex.
Sequential hermaphrodites can be divided into three broad categories:
Protandry: Where an organism is born as a male, and then changes sex to a female.[12]
Example: The clownfish (genus Amphiprion) are colorful reef fish found living in symbiosis with sea anemones. Generally one anemone contains a 'harem', consisting of a large female, a smaller reproductive male, and even smaller non-reproductive males. If the female is removed, the reproductive male will change sex and the largest of the non-reproductive males will mature and become reproductive. It has been shown that fishing pressure can change when the switch from male to female occurs, since fishermen usually prefer to catch the larger fish. The populations are generally changing sex at a smaller size, due to natural selection.
Protogyny: Where the organism is born as a female, and then changes sex to a male.[12]
Example: wrasses (Family Labridae) are a group of reef fish in which protogyny is common. Wrasses also have an uncommon life history strategy, which is termed diandry (literally, two males). In these species, two male morphs exists: an initial phase male and a terminal phase male. Initial phase males do not look like males and spawn in groups with other females. They are not territorial. They are, perhaps, female mimics (which is why they are found swimming in group with other females). Terminal phase males are territorial and have a distinctively bright coloration. Individuals are born as males or females, but if they are born males, they are not born as terminal phase males. Females and initial phase males can become terminal phase males. Usually, the most dominant female or initial phase male replaces any terminal phase male when those males die or abandon the group.
Bidirectional Sex Changers: where an organism has female and male reproductive organs, but act as either female or male during different stages in life.[12]
Example: Lythrypnus dalli (Family Lythrypnus) are a group of coral reef fish in which bidirectional sex change occurs. Once a social hierarchy is established a fish changes sex according to its social status, regardless of the initial sex, based on a simple principle: if the fish expresses subordinate behavior then it changes its sex to female, and if the fish expresses dominant or not subordinate behavior then the fish changes its sex to male.[16]
Dichogamy can have both conservation-related implications for humans, as mentioned above, as well as economic implications. For instance, groupers are favoured fish for eating in many Asian countries and are often aquacultured. Since the adults take several years to change from female to male, the broodstock are extremely valuable individuals.
Simultaneous hermaphrodites


A simultaneous (or synchronous) hermaphrodite (or homogamous) is an adult organism that has both male and female sexual organs at the same time.[12] Self-fertilization often occurs.
Reproductive system of gastropods: Pulmonate land snails and land slugs are perhaps the best-known kind of simultaneous hermaphrodite, and are the most widespread of terrestrial animals possessing this sexual polymorphism. Sexual material is exchanged between both animals via spermatophore, which can then be stored in the spermatheca. After exchange of spermatozoa, both animals will lay fertilized eggs after a period of gestation; then the eggs will proceed to hatch after a development period. Snails typically reproduce in early spring and late autumn.
Banana slugs are one example of a hermaphroditic gastropod. Mating with a partner is more desirable biologically, as the genetic material of the resultant offspring is varied, but if mating with a partner is not possible, self-fertilization is practiced. The male sexual organ of an adult banana slug is quite large in proportion to its size, as well as compared to the female organ. It is possible for banana slugs, while mating, to become stuck together. If a substantial amount of wiggling fails to separate them, the male organ will be bitten off (using the slug's radula), see apophallation. If a banana slug has lost its male sexual organ, it can still mate as a female, making its hermaphroditic quality a valuable adaptation.
The species of colourful sea slugs Goniobranchus reticulatus is hermaphroditic, with both male and female organs active at the same time during copulation.  After mating, the external portion of the penis detaches, but is able to regrow within 24 hours.[17][18]
Hamlets, unlike other fish, seem quite at ease mating in front of divers, allowing observations in the wild to occur readily. They do not practice self-fertilization, but when they find a mate, the pair takes turns between which one acts as the male and which acts as the female through multiple matings, usually over the course of several nights.
Earthworms are another example of a simultaneous hermaphrodite. Although they possess ovaries and testes, they have a protective mechanism against self-fertilization. Sexual reproduction occurs when two worms meet and exchange gametes, copulating on damp nights during warm seasons. Fertilized eggs are protected by a cocoon, which is buried on or near the surface of the ground.
The free-living hermaphroditic nematode Caenorhabditis elegans reproduces primarily by self-fertilization, but infrequent out-crossing events occur at a rate of approximately 1%.[19]
The mangrove killifish (Kryptolebias marmoratus) is a species of fish that lives along the east coast of North, Central and South America. These fish are simultaneous hermaphrodites. K. marmoratus produces eggs and sperm by meiosis and routinely reproduces by self-fertilization. Each individual hermaphrodite normally fertilizes itself when an egg and sperm produced by an internal organ unite inside the fish's body.[20]
Pseudohermaphroditism

