Effects of different dietary microalgae on survival, growth, settlement and fatty acid composition of blue mussel (Mytilus galloprovincialis) larvae

The diatom Chaetoceros calcitrans is a major component of many bivalve hatcheries, yet it is expensive and notoriously difficult to culture on a commercial scale. In an attempt to reduce dependence on the diatom C. calcitrans, mussel larvae (Mytilus galloprovincialis) were subjected to feeding experiments which altered levels of the diatom under controlled hatchery conditions. Growth, survival and settlement success of mussel larvae were determined in response to five mixed algal diets in which the relative contributions of C. calcitrans was varied over the experimental period (30 days). Fatty acid profiles of the larvae and algal diets were also assessed. The exclusion of C. calcitrans from the diet had no significant differences on larval growth and only minor differences in total fatty acid content were found between treatments. Fatty acid analysis revealed that larval survival was strongly influenced by the proportions of dietary docosahexaenoic acid (DHA), while settlement was positively correlated with higher ratios of the n-3 long-chained polyunsaturated fatty acid (n-3 LC-PUFA) (namely, DHA and eicosapentaenoic acid, EPA), to the n-6 LC-PUFA (arachidonic acid, ARA). Despite similar relative and absolute n-3 LC-PUFA levels in the larvae under different dietary treatments, the larvae receiving high levels of C. calcitrans performed significantly better in terms of survival and settlement success. These results indicate that the (DHA + EPA)/ARA ratio is a key factor in determining larval performance, rather than the total amount of these fatty acids.