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 growth 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 growth 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 within 
several days after initiation of exposure to oil. Veligers held at oil 
concentrations of “0.10” ppm showed no such morphological abnormality, but 
preliminary experiments (Pechenik, unpublished) reveal decreased ingestion 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 the 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. 
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