Physiological responses of Mytilus edulis and Cardium edule to aerial exposure

Abstract

Physiological responses of two bivalves (Mytilus edulis L. and Cardium edule L.) to intertidal conditions were studied. Specimens were collected from S. W. England in autumn/winter, 1980, and acclimatized to either intertidal or subtidal regimes before measurement of rates of heat dissipation \(\left( {{}_t\dot q} \right)\) and oxygen uptake \(\left( {\dot \upsilon _{{\rm O}_2 } } \right)\) during 5 h of air exposure, and rates of \(\dot \upsilon _{{\rm O}_2 }\), particle clearance, ammonia excretion, and food-absorption efficiency during 7 h of reimmersion. Subtidal individuals were either intermittently or continuously fed in order to distinguish the effects of periodic food supply from the effects of air exposure. Specimens of M. edulis had low aerial rates of \({}_t\dot q\) (14 to 20% of aquatic rate), and \({}_t\dot q\) was greater than the energy equivalent of \(\dot \upsilon _{{\rm O}_2 }\), indicating that they were largely anaerobic. In contrast, C. edule “air-gaped” and had higher aerial rates of \({}_t\dot q\) and \(\dot \upsilon _{{\rm O}_2 }\) (50 to 75% of aquatic rate). There were behavioural and metabolic differences in the responses of intertidally and subtidally acclimatized mussels and cockles to air exposure. Intertidal individuals of both species were more quiescent, had lower aerial rates of \({}_t\dot q\) and \(\dot \upsilon _{{\rm O}_2 }\), and showed a conditioned response at the “expected” time of reimmersion. The reduction in aerial rate of \({}_t\dot q\) was an energy-saving mechanism and the payment of the “oxygen-debt” within 2 h of reimmersion represented a significant “cost”. The “heat increment” associated with feeding and digestion was estimated as 15 to 17% of the oxygen uptake by M. edulis during all stages of recovery. M. edulis adapted to the intertidal regime by reducing its time-averaged aerial and aquatic rates of ammonia excretion. In contrast, C. edule maintained a high aerial and aquatic rate of ammonia excretion. The clearance rates of M. edulis recovered rapidly (0.5 to 1.5 h) following reimmersion, whereas those of C. edule recovered more slowly, particularly for the subtidal individuals following acute exposure (>4 h). There was no evidence of increased clearance rate or absorption efficiency by intertidal individuals to compensate for the loss of feeding time. Intertidally acclimatized individuals of M. edulis and C. edule had more energy available for growth (scope for growth) integrated over a 12 h period and higher growth efficiencies than subtidal individuals subjected to acute air exposure and intermittent feeding regimes. This was the result of reduced aerial and aquatic rates of energy expenditure, a relatively small “cost” in terms of the payment of “oxygen-debt” and a rapid recovery of clearance rate following reimmersion.