Recovery of Mytilus edulis L. from chronic oil exposure

Marinc Environmental Research 17 (1985) 250-253

Recovery of Mytilus edulis L. from Chronic Oil Exposure

John Widdows, Peter Donkin & Sheila V. Evans Natural Environment Research Council, Institute for Marine Environmental Research, Prospect Place, The Hoe, Plymouth, Devon PLI 3DH, Great Britain

Previous laboratory' studies I have shown that physiological and cellular processes of Mytilus edulis are affected by exposure to low and environmentally realistic concentrations of oil. However, there is little information concerning the rate of recovery from oil exposure and the extent to which physiological recovery may be related to the depuration of hydrocarbons from the tissues. The present study has shown a marked reduction in the feeding rate and scope for growth of mussels exposed to two concentrations of diesel oil (30 and 130 Izg/ litre ) for 8 months. During recovery' from oil exposure the depuration of hydrocarbons from the tissues was concomitant with the recovery of physiological performance. Mussels exposed to high oil concentrations ('high-oil' mussels) were found to recover more rapidly than those exposed to low oil concentrations ('lowoil' mussels), both in terms of depuration and scope for growth, and there was evidence of "catch-up' growth. Recovery of both low- and high-oil mussels was complete after approximately 55 days. Experiments were carried out in experimental basins (25m 3) with simulated wave and tidal movements at the Marine Research Station, Solbergstrand, Norway. z During the winters of 1982-83 and 1983-84, a rocky shore community, in which Mytilus edulis was an important component species, was chronically exposed to two diesel oil concentrations (30_+ 7 and 130 + 28#g/litre; mean + SD). 2 Following Contribution No. 5, Norwegian Institute for Water Research. 250 Marine" Environ. Res. 0141-1136/85/$03"30 ~ Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

Recover)" of Mytilus edulis L. from chronic oil exposure

251

approximately 8 months of oil exposure from September to May each year the physiological responses (clearance rate, respiration, food absorption efficiency and ammonia excretion) of 15 mussels from each oil basin and the control basin were measured (day 0). Mussels exposed to low and high oil concentrations were transferred to the control basin for recovery and their physiological responses determined after 5 and 10 days (short-term; May, 1983) and after 22 and 55 days (long-term; April-May, 1984). In 1984 exposed mussels were transferred to the control basin 22 and 55 days prior to measurement, thus allowing all measurements to be performed within a period of 2 days. The scope for growth of the controls was found not to vary significantly within and between the two experiments. On each sampling occasion mussels ( n = 2 x 5) were dissected into gills, digestive gland and remaining tissues, the hydrocarbons were extracted by steam distillation, 3 analysed by normalphase HPLC and tissue concentrations expressed in terms of two and three ringed aromatic hydrocarbons. Figure l (day 0) shows the inverse relationship between scope for growth and hydrocarbon exposure. During I0 days in clean water the physiological responses had only partially recovered, indicating that the mussel's performance was not directly related to the hydrocarbon concentration in the water. Analysis of hydrocarbon concentration in the tissues demonstrated a close relationship between the recovery of physiological responses and the loss of hydrocarbons from different tissues (Fig. 1). The gills depurated 40% of their accumulated hydrocarbons within 5 clays and there was a comparable recovery of clearance rate. However, the digestive gland and remaining tissues of the high-oil mussels did not depurate significant amounts within 5 days and this could account for the maintenance of a reduced absorption efficiency. After 22 days in clean water the high-oil mussels not only had depurated a higher proportion of the accumulated hydrocarbons compared with the low-oil mussels, but also their tissue hydrocarbon concentrations were lower. These observations correlated with a more rapid recovery and an ~overshoot' in clearance ( = feeding) rate and scope for growth by the highoil mussels which gave rise to 'catch-up' growth following tissue degrowth during high-oil exposure. The rapid decline in tissue hydrocarbon concentration of the high-oil mussels within 22 days was probably the combination of an enhanced rate of hydrocarbon excretion/depuration, as suggested in a previous study, 4 and a 'dilution effect' due to enhanced tissue growth (catch-up growth). The rate of recovery of low-oil mussels

John Widdows, Peter Donkin, Sheila V. Evans

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Fig. 1. Scope for growth (J/h; mean + SE) of Mytilus edulis (standard animal of 0.5 g dry mass) during recovery from 8 months of exposure to 130/~g/litre (high oil) and 30/~g/litre (low oil). Short-term recovery (0, 5, 10 days; May, 1983) and long-term recovery (0, 22 and 55 days: May, 1984) expressed relative to control mussels (no significant differences between controls in 1983 and 1984). DepuTation of 2 + 3 ring aromatic hydrocarbons (#g/gwet mass) from gills and remaining tissue (mean ___range of two pooled samples).

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Fig. 2. Relationship between scope for growth (J/h; mean +_SE) and concentration of 2 + 3 ring aromatic hydrocarbons (#g/g wet mass; mean _ range) in the body tissues of Mytilus edulis (r-" = -0"84). A synthesis of all measurements(1983 and 1984) during oil exposure and the recoveryperiod. (Broken line represents 95 % confidenceinterval of the regression line. n = 12.) was slower both in terms of tissue depuration and physiological performance, but both groups showed complete recovery after 55 days (i.e. not significantly different from controls). Figure 2 provides a synthesis of all measurements (oil exposed and recovery phases), illustrating the negative correlation (r 2 = 0.84) between scope for growth and aromatic hydrocarbon concentration in the body tissues. This confirms that the degree of stress in mussels is a simple function of the tissue concentration of toxicants. REFERENCES 1. Widdows, J., Bakke, T., Bayne, B. L., Donkin, P., Livingstone, D. R., Lowe, D. M., Moore, M. N., Evans, S. V. & Moore, S. L. Mar. Biol., 67, 15-31 (1982). 2. Bakke, T. & Sorenson, K. In Proceedings of 1985 Oil Spill Conference, Los Angeles, 1985. 3. Donkin, P. & Evans, S. V. Anal. Chim. Acta, 156, 207-19 (1984). 4. Widdows, J., Moore, S. L., Clarke, K. R. & Donkin, P. Mar. Biol., 76, 109-14 (1983).