Gender bender

oxygen depletion in oceans, caused mainly by agricultural run-off and pollution, creates dead zones where sea creatures can suffocate. A recent study claims that dead zones can also lead to male-dominated fish population, which can drive a species to extinction.

Researchers from the City University of Hong Kong raised zebra fish, a hardy species popular in aquariums, in tanks with normal and low (hypoxic) oxygen concentrations. They found low levels of dissolved oxygen -- less than 2 parts per million (ppm) -- induce sex changes in embryonic fish, leading to an overabundance of males. Hypoxic conditions reduced the activity of genes that control the production of sex hormones in embryonic zebra fish. As a result, about 75 per cent of the fish developed male characteristics. In contrast, about 62 per cent of the zebra fish spawn raised under normal oxygen conditions -- more than 5 ppm -- developed into males. The normal sex ratio of zebra fish is about 60 per cent male and 40 per cent female. With low concentrations of oxygen, zebra fish were also more likely to develop secondary male sexual characteristics -- even if they were genetically female. The findings were published on March 29 in the online edition of Environmental Science & Technology .

As the fish mature, it is unlikely they would be able to reproduce in sufficient numbers to maintain sustainable populations, researchers say. Low oxygen levels also might reduce the quantity and quality of the eggs produced by female fish, they add.

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