Nutritional value of fresh and concentrated algal diets for larval and juvenile Pacific oysters (Crassostrea gigas)

Three species of microalgae commonly used as feed for bivalves, Pavlova lutheri, Isochrysis aff. galbana (clone T-Iso) and Chaetoceros calcitrans forma pumilum, were produced with standard techniques and then harvested by a flocculation procedure. This method was effective for C calcitrans and P. lutheri though a partial deterioration of cells was observed for the latter, whereas flocculation heavily damaged T-Iso cells (diffuse cellular lysis, majority of cells clumped and/or misshapen, loss in organic matter). Quality of concentrates concentrated at I degreesC was investigated by means of gross composition analysis and pheophytin a/chlorophyll a evolution over 4 weeks of storage. P lutheri concentrates did not exhibit significant changes in gross composition over the storage period, whereas the value of pheophytin a/chlorophyll a remarkably increased, from 0.05 to 0.35. In T-Iso concentrates, a dramatic decrease in protein and carbohydrate content occurred during the first week of storage. The chemical composition of concentrated C calcitrans cells did not substantially change during the first 3 weeks of storage;, after this period, the organic matter decreased significantly (- 18%). The effectiveness of trispecific (P. lutheri +T-Iso + C. calcitrans) and bispecific (P. lutheri +T-Iso; R lutheri + C calcitrans; T-Iso+C calcitrans), fresh or concentrated (I degreesC), diets were evaluated on Pacific oyster (Crassostrea gigas) with larval and juvenile feeding trials, lasting I and 4 weeks, respectively. For larvae, concentrated diets of P. lutheri+T-Iso and T-Iso+C. calcitrans stored for 7-14 days gave better growth than the equivalent fresh diets (P<0.05). In the juvenile trial, the use of the same concentrates for a longer period (4 weeks) gave significantly lower growth compared to the corresponding fresh microalgae. In this case, the longer time of storage (8-36 days) probably prejudiced the quality of the stored concentrates. This fact was confirmed by means of larval consumption assays of fresh and concentrated microalgae using fluorimetry probes. Indeed, for P lutheri delivered as fresh microalgae, larval grazing was twice that of concentrated cells. (C) 2003 Elsevier Science B.V. All rights reserved.