MEPS 126:177-184 (1995)  -  doi:10.3354/meps126177

Mussels Mytilus edulis were collected from Whitsand Bay in southwest England during April, 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 ¹⁵N-labeled protein and nonlabeled carbohydrate. However, Types 1 and 2 contained trace amounts of [¹⁴C]carbohydrate and [¹⁴C]protein, respectively. Defecation, excretion, respiration and incorporation of ¹⁴C and ¹⁵N isotopes were measured to describe the mussel's relative utilization of dietary carbohydrate (¹⁴C from Type 1), protein-C (¹⁴C 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 ¹⁴C-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 amino-N being preferentially assimilated. The relatively poor assimilation 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, <1% of absorbed [¹⁵N]protein was excreted, and [¹⁵N]protein assimilation was 5 times more efficient than [¹⁴C]protein assimilation. Further, rates of protein synthesis were greatest in July (71.2 mg d^-1), compared with April, September and January (39.2, 58.9 and 20.5 mg d^-1). These findings suggest that seasonal variation in dietary protein utilization in M. edulis is governed chiefly by changing anabolic demands for amino-N, rather than by requirements for energy or essential amino acids.

Protein . Carbohydrate . Mytilus edulis . Microcapsule . Assimilation