<i>Karenia selliformis</i> grown under moderate light intensity exerts strong toxicity to fish

The dinoflagellate Karenia selliformis forms red tides in coastal waters and damages fishery industries in some countries, although its toxicity remains understudied. In recent years, blooms of K. selliformis have killed a vast amount of marine organisms on the Pacific coast of Hokkaido, Japan, representing the first reports of fishery damage by K. selliformis in this country. It is necessary to understand the lethal density of K. selliformis to marine organisms and environmental conditions that increase its toxicity to establish an alert level for K. selliformis red tides. In the present study, we aimed to identify the level of toxicity to fish of K. selliformis and its environmental modulators by conducting a toxicity assay with medaka fish (Oryzias latipes). First, medaka was exposed to the culture strain of K. selliformis isolated from coastal waters in Hokkaido and a culture strain of a noxious red-tide dinoflagellate Karenia mikimotoi. As a result, the toxicity of K. selliformis was extremely high compared to that of K. mikimotoi. Next, we examined growth and toxicity to O. latipes of K. selliformis cultured under several light intensities. The K. selliformis culture strain could grow at more than 20 µmol photons m−2 s−1, and its maximum growth rate reached 0.54 day−1 at 400 µmol photons m−2 s−1. The toxicities of K. selliformis grown under different light intensities positively correlated with its growth rates. Our results revealed that K. selliformis exerts strong toxicity under moderate light intensity, suggesting that its high toxicity to fish requires its active growth.