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 through the feeding process; therefore, knowledge on feeding and the assimilation 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 documented that the scallop Chlamys nobilis has a much higher PSP toxin burden than 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 diet of Alexandrium tamarense (a PSP-toxin-producing dinoflagellate) and Thalassiosira pseudonana (a non-toxic diatom), the maximum clearance and filtration rates were about two times higher in the scallop C. nobilis than in the 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 production. The scallop C. nobilis also possessed a greater ability to assimilate A. tamarense with a comparable carbon absorption efficiency to the diatom T. pseudonana. In contrast, the carbon absorption in the clam R. philippinarum was lower when feeding on A. tamarense than on the diatom. In general, the absorption efficiency decreased with increasing concentration of A. tamarense. Thus, it is likely that the higher PSP toxin levels in the scallops compared with clams can be partly explained by differences in their feeding and absorption behavior. Other processes, especially the biotransformation and biokinetics of PSP toxins, may also play a significant role in defining the inter-species differences in PSP body burden in marine bivalves.