Case study: Clownfish

Clownfish, such as Clark’s anemonefish, are tropical species that live in a mutualistic symbiotic relationship with anemones. The anemones provide safe shelter, and the fish keep the anemone clean and parasite-free. Clownfish typically live in small social groups. A group will have a single female that is relatively large and socially dominant. The female will breed exclusively with a subordinate male, typically the largest of the males. The remainder of the group is comprised of non-breeding males. Clownfish usually develop initially as males. The presence of a dominant female actually inhibits the growth of the other fish in the group, helping to maintain this size-based hierarchy.

Sex change occurs in the absence of a dominant female. This can happen when the female dies or leaves the social group, or if a male leaves the group and moves to a new habitat. When a social group is left without a female, all the males begin to grow. Typically the subordinate male transitions into the new female, but it is possible for a non-breeder to beat them to the punch. Once transitioned into the new female, the next largest non-breeder will become the new subordinate male.

Experiments in Clark’s anemonefish have demonstrated that males contain progenitors of female sex cells primed for possible sex change, but females lose their male reproductive organs completely when they transition. Clownfish show strong support for the SAM model. The sex phenotype is tied directly to size. For all except the subordinate male, growth provides no fitness advantage for the remaining individuals except through movement into the dominant female role.

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