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Gender-bending fish

Are you a boy or a girl? That seems like an easy question to answer, but for some fishes — such as the clownfish, a member of the clade called Perciformes (PER-suh-FOR-meez) — it's not so straightforward. In most vertebrate animals, the sex of an individual is permanent and is determined at the moment of conception. However, in a few vertebrates like fishes (and in plants and many invertebrate animals), a condition called hermaphroditism is common. A hermaphrodite is an organism that has both male and female reproductive organs and can perform both the male and female parts of reproduction.

In some hermaphrodites, the animal starts out as one sex and switches to the other sex later in its life. This is known as sequential hermaphroditism, contrasting with simultaneous hermaphroditism, in which the animal can produce sperm and eggs at the same time. In some sequentially hermaphroditic fish species, animals develop first as male and then switch to female (a condition called protandry), and in others, the individuals develop first as female and then switch to male (protogyny).

Indo-Pacific cleaner wrasse
Indo-Pacific cleaner wrasse
Clownfish (like Nemo from the Pixar movie) are protandrous. This species lives within sea anemones in groups of two large fish and many small fish. The two large fish are the only sexually mature fish and are a male and female breeding pair. All of the smaller fish are male. If the large breeding female is removed, her male mate changes sex to female and the next largest fish in the group rapidly increases in size and takes over the role as the sexually mature male. Can you imagine how different the movie Finding Nemo would have been had an ichthyologist (IK-thee-ALL-uh-jist, a fish expert) been asked for advice?

A good example of a protogynous fish is the Indo-Pacific cleaner wrasse. This species forms 'harems' of one large male and multiple smaller females. If the male is removed from the harem, the largest female begins courting the other fish and develops male organs within two weeks.

If you look at the different groups that have evolved hermaphroditism on the fish phylogeny, you can see that fish engaging in this is sort of gender-bending are scattered across the phylogeny. This suggests that hermaphroditism has evolved independently in fish many times.

lineages that have evolved hermaphroditism

References/Further information

Helfman, G. S., Collette, B. B., and Facey, D. E. 1997. The Diversity of Fishes. 535 pp. Blackwell Publishing, Malden, MA.

Sadovy de Mitcheson, Y. and Liu, M. 2008. Functional hermaphroditism in teleosts. Fish and Fisheries 9:1-43.

Warner, R. R. Metamorphosis. Science 82 3:42-46.

Wiley, E. O. and Johnson, G. D. 2010. A teleost classification based on monophyletic groups. In J. S. Nelson, H.-P. Schultze, & M. V. H. Wilson (eds) Origin and Phylogenetic Interrelationships of Teleosts. Pp. 123-182, Verlag Dr. Friedrich Pfeil, München, Germany.

Clownfish photo from Wikipedia user Metatron and licensed via Creative Commons; wrasse photo by Brian Gratwicke from Encyclopedia of Life and licensed via Creative Commons