Seasonal variation in the relative utilization of dietary protein for energy and biosynthesis by the mussel Mytilus edulis

Mussels Mytilus edulis were collected from Whitsand Bay in southwest England during Apnl, July and September 1993 and January 1994, and fed a defined diet of prekilled microalgae with one of 2 types of mixed protein/carbohydrate microcapsules. Both capsule types were comprised of the same w/w proportion of "N-labeled protein and nonlabeled carbohydrate. However, Types 1 and 2 contained trace amounts of ["Clcarbohydrate and [14C]protein, respectively. Defecation, excretion, respirdtion and incorporation of I4C and "N isotopes were measured to describe the mussel's relative utilization of dietary carbohydrate (I4C from Type l ) , protein-C (l" from Type 2) and protein-N (15N from Types 1 and 2). In each experiment, mussels ingested similar quantities of capsule Types 1 and 2. However, the '4C-assimilation efficiency for carbohydrate (16 to 20%) was significantly greater than the 14C-assimilation efficiency for protein (7 to 15%). In contrast, protein-N was always assimilated with greater efficiency (25 to 82%) than either carbohydrate or protein-C. The difference in assimilation efficiency between protein-N and protein-C indicates that a large proportion of amino acids from dietary protein were hydrolyzed, with amlno-N being preferentially assimilated. The relatively poor assimilatlon of amino-C indicates that mussels were not nutritionally limited by energy since amino-C generated by the energy-intensive digestion process was not fully catabolized in the tricarboxylic acid cycle (i.e. not respired). The conservation of protein-N relative to protein-C was particularly noticeable in July when rates of ingestion and ammonia excretion were lowest,