Partitioning of Rate of Heat Dissipation by Mytilus edulis into Maintenance, Feeding, and Growth Components

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Abstract
Juvenile mussels (Mytilus edulis) of ~10 mg dry mass were maintained at five ration levels, and their rates of heat dissipation were determined by direct calorimetry. Metabolic energy expenditure (heat dissipation rate) was partitioned into separate metabolic components in order to assess the relative costs of maintenance, food acquisition, digestion/absorption, and growth in relation to changes in algal ingestion rate (0.04-0.73 mg algae ingested d⁻¹). Experimentally derived estimates of metabolic costs were then compared with theoretical bioenergetic estimates of costs. The cost of food acquisition and movement through the gut was minimal (<3% of total metabolic rate) and confirmed the low theoretical estimates. The measured cost of digestion and absorption was 17% of the total metabolic energy expenditure, and maximum theoretical estimates accounted for ca. 75% of this cost. The cost of growth increased to 34% of the total metabolic rate at the highest ingestion rate, and maximum theoretical estimates of the cost of protein and nonprotein synthesis could explain only ca. 56% of the measured cost of growth. The results are summarized in terms of the effect of ration level on the overall partitioning of metabolizable energy intake into tissue deposition (protein and nonprotein) and the total metabolic energy expenditure into the costs of maintenance, digestion, and growth.