Zoologger: Transgender fish perform reverse sex flip

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Cirrhitichthys falco
Habitat: Kuchino-Erabu Island in southern Japan, swapping sexes effortlessly

When it comes to selecting mates, hawkfish keep their options open. The flamboyantly coloured reef dwellers start life as females but can transform into males after maturing. Many marine animals do this, but these fickle fish have a rare trick up their fins: they can change back when the situation suits.

Tatsuru Kadota and colleagues from Hiroshima University in Higashi-Hiroshima, Japan, have observed reverse sex changes in wild hawkfish for the first time in the subtropical reefs around Kuchino-Erabu Island in southern Japan.

Hawkfish live in harems, with one dominant male mating with several females. Kadota's team studied 29 hawkfish and found that when it comes to sex change, the size of the harem matters.

If a male hawkfish took on many females, one of the two largest females would change sex and take over half of the harem, mating as a male. Conversely, if that new male hawkfish lost a few females to other harems and was challenged by a larger male, it reverted to mating as a female, instead of wasting precious energy fighting a losing battle. "The ability to undergo bidirectional sex change maximises an individual's reproductive value," Kadota says.

Gender benders

"Because of our frame of reference, we think of gender being fated one way or another," says fish ecologist Scott Heppell of Oregon State University in Corvallis. "These animals are a lot more flexible than some species."

Hermaphroditic species are genetically predisposed to favour the male or female sexual function (and sometimes to operate both at the same time), but C.?falco has evolved to select gender based on behavioural cues. "We think about genes affecting behaviour," Heppell says. "But in this case, it's the other way around."

When a male is killed off, for example, a female can switch genders to take its place. The removal of the dominant male triggers hormones that stimulate physical changes: the female will begin to produce testosterone instead of oestrogen, which acts on germ cells, the precursors to reproductive cells.

"The largest sub-dominant individual of the opposite sex will show behavioural changes within minutes and structural changes in the tissues within days," Heppell says. "In some species, they'll be physically ready to spawn as the other sex within a week or two."

Though ecologists knew that some fish can change their sex in both directions, no one had seen a wild fish spawning after reverting to its original gender until now.

After three years of observation, Kadota's team confirmed that the hawkfish can indeed reproduce as a female after being male. Because C.?falco spawns between 4 and 8 metres below water level the researchers were able to gather eggs from mating females, which release hundreds to thousands of eggs each evening under the setting sun.

Journal reference: Ethology, DOI: 10.1111/j.1439-0310.2011.02005.x

Read previous Zoologger columns: My brain's so big it spills into my legs, Dozy hamsters reverse the ageing process, To kill a mockingbird? No, parasitise it, Chill out with the world's coldest insect, 'Werewolf birds' hook up by the full moon, Cannibal shrimp shows its romantic side, The only cross-dressing bird of prey, The biggest spider web in the world, Slime killer hagfish feasts in rotten flesh, Female monkeys indulge in synchronised sex, The toad that's part clone, part love child, The first reptile with a true placenta, The fearsome jaws of a mini movie monster.

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