We have investigated changes in specific contents of protein, glycogen and lipid, and fatty acids of blue mussels, Mytilus edulis, under different conditions in the field and in laboratory feeding experiments using different microalgae. Specific contents of glycogen and lipid increased in mussels relocated to net bags at a location in Kerteminde Bay (Great Belt, Denmark) in contrast to mussels relocated to a location in Sallingsund (Limfjorden, Denmark). The polyunsaturated fatty acid, eicosapentaenoic acid, reached 3 times higher values in the mussels in Kerteminde Bay. Mussels fed pure cultures of Crypthecodinium cohnii, which is rich in the polyunsaturated fatty acid, docosahexaenoic acid, and glycogen, gained the highest specific contents of this fatty acid and glycogen. Mussels feeding on the most protein rich of the microalgae, Bracteacoccus sp., gained the highest protein contents. The specific glycogen content of the mussels was influenced by their “condition” (body dry weight/shell length ratio) while specific protein and lipid contents were not. Starvation affected mainly the specific glycogen content. These results show that biomass composition of blue mussels is affected by living site and local phytoplankton species and that the fatty acids composition of mussels reflects the content of fatty acids in the diet. 1. Introduction The growth rate of filter feeding blue mussels, Mytilus edulis, is mainly determined by the concentration of suspended phytoplankton in the water [1, 2] while the biomass produced by the mussels can be of a variable biochemical composition. Seasonal dependent variations in biomass composition of blue mussels are well described [3–7] and spawning occurs at the expense of stored glycogen and lipid [8]. Also the availability and composition of phytoplankton species in the diet influence biomass composition of mussels [9]. Thus, the biochemical composition of M. galloprovincialis has been observed to depend on living site in the Mediterranean Sea [10–12]. Starvation will also result in changes of biomass composition of mussels because maintenance metabolism is likely to cause food reserves within different biomass components to be metabolized at unequal specific rates. Mussels acquire proteins, lipids, carbohydrates, and other components from phytoplankton and use these compounds to build their own biomass. The polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) are some of the nutritionally most valuable components of marine biomasses. The PUFA content and composition in
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