Physiological and histological responses of Phascolosoma esculenta (Sipuncula: Phascolosomatidea) to acute heat stress

Temperature is an important factor affecting the growth, development, and survival of organisms. The effects of temperature stress on aquatic organisms have received increasing attention, as these organisms are mostly poikilotherms and their body temperature are directly corresponding changes with ambient temperature, resulting in they are easily exposed in temperature stress. However, little is known about the effects of high temperature on Sipuncula. In this study, we investigated the effects of acute heat stress on malondialdehyde (MDA) concentration, the activities of antioxidant (superoxide dismutase and glutathione peroxidase) and immunity-related (acidic and alkaline phosphatase) enzymes, heat shock protein 70 ( hsp70 ) and hsp90 gene expression, and the histological structure of the sipunculid Phascolosoma esculenta . Within the coelom fluid, the MDA concentration and all detected antioxidant enzyme activities increased during high temperature stress; significant increases were also observed here and in the intestine in the hsp70 and hsp90 mRNA expression levels. These results indicated that acute heat stress caused oxidative stress; antioxidants and heat shock proteins probably act to protect P. esculenta against oxidative damage, constituting part of its physiological mechanism for adaptation to high temperatures. In addition, the increased activity of the acidic and alkaline phosphatases indicated effects on its nonspecific immune system. Furthermore, damaged tissue structures were observed in the body wall, retractor muscle, intestine, and nephridium after 96 h of 40-°C stress. The damaged cells of these tissues showed obvious condensed chromatin around the nuclear membrane. This histological damage suggests that heat stress could affect movement, food absorption, digestion, and excretion in P. esculenta . These results elucidate the effects of temperature stress on P. esculenta and its physiological response mechanisms and provide practical indicators for assessing heat stress status and determining suitable culture temperatures for P. esculenta .