GROWTH, FILTRATION AND RESPIRATION IN THE MUSSEL MYTILUS-EDULIS - NO EVIDENCE FOR PHYSIOLOGICAL REGULATION OF THE FILTER-PUMP TO NUTRITIONAL NEEDS

The specific growth rate of blue mussels Mytilus edulis fed Rhodomonas sp, algal cells in laboratory experiments increased with increasing f ood concentration to obtain a maximum value of about 9.5% d(-1), irres pective of a relatively high concentration of silt (5 mg l(-1)) added to the water. Likewise, the net growth efficiency increased with the s pecific growth rate to a maximum value of about 75%. Measurements of t he relationship between respiration and growth showed that the energy cost of growth constituted 12% of the growth. The filtration rate of a 100 mg dry wt 'standard' mussel was high and constant (about 30 ml mi n(-1)) at algal concentrations below about 6000 cells ml(-1) when meas ured in both short-time (5 h) and long-time (9 to 14 d) experiments. H igh algal concentrations of 1.3 to 2.4 x 10(4) cells ml(-1) reduced th e filtration rate by about 40%. The estimated growth, presuming maximu m filtration rate, satisfactorily described the actual growth at algal concentrations < 5000 cells ml(-1), and the concentration necessary f or maximum growth was estimated at 4500 cells ml(-1) (corresponding to 5.6 mu g chlorophyll a l(-1)). The mean specific growth rate in M. ed ulis transferred in net bags to a fjord system (Kertinge Nor/Kertemind e Fjord, Denmark) was about 6% d(-1). The results show that there is n o physiological regulation of the filtration rate to nutritional needs , and that food uptake in nature (median concentration in Danish water s = 5.1 mu g chlorophyll a l(-1)) is characterized by the full exploit ation of the capacity of the bivalve filter-pump.