When spotted hyenas were first discovered by explorers, they were thought to be hermaphrodites. Early observations of spotted hyenas in the wild led researchers to believe that all spotted hyenas, male and female, were born with what appeared to be a penis. The apparent penis in female spotted hyenas is in fact an enlarged clitoris, which contains an external birth canal.[21][22] It can be difficult to determine the sex of wild spotted hyenas until sexual maturity, when they may become pregnant. When a female spotted hyena gives birth, they pass the cub through the cervix internally, but then pass it out through the elongated clitoris.[23]
Humans



Hermaphrodite is used in older literature to describe any person whose physical characteristics do not neatly fit male or female classifications, but some people advocate to replace the term with intersex.[25][26] Intersex describes a wide variety of combinations of what are considered male and female biology. Intersex biology may include, for example, ambiguous-looking external genitalia, karyotypes that include mixed XX and XY chromosome pairs (46XX/46XY, 46XX/47XXY or 45X/XY mosaic).
Clinically, medicine currently describes intersex people as having disorders of sex development, a term vigorously contested.[27][28] This is particularly because of a relationship between medical terminology and medical intervention.[29] Intersex civil society organizations, and many human rights institutions,[30][31] have criticized medical interventions designed to make intersex bodies more typically male or female.
Some people who are intersex, such as some of those with androgen insensitivity syndrome, outwardly appear completely female or male, frequently without realizing they are intersex. Other kinds of intersex conditions are identified immediately at birth because those with the condition have a sexual organ larger than a clitoris and smaller than a penis.
Some humans were historically termed true hermaphrodites if their gonadal tissue contained both testicular and ovarian tissue, or pseudohermaphrodites if their external appearance (phenotype) differed from sex expected from internal gonads. This language has fallen out of favor due to misconceptions and pejorative connotations associated with the terms,[32] and also a shift to nomenclature based on genetics.
Intersex is in some caused by unusual sex hormones; the unusual hormones may be caused by an atypical set of sex chromosomes. One possible pathophysiologic explanation of intersex in humans is a parthenogenetic division of a haploid ovum into two haploid ova. Upon fertilization of the two ova by two sperm cells (one carrying an X and the other carrying a Y chromosome), the two fertilized ova are then fused together resulting in a person having dual genitalial, gonadal (ovotestes) and genetic sex. Another common cause of being intersex is the crossing over of the SRY from the Y chromosome to the X chromosome during meiosis. The SRY is then activated in only certain areas, causing development of testes in some areas by beginning a series of events starting with the upregulation of SOX9, and in other areas not being active (causing the growth of ovarian tissues). Thus, testicular and ovarian tissues will both be present in the same individual.[33]
Fetuses before sexual differentiation are sometimes described as female by doctors explaining the process.[34] This is technically not true. Before this stage, humans are simply undifferentiated and possess a Müllerian duct, a Wolffian duct, and a genital tubercle.
Botany


Hermaphrodite is used in botany to describe a flower that has both staminate (male, pollen-producing) and carpellate (female, ovule-producing) parts. This condition is seen in many common garden plants. A closer analogy to hermaphroditism in botany is the presence of separate male and female flowers on the same individual—such plants are called monoecious. Monoecy is especially common in conifers, but occurs in only about 7% of angiosperm species.[35] The condition also occurs in some algae.[36]
See also

Asexual reproduction
Androgyny
Gonochorism
Gynandromorph
Intersex
Self-pollination
Transsexual


Further reading

Discovery Health Channel, (2007) "I Am My Own Twin"



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