Diatom Pseudo-nitzschia cf. caciantha (Bacillariophyceae), the Most Likely Source of Domoic Acid Contamination in the Thorny Oyster Spondylus versicolor Schreibers 1793 in ha Phu Bay, Khanh Hoa Province, Vietnam

Domoic acid (DA) contamination in the thorny oyster Spondylus versicolor Schreibers 1793 was discovered in 2005, in Nha Phu Bay, Khanh Hoa Province, Vietnam. Concurrently, DA was detected in the net-plankton samples. The causative organism responsible for the DA was not detected then. In 2006, DA in S. versicolor (maximum of 43.6 μg·g^-1) and in net plankton samples (maximum of 0.78 ng·L^-1) recurred, suggesting the existence of DA producers in the bay. When DA in S. versicolor again increased in 2007, a net-plankton sample was collected, and cultures of Pseudo-nitzschia species were established for DA analysis and species identification. Eight out of eleven cultured isolates of Pseudo-nitzschia spp. showed DA production (111 244 ng·mL^-1), as confirmed by liquid chromatography-tandem mass spectroscopy. The toxic isolates examined by transmission electron microscopy shared identical morphological characteristics: a single row of poroids, hymens divided into 2–6 sectors, and mantles 1–2 poroids high. They resembled Pseudonitzschia caciantha Lundholm, Moestrup Hasle, 2003 thus we designated it as P. cf. caciantha. Our results indicated that P. cf. caciantha most likely contributed to the DA contamination in S. versicolor in Nha Phu Bay. This is the first report of DA production by P. cf. caciantha anywhere in the world.