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Our very own Nemo - a sex-changing fish!

Updated: Mar 22, 2021

Most of us would have seen ‘Finding Nemo’ and absolutely loved it, for all the right reasons.

However, if the movie was to be biologically correct, Nemo’s father would have turned into a female after his mother died. His mother would have been a male before turning into a female; and Nemo? Well, he would have first been a male and been a father to baby fish, then would have had a chance to turn into a female – mother of more baby fish!

Confused? Let me explain. Nemo belongs to a family of fish called ‘clownfish’ or ‘anemonefish’. Observations and experiments since 1970s have shown that when you keep 2 males of these fish together, one of them dominates over the other. The dominant male, then, increases his size rapidly and turns into a female! ‘She’ can then mate with the other male and together they can have baby fish!

When 3 juvenile fish were kept together, the most dominant fish turned into female; the next dominant turned male and the third fish waited for the female to disappear/die. Once she disappeared, the male turned into a female, and the third fish grew to be a father to the new female’s babies!

In nature, a male and a female form a mating pair until the female dies, after which the male changes sex to female! A few smaller fish tag along with this pair, waiting for their turn to be a part of the mating pair, as a male first and then as a female!

Clownfish form a mutually beneficial relationship with sea anemones -- they provide each other protection and food! Picture is taken from wikipedia.

Researchers suggest that one of the reasons for such a strange society to evolve might be that being a father when smaller and a mother when larger may produce the most number of young ones in such fish! Well, there are other fish which can change sex from female to males (e.g. blue headed wrasse) and a few others which sex change can be bi-directional (e.g. coral goby)! But more about them, some other time :)

For now, I am not sure if I should be sorry for spoiling 'Finding Nemo' for some of you; or happy, because next time you see Nemo, you will be in awe of what that little creature is capable of doing!

If interested, do check out these youtube links where researchers talk about experiments they have performed on clownfish!,,


Buston, Peter. "Size and growth modification in clownfish." Nature 424.6945 (2003): 145-146. Fricke, Hans W. "Mating system, resource defence and sex change in the anemonefish Amphiprion akallopisos." Zeitschrift für Tierpsychologie 50.3 (1979): 313-326. Fricke, Hans W. "Social control of sex: field experiments with the anemonefish Amphiprion bicinctus." Zeitschrift für Tierpsychologie 61.1 (1983): 71-77. Iwata, Eri, et al. "Social environment and sex differentiation in the false clown anemonefish, Amphiprion ocellaris." Zoological Science 25.2 (2008): 123-128. Munday, Philip L., Peter M. Buston, and Robert R. Warner. "Diversity and flexibility of sex-change strategies in animals." Trends in Ecology & Evolution 21.2 (2006): 89-95.

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