Effects of acute and chronic thermal stress on survival, apoptosis, and transcriptional responses of Scapharca broughtonii

Ocean warming is altering the habitats of marine invertebrates, which has resulted in an increased physiological stress to marine molluscs, especially those intertidal bivalves, such as the ark shell Scapharca broughtonii. We investigated the physiological and transcriptional responses of ark shells to acute and chronic thermal stress results showed that at 33 degrees C, a significantly higher cumulative mortality (55.7%) occurred under acute thermal stress than chronic thermal stress. The apoptosis rate of hemocytes was sustained at higher levels and the necrosis rate was increased significantly in a time-dependent manner under acute thermal stress. However, under chronic thermal stress, the apoptosis and necrosis rates exhibited similar change trends: a rapid increase followed by a gradual decline and sustained at a relatively high level until the end of the experiment. The expressions of heat shock protein genes (HSP20 and HSP90), apoptosis-related genes (TRAF6, GRP78, NIX, and Casp-3), antioxidative-related genes (GST and MRP) and cellular detoxification-related genes (HbIIB, NOS -1, HO -1, and ENO -1) were up regulated significantly under both acute and chronic thermal stresses. These results demonstrated that the anti-apoptotic system, antioxidant defense system, cellular detoxification system, and heat shock proteins (HSPs) played vital roles for ark shells in response to thermal stress. As acute thermal stress can result in irreversible damage to marine molluscs, it is thus advised that chronic thermal stress should be used to select thermal-resistant ark shell strains.