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 A', 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 v^th our observations of higher mortality 

 of adult N. obsoletus in the presence of sediment. This occurred only during 

 tlie 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 vdth 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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