In a vast dark basin full of danger at every turn such as our oceans, sex and reproduction is easier done when you don’t have to look too hard for a new partner. Clownfish (subfamily Amphiprioninae) and some species of damselfish are able to preform sex changes upon the death of a group member in order to reproduce without the need to leave the group they already assimilate with. This sex change is referred to as hermaphroditism, specifically protandrous sequential hermaphroditism as the transition is from male to female (protandrous) and this occurs sometime later in their life (sequential). At the beginning of life, all clownfish are born males. As they mature they tend to live in groups or “families” containing a dominant female, the largest one, with a male and surrounded by smaller juvenile clownfish. If by chance the female of the family happens to pass away, chemical shifts in the clownfish’s brain can trigger change in the mature male clown. The fish then starts undergoing the process of sex change and over the next few weeks transforms from male to a more dominant female. This is easier said than explained as the process requires the clownfish to undergo both physical and complex behavioral transformations.

There is very little known about the neural and transcriptional mechanisms that program sex change within the fish. The clown does tend to go through vigorous gene expression. It is seen that in brain samples, on average, the largest difference in distribution of correlation coefficient expressions happened within the transitioning process (Fig 1. Laura Casas). According to data collected in 2013, Laura Casas states the neuroendocrine mechanisms of the brain begin to change in dramatic ways. This causes the once docile clown to become much more aggressive and court the rest of the clownfish in the family as the female would. Changing their behavior steadily over this transitional period. These chemical mechanics then transmit changes to the gonads. The clownfish’s gonads experience a steady decrease in the amount of testicular tissue and an increasing amount of ovarian tissue over this transitional period (typically 40-50 days). Once the testicular tissue has receded all the way and ovarian tissue takes over the gonad region, the clown has finished its transition. The same fish yet with an entirely new behavior and completely different gender than before. The complex process repeats in the family for generations.

The reason some species of fish have evolved to be preform this ability is still unknown. Although, this process is widely believed to be developed for these smaller fish, especially the funky-like swimming clownfish, so they do not need to traverse far out into the reef in sight of a new female mate. Clownfish prefer staying close to their anemone of which they have created a mutual relationship with in order for protection from predators. These fish would not survive long if they needed to search far from comfort for a new mate. Having this natural handicap really benefits their overall population in the wild.

Citations

1. Casas, Laura, “Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis” Scientific Reports, vol. 6, Article no. 35461, 17 Oct. 2016, https://www.nature.com/articles/srep35461#MOESM1
2. Wuest, Patricia. “What Happens to a Clownfish When It Changes Sex?” Scuba Diving, 2 Sept. 2019, www.scubadiving.com/what‑happens‑clownfish‑when‑it‑changes‑sex.
3. Casas, Laura, and Fernando Saborido‑Rey. “Environmental Cues and Mechanisms Underpinning Sex Change in Fish.” Sexual Development, vol. 15, no. 1‑3, June 2021, pp. 108‑121, https://pmc.ncbi.nlm.nih.gov/articles/PMC8491468/