Experiments on pollutant toxicity are usually conducted under constant 
conditions of light, temperature, and salinity. The results of such experiments 
may not be applicable to the field, where environmental conditions vary. A 
case in point is the observed seasonal variation in oil toxicity to adult N. 
obsoletus , with toxicity being accentuated in winter. Egg capsule deposition 
patterns of oil-exposed N. obsoletus also showed temporal variability. Patterns 
were least like those for control snails when water temperature was low at the 
beginning of the breeding season, but changed substantially as water 
temperatures rose. Similarly, Krebs and Burns (17) have observed that fiddler 
crabs (Uca pugnax ) exposed to No. 2 fuel oil in the field, show abnormal 
behavior only at temperatures near the lower limit of their normal range of 
activity. 
There are several possible explanations of the greater toxicity of this oil at 
low temperatures. Low temperature may directly increase the relative 
concentration of the more toxic oil fractions present in the water, either 
through altered solubility, volatization, or shifts in bacterial activity. This 
hypothesis is currently being explored. It is also possible that seasonal changes 
in toxicity result from changes in the physiological state of the animals and/or 
from additive effects of low temperature and oil stress. 
There is currently little information on how petroleum hydrocarbons enter 
aquatic animals, but recent evidence indicates that uptake of oil through 
ingestion of contaminated food may be at least as important as diffusional 
uptake (8, 11, 18). This is consistent with our observations of higher mortality 
of adult N. obsoletus in the presence of sediment. This occurred only during 
the summer, when N. obsoletus is actively deposit-feeding (30). The sediment 
effect did not occur during the winter, when the snails are inactive. 
We observed several sublethal effects on invertebrate reproduction, 
including possible reduction in the fecundity of N. obsoletus. Although there 
was no alteration in the number of eggs per capsule, the number of capsules 
produced appeared to decline. One possible variable influencing egg capsule 
production may be date of initiation of oil exposure relative to the onset of 
oogenesis. In this study, U. cinerea were exposed to hydrocarbons after 
gametogenesis was completed, and egg capsule depostion was already 
underway, which might explain the absence of a fecundity response for this 
species. More data are needed to resolve this issue. 
The observed alteration of egg capsule deposition behavior with respect to 
substrate orientation has significant ecological consequences for N. obsoletus. 
The egg capsules and embryos of N. obsoletus are not well adapted for 
deposition in the exposed intertidal zone; successful pre-hatching development 
of this species is apparently dependent instead upon the proper placement of 
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