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Coolia malayensis is one of the commonly found benthic dinoflagellates in Hong Kong which can produce biotoxins and threaten the early life stages of marine invertebrates. Seawater temperature has been recognized as one of the primary environmental factors that affect the formation of harmful algal blooms. The present study evaluated the responses of C. malayensis, including growth, toxicity and toxin content (putative analogues of okadaic acid and azaspiracids), after exposure to a range of seven different temperatures (i.e., 16°C, 18°C, 20°C, 22°C, 24°C, 26°C, and 28°C). The highest algal density and specific growth rate were recorded at 24°C. Significantly higher F v /F m (maximum quantum yield of PSII) and total phaeo-pigment values were observed in the exponential growth phase at 28°C. The toxicity of the algal extract, which was assessed by the lethality rate of Artemia larvae, increased with temperature. The highest toxin content was detected at the second highest temperature treatment, i.e., 26°C. Overall, temperature had significant effects on the physiological activities and toxicity of C. malayensis. This study has raised attention to the potentially increasing risks posed by toxic benthic dinoflagellates during heat waves in coastal waters., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020. Published by Elsevier Ltd.)