THE EFFECT OF DIETS OF PHYTOPLANKTON AND SUSPENDED BOTTOM MATERIAL ONFEEDING AND ABSORPTION OF RAFT MUSSELS (MYTILUS-GALLOPROVINCIALIS LMK)

Measurements of feeding rate and absorption efficiency were performed to assess the nutritional value of sedimentary organics combined with phytoplankton for raft mussels (Mytilus galloprovincialis Lmk) in Ria de Arosa (Galicia, N.W. Spain), Both clearance rate and absorption eff iciency reached a maximum on mixed diets in which the proportions of p hytoplankton and sediment particles were similar. Consequently, maximu m absorption rates were reached on laboratory suspensions that closely resembled features of natural seston from the Ria, Values of absorpti on efficiency recorded with suspended sediments (around 13%) do not ap ply to the detrital component of ingestion in mixed diets, leading to the conclusion that a positive effect of phytoplankton on absorption e fficiency of sedimentary organics is taking place. On the other hand, comparison of results for mixed and monoalgal diets also suggests that the occurrence of silt in the ingesta enhances absorption of microalg ae, possibly through improving mechanical treatment within the stomach . It was concluded that mixing of bottom material with phytoplankton i n the water filtered by raft mussels greatly increases growth potentia l, provided that no reduction of phytoplankton concentration below 40% of particulate volume takes place. Mussels with different locations i n the Ria exhibit variable degrees of infection by the parasitic proto zoa Marteilia refringens. Concomitant differences in physiological beh aviour could be attributed to digestive impairment caused by parasitiz ation.