A winter (4 °C) spill condition was simulated in which young soft-shell clams, Mya arenaria, were exposed to subacute concentrations of No. 2 fuel oil-in-water emulsions for 28 d. Clams were exposed at the beginning of the experiment to single dose concentrations of 10, 50, and 100 ppm. Hydrocarbons were rapidly accumulated by clams within 1 wk after exposure. The accumulated hydrocarbons then decreased each week as the hydrocarbon content of the water decreased. Methyl substituted naphthalene isomers were the principal compounds accumulated and retained by the clams after 3 wk of oil exposure. A dose–response relationship was observed in the respiratory rates as measured by oxygen consumption (QO2). Significant differences (P =.05) in respiratory rates were found in clams exposed to low concentrations of oil. The lowest concentrations of oil caused a doubling of the respiratory rates and greater oil concentrations caused a depression in rate. The respiratory rates of the clams exposed to low oil concentrations decreased as the hydrocarbon content of the water and clam tissues decreased, but remained significantly altered from the controls. Clams were transferred to an uncontaminated system for 14 d subsequent to the 28-d oil exposure to determine effects of depuration on the respiratory rate. During the depuration period, many of the hydrocarbons present in clam tissue were again found to be methyl substituted naphthalene isomers. During this period, the respiratory rates of the clams initially exposed to 10 ppm fuel oil emulsion remained significantly altered above the controls. The respiratory rates of all groups of oil-exposed clams remained altered from the controls, but the magnitude of difference tended to decline toward the controls. A dose–response narcosis may have been evident during this period. Key words: Mya arenaria, respiration, No. 2 fuel oil, petroleum emulsion, petroleum accumulation, depuration
