the capsules on substrates (23). Interference with normal patterns of egg 

 capsule deposition behavior could substantially increase pre-hatching mortality 

 from desiccation stress (23). 



The reduction in larval grov/th rate observed at sublethal oil concentrations 

 could result from increased energy expenditure, decreased ingestion rate, 

 decreased assimilation efficiency, or a combination of these factors. Present 

 evidence suggests that the reduced grov^^h observed was due at least in part to 

 reduced food intake. Larvae of C fornicata and N. obsoletus held at "1.0" 

 ppm ceased feeding at least one to two days before they died. The larval guts 

 of C. fornicata were empty of food by the second day of each experiment, 

 even though the velar lobes remained extended and ciliary activity was 

 observed. Tissues in these individuals became dramatically shrunken v^thin 

 several days after initiation of exposure to oil. VeUgers held at oil 

 concentrations of "0.10" ppm showed no such morphological abnormality , but 

 prehminary experiments (Pechenik, unpubUshed) reveal decreased ingestion 

 rates at this concentration, relative to ingestion rates of control larvae. 



It is not yet possible to precisely predict the threshold oil concentrations at 

 which lethal or sublethal effects occur. The potential for seasonal changes in oil 

 toxicity has already been discussed. Moreover, most laboratory experiments 

 conducted to date, including many in the present study, have used static 

 exposures in which the dosing medium is replenished at one to two day 

 intervals. Due to loss of volatile fractions from these aqueous mixtures, the 

 initial hydrocarbon concentrations cited represent only maximum 

 concentrations which tlie animals experienced during a test (6, 32). Atkinson 

 et al (4) reported that 90 percent of the benzene initially present in a test 

 solution is lost from undisturbed cotton-plugged flasks within a 24 hour 

 period. Our containers were kept tightly sealed in the static experiments, 

 minimizing such loss. Some loss of hydrocarbons through volatilization could 

 have occurred during transfer of the medium from the flow-through tanks to 

 the experimental containers, however. Finally, oil concentrations are generally 

 reported as total hydrocarbon content, as measured by infrared 

 spectrophotometry. Yet, toxicity to animals is probably due to only a small 

 fraction of the hydrocarbon compounds present in the water accommodated 

 fraction used (17), a fraction which can vary qualitatively and quantitatively 

 over the period of an investigation. The concentration of specific oil fractions 

 present during an experiment is generally unknown. Better control and analysis 

 of oil exposures conditions are needed if we wish to accurately determine 

 threshold concentrations of oil which are toxic to marine organisms. 



153 



