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
Ecology Progress Series 256:293-300.
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
conclude that these effects are extremely deleterious to the
survival of oysters and "may result in severe consequences,
not only for natural populations but also for commercial hatcheries
situated in areas where nonylphenol is present in the water.
research is the first to show that "a single exposure
to a pollutant at environmentally realistic levels, administred
through water during a key stage of larval development can
cause transgenerational effects in oysters."
did they do? Nice et al. exposed larval
Pacific oysters Crassostrea gigas to two different
levels of nonylphenol (1
for a single 48-hr period during days 7-9 after fertilizing
then allowed the larvae to mature to adulthood, monitoring
growth and development monthly during maturation, and then
at adulthood calculating the sex ratio and determining the
proportion of hermaphrodites.
important background: Oyster reproductive biology
has some unusual twists. A normal oyster can change
sex between reproductive seasons. Once a given reproductive
season begins, however, the oyster cannot switch sex
until the season is over.
during during the first year of reproduction, most Pacific
oysters are males. As they grow over the years, they
are increasingly likely to switch from male to female.
hermaphroditism has been observed before in oysters,
but only at extremely low frequencies...usually below
1%. And when observed, they usually do not contain both
eggs and sperm simultaneously, and hence are incapable
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.
did they find?
from Nice et al.
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.
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.
of the larvae to nonylphenol also affected the survival rates
of their offspring
from control parents (CMxCF) survived
at much higher rates than larvae from any other combination
of parents. This result was highly significant statistically
(p < 0.001).
graph to the right shows the survival rate of the offspring
of different combinations of parentage.
from Nice et al.
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.
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
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.