About two per cent of the world's fish species – some 500 species – are thought to switch sex at some point in their adult lives.
Some, like the Asian sheephead wrasse (pictured above), routinely change from female to male. Others, like the clownfish, change from male to female, while some, such as coral-dwelling gobies, switch back and forth depending on what’s happening in the environment.
This is possible because, unlike mammals and birds, the sex of many fish species isn’t determined by the presence of sex chromosomes.
Instead, environmental cues trigger changes in gene activity, which then change levels of key enzymes and hormones. In particular, one enzyme, called aromatase, is known to change male sex hormones into female sex hormones, which can then transform gonads into ovaries.
Sometimes, the environmental cue can be social. Clark’s anemonefish, for example, is a type of clownfish that lives inside protective anemones.
The fish live in small groups, with a breeding male and female pair, and a number of nonbreeding subordinates. If the breeding female dies, however, the largest subordinate male will switch sex and take her place.
Other times, the cues can be direct changes to the water.
It has been shown that when certain pollutants are flushed into rivers, they cause male fish to develop female characteristics, such as egg production.
Meanwhile, a 2008 study found that an increase in water temperature of just 1–2°C can skew the sex ratio of some fish towards more males.
Some of these transitions are helpful. When clownfish switch sex, it’s because evolution has selected this as a useful strategy that helps the fish to keep breeding and the species to survive. Not so, sex changes that are driven by human activity.
When we dump chemicals into rivers or cause the oceans to warm, we play a dangerous game with the futures of all the species that live there.
This article is an answer to the question (asked by Alex Jackson, via email) 'How can animals switch sex?'
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