Pseudo-nitzschia species, a possible causative organism of domoic acid in Spondylus versicolor collected from Nha Phu Bay, Khanh Hoa Province, Vietnam

Recently, it was reported that a significant level of domoic acid, a causative toxin of amnesic shellfish poisoning (ASP), was detected in bivalves belonging to the genus Spondylus collected from various tropical areas, including Vietnam, whereas no significant domoic acid was detected in other bivalve species. These facts suggest that the causative plankton species for domoic acid occur widely in tropical waters and that Spondylus spp. accumulate domoic acid more effectively than other bivalve species. In the monitoring on seasonal change of domoic acid levels in S. vesicolor and plankton net samples in Nha Phu Bay, Vietnam, domoic acid level of S. versicolor was found to increase when domoic acid was detected in the plankton net samples. Under light microscopic observation of the plankton samples, cells of Pseudo-nitzschia spp., the size of which was smaller than toxic Pseudo- nitzschia such as P. multiseries, were observed. When the plankton net samples were fractionated by successive filtration through the sieves and membrane filters with different pore sizes, most of the domoic acid was recovered in the particle fraction with smallest size (0.6-10 mm). Some strains of unicellular cultures established from the cells in this fraction showed the production of domoic acid, though the productivity was low. All the domoic acid-producing strains seemed to be the same species. These results indicate that Pseudo-nitzschia species is also causative for domoic acid in tropical bivalves.