Dietary effects of the red-tide raphidophyte Heterosigma akashiwo on growth of juvenile Manila clams, Ruditapes philippinarum

We studied the effects of Heterosigma akashiwo, a harmful algal species, in the diet of juvenile Manila clams (Ruditapes philippinarum), because it often blooms in coastal waters where aquaculture of the clams is carried out. Weight of soft tissue of clams increased when they were fed H. akashiwo added at 8,000 cells mL-1, with weight growth about 8% higher than that of clams fed only Chaetoceros neogracile. In addition, a significant increase in the body-mass index and glycogen content was observed when they were fed H. akashiwo compared with C. neogracile. Analysis of the chemical composition of H. akashiwo, C. neogracile, Pavlova lutheri and Nannochloropsis sp. showed a higher total sugar and acidic sugar content in H. akashiwo than in the other species, whereas the protein content was almost the same for all species except Nannochloropsis sp. These observations suggest the possibility that the sugar content of phytoplankton is an important factor affecting the growth of juvenile clams.