Influence of irradiance on the nutritional value of two phytoplankton species fed to larval Japanese scallops (Patinopecten yessoensis)

Japanese scallop (Patinopectin yessoensis Jay) larvae grew faster and were larger after 18 d when fed a diet of high-light(HL)-grown Chaetoceros simplex or HL Pavlova lutheri relative to diets of the same phytoplankton species grown at low light (LL). When provided as saturating rations to larval scallop, these diets could be ranked: HL C. simplex>LL C. simplex>HL P. lutheri>LL P. lutheri. In both phytoplankton species, HL-grown cells contained more of the short-chain saturated fatty acid (FA), 16:0 than LL-grown cells. Scallop growth rates were a significant function of the amounts (mg g^-1 dry wt) and the proportions (as percentage of total FAs) of the FAs 14:0 and 14:0+16:0 (total saturated FAs) in their diet. The proximate biochemical composition of HL- versus LL-grown phytoplankton showed no significant differences in protein, total lipid, carbon, carbohydrate or nitrogen per cell which were consistently associated with the greater nutritional value of HL cells. In spite of this high variability in proximate composition, the larval growth rate was a significant function of the average carbon content, nitrogen content and cell volume of the phytoplankton cells. Increased amounts of the essential polyunsaturated FAs 20:5 3 and 22:6 3 in the phytoplankton were negatively correlated with larval scallop growth rates. Thus HL-grown phytoplankton cells were nutritionally superior to LL-grown cells. This nutritional superiority seems to be determined by the fatty acid composition of the cells which, in turn, is controlled by variation in irradiance. The general tendency of predator FA profiles to resemble that of their prey was not observed in larvae fed P. lutheri. The much greater amounts of 18:4 3, 20:5 3, and 22:6 3 FA in P. lutheri relative to C. simplex were not evident in the scallop larvae fed these cells.