SEASONAL PATTERNS IN UTILIZATION OF DIETARY-PROTEIN BY THE MUSSEL MYTILUS-TROSSULUS

Utilization of dietary protein by adult mussels Mytilus trossulus collected from Yaquina Bay, Oregon, USA, was measured under controlled laboratory conditions during February, May, August, and November 1991, Mussels were fed C-14-labeled protein microcapsules ([C-14]PM), which were added to either laboratory-cultured microalgae or natural seston. Utilization of dietary protein, represented by [C-14]PM, was assessed by measuring C-14-activities of each component of the mussel's carbon budget and calculating the ingestion rate, digestion efficiency, and assimilation efficiency of [C-14]PM. Mussels utilized dietary protein differently during the year; ingestion rates during May were more than 3 times greater than during November. Digestion and assimilation efficiencies ranged from 16 to 53 % and 5 to 31 %, respectively, and were greatest during February and November. In all experiments, dietary protein was ingested at greater rates and assimilated with lower efficiencies when [C-14]PM were fed to mussels in combination with a standard diet of microalgae rather than with naturally occurring seston. However, the seasonal pattern of protein utilization was similar for mussels fed on the 2 diets, suggesting that utilization of dietary protein was not a simple response to seasonal differences in seston composition. Greatest net assimilation rates for [C-14]PM occurred just before peak reproductive condition (May) when more than 90 % of the area in mantle tissue sections was comprised of gametes. Estimated assimilation rates of bioavailable protein by mussels in situ were more than 7 times greater during February than in August, when mussels were less reproductively active. Proteolytic activity of crystalline style and digestive diverticula gut extracts was significantly greater during the period January to March than during July to September. Mussels also conserved assimilated protein from catabolism during the peak in reproductive activity (May), when the ratio of C-14-respiration to C-14-incorporation was lowest and the ratio of oxygen consumption to nitrogen excretion was highest. High assimilation rates of dietary protein and metabolic conservation of assimilated protein during late winter and spring were associated with greatest tissue protein contents during March to May. These results suggest that seasonal variation in the utilization of dietary protein by M. trossulus is closely linked to the reproductive cycle.