Evolutionary adaptation of anaerobic and aerobic metabolism to high sulfide and hypoxic hydrothermal vent crab, Xenograpsus testudinatus

The vent crab, Xenograpsus testudinatus (xtcrab), is adapted to inhabit shallow-water, high sulfide and hypoxic hydrothermal vent. Our previous study revealed sulfide tolerance of vent xtcrabs which sulfide: quinone oxidoreductase (xtSQR) paralogs aid in sulfide detoxification. However, the mechanisms of how xtcrab adapts to high sulfide-hypoxic conditions in the vent area remain to be explored. In the present study, we tested the tolerance of xtcrab to sulfide-induced hypoxia, and investigated their aerobic and anaerobic responses in situ and in the laboratory. Comparisons were made to a non-vent, intertidal species, Thranita danae (tdcrab). We analyzed the several factors related to aerobic metabolism (SQR, cytochrome c [CYTC], complex IV [COXIV]), the product of anaerobic metabolism (hemolymph lactate levels) and glucose levels. Our results showed a higher survival tolerance to hypoxia of xtcrabs than tdcrabs. Hemolymph lactate levels increased more rapidly in xtcrabs than tdcrabs exposed to experimental hypoxia, revealing a rapid induction of anaerobic metabolism in hypoxic xtcrabs. Lactate measurement in xtcrabs returned from aquaria to original capture sites (vent habitats), further assessed the remarkable ability of xtcrabs to rapidly switch on and off their anaerobic metabolism. To assess aerobic metabolism, long-term exposure of xtcrabs to hydrothermal vent habitat increased gill xtCYTC transcripts and protein levels together with steadily enzymatic activity of COXIV. This revealed ability of xtcrabs to maintain functional capacity of aerobic respiration in hypoxia. Phylogenetic analysis showed that xtSQR paralogs in xtcrabs were more distant compared to tdSQR paralogs in tdcrabs. The increase of transcripts and enzymatic activity of gill xtSQR, and co-localization of xtSQR and xtCYTC also contribute to maintain aerobic metabolism by preventing sulfide toxicity on mitochondrial respiratory function. Overall, our study suggests that multiple strategies including detoxification of sulfide by gill xtSQR, and a quick/dynamic switch between aerobic and anaerobic metabolisms may play important roles in the metabolic adaptations of xtcrabs to extreme hydrothermal vent environment.