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Fish that change sex - Groupers

Green Country Vol. 119 (Apr 2016)

We humans, like most animals in nature, are classified into male and female. Mammals and birds have chromosomes that decide their sexes, while some reptiles and amphibians have their sexes fixed according to the environment (such as temperature) at their hatching time. Some animals, on the other hand, are hermaphrodites that contain both male and female reproductive organs. Groupers are among them, and can even change sex to increase the survival rate of the population. However, the greatest threat to groupers comes from humans. Today, some species are already endangered due to over-fishing.

Groupers, members of the Serranidae family, are widely distributed in tropical and temperate waters. There are over 500 species of Serranidae worldwide, more than 160 of which are groupers. The ferocious predators are mostly found in coral communities and rocky reefs, and feed on fish, shrimps and crabs.

Most fishes are dioecious, having only male or female reproductive organs. Groupers are among the few fish that can "change sex". They contain both testes and ovaries. They are female at birth, as ovaries start to grow in the embryos. The female fish mature up to the stage of reproduction. After laying eggs, testes begin to grow, and eventually the fish becomes a male. How amazing!

Turning from female to male in a few days

Changing sex may be a survival strategy by increasing genetic diversity by "changing the sexes". Apart from Giant Grouper, which can live to over a hundred years, most groupers take some 10 years for the female to mature. That is, it takes 10 years for a female to change into a male. In this way, an age difference is maintained between the males and females within the same population, reducing the chance of reproductive failure due to inbreeding and increasing the survival of the whole population.

More surprisingly, if a male dies within a population, a senior female may turn into male in a few days to stabilise the sex ratio within the group to enhance reproductive success.

Over-consumption of fish

Groupers gather in groups to lay eggs. They are top consumersin the marine food chain. The low number of individuals means that it is hard to find a mate. The female lays eggs together in certain waters to prevent them being flushed away, and hence increase the chance of fertilisation when the male releases sperm.

Although groupers have special way of raising the number and survival rate of the population, the huge demand for groupers, particularly Orange-spotted Grouper, Duskytail Grouper and Brown-marbled Grouper – which are upmarket cuisine on the Chinese dining table – has caused overfishing of them. Fishermen and gourmets even target both large and small fish. Many female fish that are yet to change sex are caught, creating a situation with more females than males. In a research paper from the University of Hong Kong in 2013, 20 species of groupers were listed as endangered. Next time you are about to consume a grouper, please stop and consider choosing one from sustainable harvesting or aquaculture, instead of the endangered or vulnerable ones.

Text|Hayward Ng

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©Ken Ching
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© Ken Ching
Groupers are female at birth and turn into males when mature. Blacktip Grouper (Epinephelus fasciatus) is one of the most familiar groupers to Hong Kong people.

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Sabah Giant Grouper (above), a common fish sold in the market, is not a natural grouper. It is in fact a cross-breed between Giant Grouper (Epinephelus lanceolatus) (below), created by researchers at the University of Sabah. Artificial aquaculture may fulfil the huge demand for groupers.
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