4.3.2 High oxygen transport ability improves the hypoxia tolerance of C. japonica
Although hemocyanin has a low affinity for oxygen, it is still an essential oxidizing medium for crustacean tissues in the absence of oxygen (Figure 11) (Charlotte, 1980; Fujieda et al., 2010). Desiccation can inhibit oxygen supply and induce respiratory acidosis in crustaceans (Defur, 1988). Respiratory acidosis further reduces the ability of oxygen and hemoglobin to bind, which exacerbates oxygen deficiency and tissue hypoxia (Allen et al., 2012). However, desiccation-tolerant crustaceans can regulate ion (including Ca2+ and Mg2+) concentration to improve the oxygen-binding ability of hemocyanin and promote oxygen transfer between tissues (Charlotte, 1980). The expansion or positive selection of the IP3and PRPS1 genes means that C. japonica can maintain Ca2+ and Mg2+ homeostasis to ensure the oxygen-binding ability of hemocyanin to resist hypoxia in drought environments. Hypoxia-inducible factors (HIFs) also play an important role in hypoxia response by binding with hypoxia response elements near the downstream gene promoter for transcriptional activation (Hirota, 2002; Martens et al., 2007). The STK gene in C. japonicacan enhance HIF phosphorylation and increase the molecular weight of HIFs (Richard et al., 1999). In addition, the GSR gene, which prevents the oxidative decomposition of hemocyanin, was also positively selected or expanded. Glutathione reductase encoded by the GSRgene can effectively maintain reduced glutathione in cells, which plays an important role in preventing the oxidative decomposition of hemocyanin (Kanzok et al., 2001). In conclusion, the maintenance of hemocyanin level and oxygen-carrying capacity will help C. japonica cope with hypoxia in drought environment.