Male, Female Or Both? Study Shows Chemicals, Temperature Can Confuse Crustaceans

Reports of blue crabs exhibiting both male and female sex characteristics in the Chesapeake Bay and other water systems raise a red flag about the environment in which the crabs live, says Dr. Gerald A. LeBlanc, professor of environmental and molecular toxicology at North Carolina State University.

And while the definitive cause for these crustaceans’ sexual confusion is unknown, LeBlanc and post-doctoral student Allen Olmstead decided to do some tests on their lab-friendly crustaceans – daphnia, or water fleas – to ferret out some clues to what’s going on in surface waters containing sexually ambiguous crustaceans.

They found that exposing water fleas to moderate levels of a specific hormone promoted gynandromorphism, or the exhibition of dual sex characteristics, and that the effect was heightened when water fleas were exposed to warmer temperatures at the same time. In addition, numbers of gynandromorphic daphnia rose when they were exposed to a particular pesticide – pyriproxyfen – and the effect was increased when the water fleas were also exposed to warmer temperatures.

The paper describing the research was published in the December 2006 edition of the International Journal of Biological Sciences.

Gynandromorphic crustaceans have “both sets of plumbing,” as LeBlanc puts it. Economically important animals – crab, lobster, shrimp and crayfish populations – have been affected by this phenomenon, albeit in low numbers, he says. But if these confused animals produce fewer babies – and LeBlanc says studies of these ambiguous animals indeed show reduced baby broods – the impact could eventually be significant.

The researchers started by looking through their own data, specifically the birth records of about 9,000 daphnia raised in LeBlanc’s lab over the course of 15 years. None of the 9,000 water fleas exhibited gynandromorphism.     

Water fleas are able to reproduce asexually, with females not needing males to produce offspring. These females commonly give birth to other females, who also reproduce asexually. In previous research, LeBlanc discovered that exposing females to high levels of methyl farnesoate, a hormone, caused female water fleas to give birth to males. So LeBlanc and Olmstead began tinkering with hormone levels.

Exposing water fleas to moderate levels of methyl farnesoate resulted in more dual-sex daphnia, although not many. However, when the researchers exposed water fleas to moderate levels of the hormone in warmer temperatures – some 10 degrees Celsius higher than the temperatures at which the water fleas are normally maintained in the lab – the number of gynandromorphic water fleas increased 46-fold.

The researchers also studied the effects of exposing water fleas to varying levels of pyriproxyfen, a pesticide used to keep pests like mosquitoes from breeding. LeBlanc and others had previously published research suggesting that exposure to this particular pesticide caused more male water fleas to be born, so its effects were similar to that of exposure to the hormone methyl farnesoate.

In this study, the NC State scientists found that exposure to pyriproxyfen mimicked the results of tests with methyl farnesoate, although lower amounts of the pesticide were needed to confuse water fleas and produce some with both sets of plumbing. And, as with methyl farnesoate, exposure to the pesticide coupled with increased temperatures caused even more water fleas to be born with dual sex characteristics.

“The incidence of gynandromorphism seen in the environment is too low to endanger crustacean populations, but something is going on when we see these sexually compromised animals,” LeBlanc says. “It’s a warning signal that something is happening in the environment. We’re now focusing on the protein receptor that methyl farnesoate binds to in order to learn more about this phenomenon.”

The research was funded by a grant from the U.S. Environmental Protection Agency.