The Plastic Trait: Hermaphroditism Under the Sea

Now, let’s be mature about this. The fundamental aim of fish is to reproduce as many offspring as possible during their lives.

Scientists’ observations of certain species of fish have led them to conclude that a single gene can code different expressions in different environments, which doesn’t change the actual gene but changes the sex of that species, which can be advantageous for their ultimate reproductive aim. This phenomenon is known as sequential hermaphroditism.


A gonochronistic species, for example, the Mediterranean Parrotfish, remains a single sex in their adult life, but they are hermaphroditic in their juvenile stage.

Synchronous species, such as the sea bass, exhibit characteristics so that the individual specimen develops both female and male gonads at the same time.

Successive protandrous species, such as the Anemonefish and the Damselfish, begin their individual lives as males and develop into females.

The species identified as successive protogynous are born as females and later develop into males. These fish live in harems of one male to many females in order to prevent genetic drift. The females are dully colored while the males are bright in order to maintain this ratio.

Hormones influence the gender of a fish before and after it is born as well. Oestrogen and testosterone are commonly known hormones that occur in humans too. Androgens influence the sex of males, and aramotases are the enzymes that catalyze the conversion of androgens to estrogen (KT-11).

There’s also a social control over protogynous fish. In order for a male fish to act behaviorally like a male, it depends on its brain functioning like an actual male brain first, which can be influenced by social interactions. Factors such as sex ratios (the perception of the amount of male and female interactions) and overcrowding (causes fish to die young) also can influence the fish population.

Sometimes other factors come into play as well. For example, the rearing temperature can influence the gender of gobies. A cooler temperature leads to a larger proportion of males. Same with the photoperiod. If the photoperiod falls below 12 hours, some synchronous species will become secondary males.

Interested? Read more about coral reefs in posts to come!


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