Feeding and absorption of the toxic dinoflagellate Alexandrium tamarense by two marine bivalves from the South China Sea

Paralytic shellfish poisoning (PSP) toxins can be accumulated by bivalves t hrough the feeding process; therefore, knowledge on feeding and the assimil ation of PSP-toxin-containing algae is critical to understand the kinetics of PSP toxins in these bivalves. In the South China Sea, it has been docume nted that the scallop Chlamys nobilis has a much higher PSP toxin burden th an the clam Ruditapes philippinarum. Experiments were therefore carried out to assess whether the difference in toxin burden between these two species of bivalves was due to differences in feeding and absorption. In a mixed d iet of Alexandrium tamarense (a PSP-toxin-producing dinoflagellate) and Tha lassiosira pseudonana (a non-toxic diatom), the maximum clearance and filtr ation rates were about two times higher in the scallop C. nobilis than in t he clam R. philippinarum. Furthermore, the clams produced pseudofeces at a lower cell density than the scallops. However, we found that the clams were unable to selectively exclude the toxic dinoflagellates by pseudofeces pro duction. The scallop C. nobilis also possessed a greater ability to assimil ate A. tamarense with a comparable carbon absorption efficiency to the diat om T. pseudonana. In contrast, the carbon absorption in the clam R. philipp inarum was lower when feeding on A. tamarense than on the diatom. In genera l the absorption efficiency decreased with increasing concentration of A. t amarense. Thus, it is likely that the higher PSP toxin levels in the scallo ps compared with clams can be partly explained by differences in their feed ing and absorption behavior. Other processes, especially the biotransformat ion and biokinetics of PSP toxins, may also play a significant role in defi ning the inter-species differences in PSP body burden in marine bivalves.