How increasing temperature affects the innate immune system of sea urchin Paracentrotus lividus (Lamarck, 1816) reared in a RAS system

• The effect of increasing temperature was evaluated in the sea urchin immune system. • The coelomocytes density and lysozyme activity were affected by temperature increase. • Protease and nitric oxide production were not affected by temperature increase. The purple sea urchin, Paracentrotus lividus , is the most exploited and economically important in Southern Europe due to the high value of its gonads. Temperature generally affects several physiological functions of marine invertebrates and the ocean warming has been linked to increasing frequency and severity of disease outbreaks in several echinoderms. Sea urchins have an innate immune system consisting of coelomocytes, the cellular components responsible for the immune response, supported by proteases and lysozymes. The present study aimed to investigate the effect of increasing seawater temperature on the immunological response of P. lividus . In this experiment, the animals were exposed to an increase in temperature up to 24 °C for 36 days, after which cellular and humoral immunity parameters were measured. The number of coelomocytes in the animals increased with the temperature rise, mainly the phagocytes and the colourless granulocytes. In the humoral response of the animals, only the concentration of lysozyme responded to the increase in temperature.