Lethal Effects and Behavioral Responses of Sea Cucumber (Apostichopus Japonicus, Selenka) to Thermal and Hypoxia Stresses

Over recent years, the increasing frequency of extreme weather caused by climate change is rising ocean temperatures, which in turn could reduce dissolved oxygen (DO) levels in seawater. At present, the thermal and hypoxia stresses are the main reasons affecting the development of sea cucumber aquaculture in China. In this study, sea cucumbers were exposed to thermal (T = 16 °C, 22 °C, 24 °C, 26 °C, 28 °C, 30 °C, 32 °C, and 34 °C), hypoxia (DO = 8 mg/L, 6 mg/L, 4 mg/L, 2 mg/L, and 0.5 mg/L), and the combination of thermal and hypoxia stresses (DO = 2 mg/L, T = 16 °C, 22 °C, 24 °C, 26 °C, 28 °C, 30 °C, 32 °C, and 34 °C) for 96 h. Results showed that the median lethal dose (LD50) of sea cucumber under thermal stress, hypoxia stress, and the combination of thermal and hypoxia stresses was 28.25 °C, 1.03 mg/L, and 24.59 °C, respectively. Additionally, during the exposure period, we observed and counted the changes in 15 physiological behavior endpoints of sea cucumbers at different exposure time points. Results showed that different environmental stresses could cause sublethal effects on sea cucumbers, such as the ability of tube-feet adhesion reduced, the parapodia soft or flat, body wall ulceration, and secretion of transparent mucus and tentacle stretched. Our results highlighted its usefulness in providing the threshold warning information for the sea cucumber aquaculture water environment when extreme heat weather occurred for the sea cucumber aquaculture industry.