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 food concentration to obtain a maximum value of about 9.5 % d-1, irrespective of a relatively high concentration of silt (5 mg l-1) added to the water. Likewise, the net growth efficiency increased with the specific growth rate to a maximum value of about 75 %. Measurements of the 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 min-1) at algal concentrations below about 6000 cells ml-1 when measured in both short-time (5 h) and long-time (9 to 14 d) experiments. High algal concentrations of 1.3 to 2.4 x 104 cells ml- 1 reduced the filtration rate by about 40%. The estimated growth, presuming maximum filtration rate, satisfactorily described the actual growth at algal concentrations <5000 cells ml-1, and the concentration necessary for maximum growth was estimated at 4500 cells ml-1 (corresponding to 5.6 μg chlorophyll a l-1). The mean specific growth rate in M. edulis transferred in net bags to a fjord system (Kertinge Nor/Kerteminde Fjord, Denmark) was about 6% d-1. The results show that there is no physiological regulation of the filtration rate to nutritional needs, and that food uptake in nature (median concentration in Danish waters = 5.1 μg chlorophyll a l-1) is characterized by the full exploitation of the capacity of the bivalve filter- pump.