In search of a superior diet for post-settlement bay and sea scallops (Argopecten irradians and Placopecten magellanicus): How do diets containing different Pavlovaspp. stack up?

Poor growth and survival of scallops is often observed in hatcheries during immediate post-settlement stages as scallops undergo substantial morphogenesis of feeding organs and may be particularly vulnerable to dietary deficiencies. The goal of our studies is to develop high-performance algal diets for cost-effective implementation in bivalve hatcheries and to identify particular dietary requirements, especially of lipids, which may enhance production. Two commercially important scallop species, Argopecten irradians and Placopecten magellanicus, were grown in recirculating systems during four separate trials, each lasting 3-4 weeks. Binary diets containing the flagellates Pavlova lutheri and Pavlova sp. CCMP strain 459 (Pav 459), in combination with the diatom Chaetoceros muelleri (CHGRA), were investigated to determine their ability to support growth of scallop postlarvae. Substitution of Pav 459 for the more commonly used P. lutheri yielded a 51% and 31% increase in shell growth rate for bay and sea scallops, respectively, indicating that Pav 459 is superior for scallop postlarvae. We further investigated the nutritional value of diets containing Pavlova pinguis for sea scallop postlarvae. A binary diet of P. pinguis and CHGRA produced growth rates equal to those observed for the Pav 459/CHGRA diet in multiple growth trials (initial shell heights of 350 mu m and 1.6 mm). However, unialgal diets of Pav 459, P. pinguis and especially CHGRA yielded significantly lower growth rates than the mixed diets, indicating that both components of the binary diet contribute substantially to scallop nutrition. Pavlova pinguis, Pav 459, and CHGRA are all characterized by relatively high levels of n-6 polyunsaturated fatty acids (PUFAs). CHGRA contains high levels of arachidonic acid (AA; 20:4n-6), while Pav 459 and P. pinguis (in contrast to P. lutheri) are characterized by high levels of docosapentaenoic acid (DPA; 22:5n-6), which may account for the differential growth performance. Unlike n-3 PUFAs such as docosahexaenoic acid (DHA) and eicosa-pentaenoic acid (EPA), little is known about the role of n-6 fatty acids in bivalve mollusks, however, AA is a precursor of bioactive compounds such as prostaglandins, and DPA has been hypothesized to play a role in the maintenance of membrane function. Enrichment of n-6 fatty acids, specifically AA and DPA, was observed in tissues relative to the diet in both scallop species and across dietary treatments whereas no enrichment o