The first episode of Blue Planet II features an intriguing segment in which the Asian Sheepshead Wrasse (Semicossyphus reticulatus) changes from a female to a male and then competes with other males for territory and mates. The fish has male and female organs, which enables it to change from a female to male during its lifetime. Prior to this sex change, males fight other males for territory and then court females in that new territory . After mating a few times, older females will then make their transformation, and then fight the male they once mated with.
Despite this shocking segment on Blue Planet II, sex change is actually a relatively common phenomenon in fish species. In many species, when males die or are removed from a population, females will change sex immediately so the population can still grow and individuals can reproduce . In other populations, females will change sex over a longer period of time . Since many fish species have evolved this sequential hermaphroditism, there must be a selective advantage to this ability. It is likely that this sex changing ability became fixed in the population due to transforming individuals having a higher reproductive potential than strictly single-gendered fish .
An important distinction must be made when discussing this sex changing transformation. Though the fish is changing from male to female, this does not mean that the fish is evolving. Rather, evolution is what shaped this sex changing process, and it spread through the population due to sex changing individuals having a reproductive advantage. Therefore, every member of the population shares a common ancestor that developed this ability for sequential hermaphroditism.
Despite knowing that many fish species undergo sex changes during their lifetimes, scientists are still unsure of its evolutionary origin. Therefore, research is being completed on this evolutionary phenomenon to determine how and when this trait evolved in these populations.
by Libby Dunne
Blue Planet II, Episode 1, starting at 30:00.
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- RR Warner. 1975. The adaptive significance of sequential hermaphroditism in animals. The American Naturalist 109, 61-82.