Physiological energetics of the horse mussel Modiolus modiolus in a cold ocean environment

A population of Modiolus modjolus (horse mussel) inhabiting a sub-arctic environment in Logy Bay, Newfoundland, Canada, was studied for 2 yr. The main objective was to gain insight into the relationship between environmental factors (temperature and components of the seston) and the physiological response of this species. The highest values for energy acquisition (ingestion and absorption rates) by M. rnodiolus coincided with the spring phytoplankton bloom occurring during April-May in Logy Bay, whereas energy expenditure (oxygen uptake and ammonia excretion rate) was greatest during the summer (July and August). The result was a clear seasonal fluctuation in the 2 physiological integrations, scope for growth (SFG) and net growth efficiency (K,), for which lower values were associated with a high metabolic rate, high temperature and low quality of the food supply. Conversely, higher values of SFG and K2 were associated with a low metabolic rate, low temperature and a n energy-rich food supply provided by the phytoplankton bloom. M. modiolus appears to be a species which is able to compensate physiologically for the poor food conditions occurring in Logy Bay during a large part of the year. It does this by reducing feeding activity during fall and winter, when there is a poor food supply. Conversely, it increases clearance rate during spring and early summer, when the seston is mainly composed of phytoplankton. Furthermore, M modiolus has a high absorption efficiency, which may be an adaptation to the low seston concentration in Logy Bay Such physiological compensations minimise the periods dunng which SFG is negative, and enable the horse mussel to survive in a n environment characterised by an intermittent and often inadequate food supply.