Nice, HE, D Morritt, M Crane and M Thorndyke. 2003. Long-term and transgenerational effects of nonylphenol exposure at a key stage in the development of Crassostrea gigas. Possible endocrine disruption? Marine Ecology Progress Series 256:293-300.

In experiments exposing larval oysters to a single dose of nonylphenol at levels commonly found in the environment, a team of English scientists finds that nonylphenol can alter the sex ratio of oysters, cause some to become hermaphroditic, and dramatically impair survivorship of offspring.

When the oysters reached adulthood, they were bred in various combinations: controls with controls, 1 µg/liter-treated animals with 1 µg/liter-treated animals; 100 µg/liter-treated with 100 µg/liter-treated, and all cross-combinations (e.g., controls with 100 µg/liter-treated animals). The scientists then examined the development of embryos and larvae produced by these combinations 48 hrs after fertilization.

What did they find?

adapted from Nice et al.

At 10 months post-fertilization , when the oysters reached sexual maturity for their first breeding season, not only was the sex ratio of treated animals shifted toward a more females than the controls, many individuals were functional hermaphrodites... 30% in the group treated with 100 µg/liter and 17% in the lowest dose of nonylphenol tested. No hermaphrodites were observed in the control group.

These differences in sex ratio and percentage of hermaphrodites are highly significant statistically. The scientists observed no difference, however, in growth rates comparing treated to control animals.

What does it mean? Nonylphenol has a profound effect on sexual development of oysters, even at low, environmentally relevant doses. Because the tests found positive results even at the lowest dose tested, ascertaining a "no-effect" level is impossible based upon these data. Clearly it is lower than 1 µg/liter. Other molluscs have been found to be exquisitely sensitive to another endocrine disrupting compound, bisphenol A.

The authors summarize a suite of studies indicating that sex determination of oysters is likely to be influenced naturally by pheremones. Their results in this study suggest that nonylphenol may be interfering with that chemical communication system, to the detriment of the exposed oysters.

The extreme sensitivity of oysters to nonylphenol, combined with the frequent presence of nonylphenol in sewage discharge waters, raises concerns about the effect of nonylphenol and related compounds on natural and farmed populations of oyster. Some natural populations of oysters have undergone dramatic population crashes, which typically are attributed to overharvest (e.g., in the Chesapeake Bay). These results suggest that endocrine disruption should be examined as a contributing factor.