Why some fish change sex during their lifetimes

Transphobes claim that in nature, there are only two sexes which are fixed from birth. There are many reasons why they are wrong, such as how some fish and invertebrates change sex over their lifetimes. There are 305 species of sex-changing fish, but this article will only describe four: two where individuals transition from female to male, and two which do the opposite. Though ‘transgender’ is not the technical word to describe these fish, they prove that not only is changing sex a normal phenomenon, but natural selection has chosen it many times.

Why would natural selection favour sex changes?

Sex change usually evolves when it is beneficial for an animal to be one sex when smaller and younger, and the opposite sex when larger and older (Figure 1). This is called the size-advantage hypothesis. In harems, where one male mates with many females, bigger males get the best harems, so they are the most successful. Female-to-male transitions, called protogyny, are expected in this situation – the biggest female changes sex to replace the dominant male in a harem. Bigger females have higher reproductive success in monogamous species, favouring male-to-female transitions called protandry. However, different sexes may have different growth and mortality rates. Sex change evolves in fish if the reproductive benefit of changing sex outweighs any survival costs.

Protogynous sex change in wrasses

Some fish in the wrasse family start off female and later change to male, which is called protogyny. A 1972 study on Labroides dimidatus wrasses found that when the dominant male in their harem died, the biggest female changed sex to replace it. This transitioning fish could perform the male aggressive display within 2 hours of the previous male dying. It took a few days for the reproductive organs of the new dominant fish to change from female to male completely. If a neighbouring dominant male took over the harem before the dominant female could fully transition, she would de-transition back into a female. 

According to a 1996 study, Thalassoma bifasciatum wrasses do something similar. Within 2 days of artificially removing the male from the harem, the dominant female was coloured like a male and doing courtship behaviours like a male would. After 4-5 days, these behaviours were at the same frequency as control males. The ovaries of the dominant females in this study were surgically removed, so those behavioural sex changes happened without hormonal influence from the ovaries. Therefore, wrasses can both biologically and socially transition from female to male.

Protandrous sex change in anemonefish

In contrast, anemonefish transition from male to female. As the size-advantage hypothesis predicts, societies of the Amphiprion bicinctus fish consist of a monogamous breeding pair and many juveniles. If the breeding female dies, the breeding male changes sex to replace her, and a juvenile develops into the new breeding male. Male reproductive tissue shrinks during the protandrous transition, and the ovary proliferates, although the egg cells do not mature (Figure 2). Gene expression in the ovaries of transitioned fish had an intermediate profile between the reproductive organs of control male and female fish. 

In 2022, researchers investigated the timing of sex change in a different anemonefish, Amphiprion ocellaris. In this species, two male fish fight when paired and whoever wins changes to a female. Winners had higher testosterone levels than losers, and intermediate levels of sex hormones between control males and females. Sex-changing winners were behaviourally and hormonally more similar to males, even after their reproductive organs had completely switched to females. This means the almost instant changes in wrasses take much longer in anemonefish.

Conclusion

Sex-changing fish make the most of an opportunity to advance in society – changing their behaviours, organs, and hormone levels to become the dominant breeding fish. Anemonefish and wrasses change sex in opposite directions and at different rates following a disturbance in the social order. With sex changes in fish being social and biological processes, perhaps we should view human sex changes similarly. If anyone thinks transgender people go against biology, not only are they wrong, but changing sex is beneficial and selected for in nature.

Written by Simran Patel

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