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Apr 5, 2006

Ocean “dead zones” cause sex changes in fish, researchers say

March 31, 2006
Courtesy American Chemical Society
and World Science staff

Oxygen depletion in the world’s oceans, caused by pollution, could spark the development of far more male fish than female, threatening some species with extinction, a study has found.

A study found that zebra fish raised in tanks with low concentrations of oxygen were more likely to develop male sexual characteristics—even if they were genetically female. (Courtesy NASA)

The researchers, Rudolf Wu and colleagues at the City University of Hong Kong, said the findings raises new concerns about vast areas of the world’s oceans called “dead zones.” 

In these areas, there isn’t enough oxygen dissolved in the water to support most sea life. 

Fish and other creatures trapped there often die. Those that escape may be more vulnerable to predators and other stresses. 

The new study, Wu said, suggests these zones potentially pose a third threat to these species: an inability of their offspring to find mates and reproduce.

The researchers found that low levels of dissolved oxygen, also known as hypoxia, can induce sex changes in embryonic fish. This leads to an overabundance of males that makes it harder for them to find mates and reproduce.

Low oxygen levels also might reduce the quantity and quality of the eggs produced by female fish, diminishing their fertility, Wu added.

In experiments, Wu and his colleagues found low levels of dissolved oxygen—less than 2 parts per million—reduced the activity of certain genes that control the production of sex hormones and sexual differentiation in embryonic zebra fish. 

As a result, 75 percent of them developed male characteristics. This isn’t as bad as it sounds, Wu said, because zebra fish populations are normally about 60 percent male and 40 percent female—but it’s still a problem.

At least some of the excess “male” fish were actually genetically female, but had male sexual characteristics, Wu said, suggesting the oxygen shortage somehow affects sexual development after the genes are in place. It appears to do so by enhancing testosterone levels, his team reported.

“Reproductive success is the single most important factor in the sustainability of species,” Dr. Wu said. “In many places, the areas affected by hypoxia are usually larger than the spawning and nursery grounds of fish.”

Hypoxia is considered one of major threats to marine life and genetic diversity, Wu said. It occurs when excessive amounts of plant nutrients, particularly nitrogen, accumulate in oceans, freshwater lakes and other waterways. This often occurs as a result of agricultural runoff or pollution, but sometimes happens naturally.

These nutrients trigger the growth of huge blooms of algae. These eventually die, sink to the ocean floor and decompose, a process that removes oxygen from the surrounding water. 

Dead zones are developing along the coasts of the major continents, and they are spreading over larger areas of the sea floor, Wu said. The United Nations Environmental Programme estimates nearly 150 permanent and recurring dead zones exist worldwide, including 43 in U.S. coastal waters.

The study was published March 29 on the Web site of the American Chemical Society journal Environmental Science & Technology. The study is scheduled to appear in the May 1 print issue of the journal.

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Posted: Apr 5, 2006 6:31pm


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