Enhancement of dietary effect of Nannochloropsis sp. on juvenile Ruditapes philippinarum clams by alginate hydrolysates

Abstract The eustigmatophyte Nannochloropsis sp. can be produced on a large scale at low cost. However, its dietary usefulness for juvenile bivalves is less than that of other algae. Recently, we reported that growth of juveniles of the clam Ruditapes philippinarum was dramatically promoted by supplementing a diet of the diatom Chaetoceros neogracile with alginate hydrolysates (AHs) at the concentration of 4 mg L −1 . In this study, we examined the effect of AHs on Nannochloropsis sp. as a clam diet. Ten-day rearing experiments in 500-mL beakers showed that AHs have a beneficial effect on clam culture regardless of water temperature. Shell growth in clams given AHs at the concentration of 4 mg L −1 along with Nannochloropsis sp. at the concentration of 20 × 10 4 cells mL −1 was higher than in any other test groups at 15 or 25 °C. In 20-day rearing experiments in 30-L tanks, the average shell length in the groups given AHs at the concentration of 4 mg L −1 along with Nannochloropsis sp. at the concentration of 30 × 10 4 cells mL −1 was significantly greater than that in the groups given C. neogracile and Nannochloropsis sp. alone ( P ≪  0.05). Furthermore, the total weight of clams given AHs at the concentration of 4 mg L −1 along with Nannochloropsis sp. at the concentration of 30 × 10 4 cells mL −1 was greater than that in the group given only Nannochloropsis sp. ( P ≪  0.05). Hence, the enhanced dietary effect of a combination of Nannochloropsis sp. and AHs will help to shorten the rearing time of R. philippinarum and to provide a stable supply of algal diet.