Adaptability of clams to prolonged heatwaves: Influences on gene expression, antioxidant capacity, and histology of Chinese cyclina Cyclina sinensis

An increase in the duration of marine heat waves (MHWs) has a serious negative impact on the survival of marine organisms. However, the impact of MHWs on Cyclina sinensis remains largely unknown. Here, this study assessed the effects of acute (25 degrees C or 35 degrees C for 12 h) and prolonged heatwave stress (no heatwave (25 degrees C for 4 weeks), short heatwave (25 degrees C for 3 weeks then 1 week at 35 degrees C), and long heatwave (35 degrees C for 4 weeks)) on the physiology and metabolism of C. sinensis. Acute heat stress alters the expression of immune-related genes in the hepatopancreas, including LSZ1, DPP1, SRB1, Casp3, BIRC2, Arf, CLEC4M, CLEC10A and Gadd45/3. The transcript levels of chaperones genes and antioxidant enzymes were up-regulated after acute heat stress treatment (35 degrees C). Under prolonged heatwave stress challenge, HSP27, HSP70B2 and HSP90/3 mRNA in hepatopancreas and mantle were up-regulated in short heatwave group. The mRNA levels or activities of SOD in the hemolymph, hepatopancreas, and gills, as well as CAT in the hemolymph, hepatopancreas, and gills, were higher in clams from the short heatwave group than those from the no heatwave group. Surprisingly, the mRNA levels and activities of SOD or GSTP1 in the gills of clams in the short and long heatwave groups were significantly lower than those in the control group. The levels of GSH in the hepatopancreas and gills of clams in the short and long heatwave groups were lower than those in the no heatwave group, but the level of MDA was higher than that in the no heatwave group. Moreover, prolonged exposure to heatwaves resulted in the breakage and shortening of gill filaments and thinning of the hepatopancreas wall in clams. These findings suggest that heatwave cause oxidative stress and induce tissue damage in mollusks.