In the pilot of Blue Planet II, we are introduced to the dramatic scenery of Northern Japan, where life teems beneath the surface. Out of a patch of flora emerges the rather odd looking male Kobudai, a one meter long fish weighing around 15 kilos. His bulbous head and protruding chin give a sort of Nigel Thornberry-esque look, but to the much smaller female Kobudai, he is actually quite attractive. He reigns over about a dozen females residing in his territory. Older females – those around 10 years old – do not give the male the time of day. Why?

Once older females reach a critical mass, certain enzymes lose function, and male hormones begin to emerge. Over the course of a few months, the female undergoes a complete metamorphosis, emerging as a male. The new male can then engage in male-to-male competition with a more established male to gain access to a new pool of females. In this polygynous society, changing into a male allows females to have more offspring, thereby increasing their fitness.

This ability for an organism to change sexes, due to having both female and male reproductive organs, is known as hermaphroditism. This phenomenon, although remarkable, is not unique to Kobudai; in fact, it has been observed in about 2% of fish species (which amounts to approximately 500 species) [1]. The type of hermaphroditism observed in Kobudai, in which an individual begins their reproductive life as a female and later switches to male, is known as protogyny. Protogyny is usually seen in social species with polygynous mating systems, such as bluehead wrasse. Bluehead wrasse are small reef fish in which a dominant male defends a territory containing many females. In the case that this dominant male leaves his territory, the largest female of the group typically undergoes sex change [2]. In this species, sex change is caused by a social cue that allows a member of a group to replace the missing leader.

Protandrous fish work in the opposite direction as species that undergo protogyny. Starting as a male and eventually turning female, protandrous species go under gonadal reconstruction, which can be dictated by different factors, such as size or social circumstances. While not shown in the documentary, species such as clown fish participate in this type of mechanism. In their case, once the female leaves the clown fish’s environment, it then begins to transition into a female [3]. There are also serial bidirectional sex-changers that are able to change from one sex to another based on the situation. This may be seen in species, such as Gobiodon, that face high predation pressures and, therefore, finding a mate becomes risky and dangerous. It is more advantageous for these species to switch genders when a mate dies or when the sex-ratio becomes skewed [4].

by Asher Ripp, Gustavo Capo, Betsy Benitez and David Kim

Blue Planet, Season 1, episode 1, starting at 29:28

References

1. Avise JC & JE Mank. 2009. Evolutionary perspectives on hermaphroditism in fishes. Karger 3: 152-163.
2. Warner RR & SE Swearer. 1991. Social control of sex change in the bluehead wrasse, Thalassoma bifasciatum (Pisces: Labridae). The Biological Bulletin 181: 199-204.
3. Casas L, Saborido-Rey F, Ryu T, Michell C, Ravasi T & Xabier I. 2016. Sex change in clownfish: molecular insights from transcriptome analysis. Nature. 35461: 1-8.
4. Munday P, Caley M & G Jones. 1998. Bi-directional sex change in a coral-dwelling goby. Behavioral Ecology and Sociobiology. 43(6): 371-377.