Relative insensitivity of butter clam neurons to saxitoxin: a pre-adaptation for sequestering paralytic shellfish poisoning toxins as a chemical def ense

Neurons from the butter clam Saxidomus giganteus and its congener the Washington clam S. nuttalli were shown to be 10 to 100 times more resistant to the dinoflagellate toxin saxitoxin (STX) than those from 4 CO-occurring infaunal bivalves, Mya arenaria, M, truncata, Tresus capax and Protothaca staminea. Only neurons from the venerid clam Humilan'a kennerlyi proved more resistant than those of Saxidomus spp. No difference in sensitivity to STX was found between butter clams from 3 sites with different histories of toxin contamination (chronically, occasionally or never toxic) suggesting that the resistance of this species to STX is innate rather than acquired with increased exposure. The resistance of butter clam neurons to STX may have permitted the evolution of this species' unique ability to sequester high concentrations of STX, especially in the siphon, for long periods (> 2 yr). Neither of the 2 other venerid clams analyzed, H. kennerlyi and P. staminea, were found to concentrate STX or derivatives in their siphons. Because predators have been shown to reject bivalve prey contaminated with STX, long-term retention of this toxin may have been favored in the butter clam as an acquired chemical defense.