A Plausible Explanation of Sex Change in Fishes

A glimpse of hermaphroditism in the animal kingdom.

We are a curiosity-driven species, having charted even the most remote environments. We even sent people to the Moon! And yet, more than 80% of the earth's oceans remain unexplored.

The study of these bodies of water is of particular interest to biologists, due to the theory that life originated in the oceans. The first fish is estimated to have appeared on Earth sometime around 500 million years ago, while mammals have been around roughly 200 million years ago. 

This means fish show a great deal of diversity, an effect compounded by the variety of environments they live in.

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This leads to many interesting biological phenomena for the evolutionary biologist to study. It might help provide more details to the question of what evolutionary advantage is granted by having different sexes.

An organism that can change its gender

One interesting subject on that note is hermaphrodites.

A hermaphrodite is an organism that has both male and female reproductive organs and produces gametes normally associated with both male and female sexes. It is common in flowering plants and invertebrates. It is named after Hermaphroditus, who, in Greek mythology, is the son of Aphrodite and Hermes. 

One specific type of hermaphrodism is sequential hermaphrodism. These are organisms that can change their sex based on environmental triggers. There are three kinds of sequential hermaphrodites:

  • Protandry — an organism that is born as a male and changes into a female. Eg. Clownfish
  • Protogyny — an organism that is born as a female and changes into a male. Eg. Wrasses
  • Bi-directional sex changers — organisms that can change their sex in either direction. Eg. Lythrypnus dalli, the blue-banded goby
A Plausible Explanation of Sex Change in Fishes
Source: Rickard Zerpe/Flickr

What drives sex change in fish?

As an embryo, all fish are sensitive to physical conditions such as acidity and temperature of the water, and this can affect their gender. Temperature can be a major factor in deciding the sex of a fish. 

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Warmer waters will typically lead to the birth of a larger number of males in some fish, and several species of adult fish can also change sex based on environmental cues. The social environment can also lead to a change in sex in adult fish.

Around 500 species of fish are known to change sex in adulthood. Unlike mammals, the sex of a fish is not determined solely by chromosomes. Sex change can be triggered by the release of certain hormones. 

The Asian Sheepshead Wrasse

One of the most commonly known sex-changing fish is the Asian sheepshead wrasse, also called Kabudai. A Kabudai undergoing a transformation from female to male was caught on camera and presented in an episode of BBC’s Blue Planet - II.

The video brought this phenomenon to the wider public interest.

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The Kabudai, or Semicossyphus reticulatus, is a species of wrasse that live in the western Pacific Ocean. A female Kabudai can change sex from female to male in a couple of months. 

The emerging male Kabudai is more aggressive and has a bulbous chin and head, compared to the female.

One theory for why these fish change sex is because it can pass on more genes as a male. According to this model, the smaller-sized younger fish have more reproductive value as females, while the larger size of the male gives the older fish a greater chance to pass on its genes. 

Other gender-changing fish

Bluehead Wrasses

Bluehead Wrasses are another species of sex-changing wrasses. They live in groups on coral reefs in the Caribbean. These wrasses may be born as either males or females, but the females can change sex later in life. 

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Unlike the Kabudai, the Bluehead wrasse can change sex in under 20 days. This change is reversible.

Bluehead Wrasse
Source: James St. John/Wikimedia Commons

The Bluehead Wrasse was recently the subject of collaborative research led by a team of New Zealand scientists and Professor Jenny Graves, a geneticist at La Trobe University and the winner of the Prime Minister's Prize for Science in 2017. 

Their findings were published in the July 2019 edition of Science Advances journal.

The research group used high-throughput RNA-sequencing and epigenetic analyses to reveal the process of how and when certain genes are turned off and on in the brain and gonad, allowing the sex change to occur. The change in sex takes effect through changes in chemical markers that control the gene expression of DNA.

They found that the sex change involved completely rewired the genetic code of the gonad. Genes in the ovary are first turned off. Then a new genetic pathway is turned on, which promotes the formation of the testis.

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Clownfish

The clownfish are another species familiar to many from the movie Finding Nemo.

They are protandrous fish that live in warm waters, usually existing in symbiotic mutualism with sea anemones. This gives them the name ‘Anemonefish.’ Protandrous fish constitute the minority of sex-changing fish. 

Clownfish live in small groups, with one dominant breeding pair, and a number of non-breeding subordinates. When the female dies, the dominant male changes sex and chooses a new mate from among the subordinates.

Unusual sexual behavior in animals: can hamsters change their gender?

It is an urban myth that hamsters can change sex. However, there are some amphibians and reptiles that show some form of hermaphroditism.

Some slugs and frogs are simultaneous hermaphrodites. As opposed to sequential hermaphrodites, simultaneous hermaphrodites have both male and female sex organs at the same time. 

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Stranger yet, banana slugs are known to engage in apophallation, biting off the partner’s penis. The apophallated slug can still mate as a female. It has been suggested that apophallation may be a strategy to prevent the partner from mating again as a male, increasing its allocation of resources to the production of eggs.

Some lizards and one species of snake can undergo parthenogenesis. This is a form of asexual reproduction in which the egg cell is either fully or partially cloned, enabling the female to self-fertilize. All offspring produced this way are female.

Conclusion

Studying biological processes such as these can provide valuable information on genetic processes, possibly even leading to many practical applications.

The study on Wrasses provides insight into mechanisms for gene activation. It also provides information on the way that chemical markers on DNA control gene expression and help cells remember their specific function in the body.

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Studies like these could one day help to unlock methods for controlling gene activity. 

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