Algal exposure assays revealed that all tested strains of H. circularisquama and H. illdefina were markedly toxic to juvenile pearl oysters. H. triquetra, and Heterocapsa sp. 3 and sp. 5 caused low levels of mortality in juveniles (1.7-7.5%), whereas H. lanceolata, H. horiguchii and Heterocapsa sp. 4 caused no mortality. Three strains of H. circularisquama alone exhibited strong toxicity against bivalves among Heterocapsa species isolated in Japan.
The sequencing of internal transcribed spacer (ITS) regions including 5.8S rDNA and phylogenetic analysis were performed for isolates of Heterocapsa species. Alignment of the sequences demonstrated that ITS regions were highly conserved among H. circularisquama strains, while the distance between H. circularisquama and other species ranged from 0.165 to 0.254. Except for H. horiguchii and Heterocapsa sp. 4, the phylogenetic tree obtained was congruent with morphological identification. Heterocapsa species were subjected to PCR amplification with a primer set designed based on a specific signature sequence for H. circularisquama in the ITS 2 region. A specific band was detected for all strains of H. circularisquama but not other Heterocapsa species, indicating that the ITS region is suitable as a species-specific marker for H. circularisquama.