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

Abstract

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, <l % of absorbed ( ' 5~] p r o t e i n was excreted, and [I5N]protein assimilation was 5 times more efficient than [I4C]protein assimilation. Further, rates of protein synthesis were greatest in July (71.2 mg d-'), compared with April, September and January (39.2, 58.9 and 20.5 mg d-l). 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.

Evaluation of microalgal and formulated diets for the culture of the New Zealand pipi clam Paphiesaustralis

“…These results suggest that the adults may have initiated their gonadal development. This conforms with other studies where accumulations of proteins in bivalve tissue are simultaneously an indicator to maturation of gonads (Devi et al 1985;Kreeger 1993;Rivonker and Parulekar 1995), and as a reserve prior to spawning activity (Nagabhushnam and Mane 1978;Gabbott and Peek 1991). Proteins and lipids were continuously metabolised to support energy demand whenever needed (food scarcity or insufficient carbohydrates).…”

Seasonal variation in C, N and P budgets and tissue composition of the mussel Mytilus edulis

“…Others have also been able to correlate respiration rate with gametogenic cycle ( R a and Kreeger et al (1995) reported ammonium excretion rates comparable to those we found, with high values in periods when glycogen reserves were depleted. Kuenzler (1961) reported an average phosphate excretion rate of 0.086 pm01 P-PO, g-' h-' for Modiolus demissus, while for Mytilus edulis, Kautsky & Wallentinus (1980) reported rates from 0.01 to 0.77 pm01 g-' h-' in February and August, and Asmus et al (1994) measured phosphate excretion rates in mussels ranging from 0.07 pm01 g-' h-' in September to 0.17 pm01 g-' h-' in May.…”

“…They concluded that energy or carbon-based SFG overestimated growth under nitrogen-limited conditions. Kreeger et al (1995) demonstrated excretion of the carbon fraction of amino acids which were hydrolysed and transaminated for biosynthesis of proteins, and suggested that mussels were more likely to have been nutritionally limited by their quantitative amino-N requirements rather than by their energy demand. In our study, seasonal NGE for P was always positive and, on average, was higher than for N. However, possible underestimation of P excretion in winter might account for the high NGE for P. A high NGE indicates that a relatively small fraction of the absorbed material is used for maintenance, and a relatively large fraction is available for growth and reproduction.…”

Growth, filtration and respiration in the mussel Mytilus edulis:no evidence for physiological regulation of the filter-pump to nutritional needs

“…g . Winter 1973, Navarro & Winter 1982, Bayne et al 1987, 1988, 1993, Hawkins & Bayne 1992, Stenton-Dozey & Brown 1992, Willows 1992, Navarro et al 1994, Kreeger et al 1995. Earlier contradictions to the theory of 'physiological control' or 'regulation of filtration rate' have in particular been advanced by Jsrgensen (1990) Riisgdrd et al (1994) note (Jsrgensen 1996)-which gives a comprehensive review of the most recent literature.…”