4.4| Distinct response of C. dehaani to ambient minor cations
The response of C. dehaani to ambient minor cations is different from that of H. tridens and M. japonicus. First, under isosmotic conditions, the decrease of survival rate of C. dehaaniin 513.3 mmol/L NaCl was less drastic (Fig. 1). Consistent with this result, there was no significant difference in hemolymph Na+ and K+ concentrations between crabs bathed in 513.3 mmol/L NaCl and 513.3 mmol/L NaCl+MCK solutions (Fig. 3C, F). In addition, NKA expression was enhanced but less drastic compared to H. tridens and M. japonicus , and CAc expression was not activated in 513.3 mmol/L NaCl solution (Fig. 5). Furthermore, ambient minor cations did not affect the survival rate (Fig. 7C) and hemolymph Na+ and K+concentrations under hypo-osmotic conditions (Fig. 8C, G). These results strongly indicate that C. dehaani is less susceptive to ambient minor cations compared to H. tridens and M. japonicus.Compared to H. tridens and M. japonicus , C. dehaaniis adaptive to fresh water and habitats of this species is not restricted within estuaries but extends more upstream regions (Irawan and Kijima, 1993; Kobayashi, 2000). Susceptibility to ambient minor cations would be less adaptive for such species and lost in C. dehaani. Alternatively, the difference of sensitivity to ambient minor cations is due to phylogenetic difference of these species. Comparative studies using other fresh water–adaptive crabs, such as Eriocheir japonica and E. sinensis (Varunidae), will elucidate the relationship between sensitivity to ambient minor cations and adaptation to fresh water.