The lethal effects of the harmful dinoflagellate <i>Karenia mikimotoi</i> on two bivalves, Yesso scallop and Sakhalin surf clam, and two echinoderms, short-spined sea urchin and Japanese common sea cucumber

Red tides of harmful dinoflagellates Karenia spp. have recently occurred along the coast of Hokkaido, northern Japan, causing concern about the mass mortality of aquatic organisms. This study aimed to verify the lethal effects of Karenia mikimotoi on reared juveniles of two bivalves, the Yesso scallop Mizuhopecten yessoensis and Sakhalin surf clam Spisula sibyllae, and two echinoderms, the short-spined sea urchin Strongylocentrotus intermedius and Japanese common sea cucumber Apostichopus japonicus, which are industrially important species in Hokkaido. Their juveniles were exposed to seawater with four different densities of red tide-causing K. mikimotoi from 0 to 4.5×104 cells mL−1 for 48 h and then kept in the seawater without K. mikimotoi for 96 h for follow-up. The survival of the individuals was recorded every two days. The analysis using Firth’s bias-reduced logistic regression model showed that K. mikimotoi significantly affected the survival of the Yesso scallop, Sakhalin surf clam, and short-spined sea urchin within 96-h follow-up of the 48-h exposure, but did not significantly affect the survival of the Japanese common sea cucumber. The median lethal doses of K. mikimotoi at the end of follow-up were estimated as 2.40×104 cells mL−1 for the Yesso scallop, 3.32×104 cells mL−1 for the Sakhalin surf clam, and 4.45×104 cells mL−1 for the short-spined sea urchin. These results suggested that the sensitivities of species to K. mikimotoi toxicity differ.