In Blue Planet II, Episode 1: One Ocean, we are introduced to the Kobudai fish, an example of the sexual fluidity of fish. After 10 years, a large female Kobudai fish undergoes a several month-long transformation into a large, bulbous and aggressive male. These are known as sequential hermaphrodites, and in the Kobudai fish case, this is specifically known as a protogynous sex change, the Greek word for “female first”, as they change from female to males during their lifetime [1]. The footage captured was remarkable, but it is not unique to the Kobudai fish. Around two percent of fishes exhibit some type of sexual dimorphism throughout their lifetime [2]. Some change from female to male, like the Kobudai. Others, like the clownfish, do the opposite and change from male to female. And if that wasn’t complicated enough, some fish, like coral-dwelling gobbies, switch back and forth between sexes depending on the circumstances.
The reason why fish have the ability to change sex so fluidly is still unknown. One possible reason for the wide range of gender-bending capabilities in fish is the fact that fish have been around for more than 500 million years and there are more than 26,000 species. This wide variation allows for evolutionary radiation and adaptation to a diverse set of niches, which permits for variation of all sorts, including the way fish reproduce. The ability that some fish have to change sexes throughout their lifetime produces an evolutionary advantage in being able to effectively double reproductive output and maximize the ability to pass on genes, even in changing environments [3].
How can these fish accomplish this mastery in sex change? For one, unlike birds and mammals, the sex of most fish is not determined by chromosomes. Social and physical factors, such as sex ratio in the population and temperature in the environment, can affect the sex of the organism. Also, recent studies have shown that aromatase, an enzyme that catalyzes the conversion of testosterone to estradiol, can have a lot to do with the sex change experienced in fish. It can act like an on and off switch that pushes sex into a specific direction depending on its activity, due to the fact that fish gonads contain the precursors for both ovarian and testicular tissue [4]. There is still much to learn about this phenomenon as evolution keeps surprising us.
by Lucas Mondo and Simon Burdeaux
Blue Planet II, Episode 1: One Ocean, starting at 30:00
References
- Warner, R.R. 1975. The adaptive significance of sequential hermaphroditism in animals. The American Naturalist 965: 61-82.
- Berglund, A. 1990. Sequential hermaphroditism and the size-advantage hypothesis: an experimental test. Animal Behaviour3: 426-433.
- Avise, J.C., and J.E. Mank. Evolutionary perspectives on hermaphroditism in fishes. Sexual Development2-3 (2009): 152-163.
- Todd, E.V. et al. 2016. Bending genders: the biology of natural sex change in fish. Sexual Development5-6 (2016): 223-241.
The Evolution of Planet Earth 