Biochemical characterization and nutritional value of three Pavlova spp. in unialgal and mixed diets with Chaetoceros muelleri for postlarval sea scallops, Placopecten magellanicus

Low growth and survival of sea scallops, Placopecten magellanicus, are often observed during postlarval stages when they undergo a period of protracted morphogenesis and thus may be particularly susceptible to nutritional deficiencies. Diets containing flagellates from the genus Pavlova (especially Pavlova sp. CCMP 459) have proven successful for scallop rearing in previous studies. To further examine their contribution to growth performance, Pavlova sp. (CCMP 459) and Pavlova pinguis, both as unialgal diets and in combination with the diatom Chaetoceros muelleri (CHGRA), were tested as diets for postlarval (~ 350 µm and 1.6 mm) sea scallops in two growth trials each lasting ~ 4 weeks. The Pav 459- and P. pinguis-CHGRA combination diets yielded high and comparable growth of postlarvae whereas previous work found substantial decreases in growth rate (31%) when another algal species of the same genus, Pavlova lutheri, was substituted for Pav 459 in the combination diet. In the present study a unialgal diet of P. pinguis yielded growth rates 32% lower than the combination, and unialgal diets of C. muelleri consistently ranked lowest of all diets tested (54–65% lower than the combination). Algal sterol composition may influence scallop growth performance, as CHGRA contains high levels of cholesterol, also present although to a lesser extent, in Pav 459 and P. pinguis. P. pinguis, Pav 459, and C. muelleri are also all characterized by relatively high levels of n-6 polyunsaturated fatty acids (PUFA). Pav 459 and P. pinguis (in contrast to P. lutheri) are characterized by high levels of docosapentaenoic acid (DPA, 22:5n-6), and C. muelleri contains high levels of arachidonic acid (AA, 20:4n-6) which may account for the higher growth obtained on this mixed diet. Enrichment of Σn-6 fatty acids, as well as of the individual n-6 fatty acids AA and DPA, was observed in scallop tissues relative to the diet across dietary treatments. However, no enrichment of Σn-3 fatty acids was observed, which provides evidence of selective uptake, retention and/or metabolism of n-6 fatty acids. This study strongly supports the role of n-6 DPA as an essential nutrient for scallop early life history stages.