Changing sex in the animal kingdom
Ah, Finding Nemo! The story of a single father travelling across oceans to find his only son. It is a classic, heartwarming Pixar movie. However, in reality, the story may have unfolded differently, due to the clownfish’s unique ability to change sex from male to female. According to BBC Science Focus, clownfish typically live in small groups with only one male-female breeding pair. When the dominant female dies, the largest male will become a female and take her place. This means that after Nemo’s mom died, Nemo’s dad would have changed sex into a female and found a male to mate with.
This behaviour is not uncommon in fish. About two percent of fish, some 500 species according to /Science Focus/, can change their sex. Many of them are what is known as ‘sequential hermaphrodites,’ meaning they undergo a permanent sex change at one point in their adult lives.
Most of these fish are ‘protogynous,’ which is Greek for ‘female first,’ as BBC Earth reports. As the name suggests, these fish start their lives as females and become males later. This includes species such as the Asian sheephead wrasse, other wrasses, and many species of parrotfish. In more extreme cases, such as the Potter’s angelfish, all fish begin their lives as females and transition to males later on. The opposite, transforming from male to female or being ‘protandrous,’ is less common, but occurs in a variety of fish such as the Australian barramundi, gilthead seabream, black porgy, and clownfish.
Then there are ‘bidirectional hermaphrodites,’ also called ‘serial hermaphrodites,’ which refers to fish that can change sex back and forth depending on their environment. An example of this are the coral gobies that live in the crevices of coral reefs. These fish are mostly sedentary and thus have few opportunities to mate and reproduce, which explains their bidirectional sex changes. When any two coral gobies encounter each other, they can always form a male-female pair and produce offspring regardless of their original sex.
For most sequential hermaphrodites, sex transformations are also related to reproduction. According to BBC Earth, the “size advantage hypothesis” appears to explain changes in sex for both protogynous and protandrous fish. In protogynous species, such as the Asian sheephead wrasse, dominant males may possess a harem of females which they mate with. It is more advantageous to become male later in life when the fish has grown larger and can better fight to defend its harem. They are able to monopolise and mate with large groups of females, greatly increasing reproductive output. On the other hand, for protandrous species such as clownfish, large females tend to be more fertile than smaller ones, making it more efficient to produce eggs later in life when the fish is larger.
The reason fish can accomplish these sex changes is due to the enzyme aromatase. According to Professor Stefano Mariani of the University of Salford, aromatase can “change androgen hormones into the estrogenic hormones that can transform gonads into ovaries.” Furthermore, unlike birds and mammals, whose sex is determined by chromosomes, fish have their sex determined by a variety of factors, including not only aromatase, but also temperature and water quality. For many species of fish, as well as reptiles such as turtles, alligators, and crocodiles, the temperature of developing eggs determines their sex. Most fish will develop as male in warmer water, while warmer temperatures tend to produce more female sea turtles. Unfortunately, exposure to chemicals from human activity can also trigger sex changes, resulting in unbalanced populations which may have a detrimental impact on the species. The Guardian reported that “more than 80 percent of the male bass fish in [the Potomac river] exhibited female traits such as egg production because of a ‘toxic stew’ of pollutants.”
Past research suggests that a similar phenomenon may have been occurring with frogs. According to National Geographic, “a paper in the Proceedings of the National Academy of Sciences found that pollution-laden runoff into suburban ponds in the U.S. might be turning larval male amphibians into females.” However, a more recent study discovered that frogs change sex even in natural, unpolluted environments, suggesting they are reacting to shifts in temperature or other environmental changes as opposed to human pollution.
