Abstract
Heterocapsa circularisquama Horiguchi is lethal to shellfish, particularly bivalves such as pearl oysters (Pinctada fucata Gould). No detrimental effects of this flagellate on fish have been observed thus far. In this study, we found that H. circularisquama causes mammalian erythrocytes to lyse. Among the erythrocytes tested, rabbit erythrocytes showed the highest susceptibility, whereas erythrocytes from cattle, sheep, and human were relatively insensitive. Heterocapsa triquetra Stein, which is morphologically similar to H. circularisquama but not toxic to bivalves, showed no hemolytic activity toward rabbit erythrocytes. Culture supernatant or ultrasonic‐ruptured cells of H. circularisquama showed only weak hemolytic activity. Hemolytic activity was found in the ethanol extract of H. circularisquama cells, suggesting that the hemolytic agents may be more stable in ethanol than in aqueous solution. Both an intact flagellate cell suspension and the ethanol extract caused morphological changes and eventual collapse of unfertilized eggs of Pacific oyster. Furthermore, the ethanol extract was lethal to the microzooplankton rotifer Brachionus plicatilis Müller, which is highly sensitive to H. circularisquama. Our results suggest that a hemolytic toxin produced by H. circularisquama may be one of the causative agents responsible for the shellfish toxicity.
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