How do you find a mate in the vast blue desert of the ocean? A fish could swim for months and when they finally stumble upon a member of their species, they turn out to be the same sex. Evolution has endowed more than 500 species of fish with a cunning solution: switch sex to complement your new partner – whatever their sex may be.

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Clownfish swimming in coral reef. © tiaramaio/Getty

Seeing Finding Nemo with fresh eyes

The most famous of these oceanic hermaphrodites are the clownfish (also known as anemonefish): the stars of Finding Nemo – a film about a young clownfish who loses his mum to a barracuda and ends up going on an incredible, ocean-going adventure before being reunited with his dad, Marlin.

Needless to say the animated movie takes more than a pinch of artistic licence. These monogamous reef dwellers set up home together in an anemone, whose stinging tentacles offer the couple, and their eggs, protection.

A biologically accurate version of the blockbuster would therefore have seen Marlin the dad transition into a female and start having sex with his son Nemo, which might have made for a less popular family film.

The bigger female is the boss in the relationship – it’s her job to defend the territory while the male cares for the eggs. The fish live up to 30 years in the same anemone, often with a bunch of juvenile males in attendance. If the female is killed, it triggers Mr Clownfish to transform into the new dominant female, and one of the juvenile males to mature into her mate.

A biologically accurate version of the blockbuster would therefore have seen Marlin the dad transition into a female and start having sex with his son Nemo, which might have made for a less popular family film.

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Observations in the lab

Neuroscientist Justin Rhodes of the University of Illinois has a lab full of clownfish. His research has shown that the male-to-female sex change starts first in the brain and only after months or even years later do the fish’s gonads catch up and become fully female. Rhodes introduced me to a transitioning fish via Skype. It was half way through the change, which meant it had a feminised brain but male gonads. This could be considered confusing for the fish, but Rhodes is certain that if asked, the fish would say it’s female.

“That’s the great thing about these fish, they’ll tell you,” he told me.

All Rhodes has to do is put the fish in a tank with another female. In the wild, highly territorial females are guaranteed to fight. It is unmistakeable behaviour. Quite different to a female encountering another male, which as the submissive sex, does not instigate a deadly territorial war.

Clownfish amongst the tendrils of an anemone
Clownfish amongst the tendrils of an anemone on the Great Barrier Reef, Australia. © Reinhard Dirscherl/Getty

A lesson in the complexity of sex

The simple experiment shows that these transitional clownfish both behave like females and are recognised as females by other fish, even though their biological sex remains male. These transitioning hermaphrodites teach us lessons on the complexity of sex by exposing the flaw in assuming a linear relationship between gonadal sex, sexual identity, sexuality and sexed behaviour, even in a fish.


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Main image: A clownfish illustration © Holly Exley

Authors

Author, broadcaster, Nat Geo Explorer and zoologist

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