4| DISCUSSION
In this study, three species of euryhaline crabs, H. tridens ,M. japonicus , and C. dehaani were used. Habitats ofH. tridens and M. japonicus are restricted within estuaries, but C. dehaani is a species which invaded more upstream regions and is adaptive to fresh water (Irawan and Kijima, 1993; Kobayashi, 2000). These species belong to three different families: Varunidae (H. tridens ), Ocypodidae (M. japonicus ), and Sesarmidae (C. dehaani ), although H. tridens and C. dehaani belong to the same superfamily, Grapsoidea. The different responses to environmental water of these species are possibly due to the differences in the microenvironment of their habitats or phylogeny.
4.1| Effect of ambient minor cations in seawater on the survival of euryhaline crabs
It has been reported that not only salinity but also ionic composition of the environment significantly affects the survival of euryhaline crustaceans. For example, the mortality of the euryhaline prawn P. mondon reaches 100% during the first 48 h in 0.17% NaCl solution, although it remains low in diluted artificial seawater of the same salinity (Cawthorne et al., 1983), indicating that some ambient minor ions, besides Na+ and Cl, are essential for its survival. In addition, analysis of ionic profiles of inland well water for cultivation of prawns shows that the presence of K+, Mg2+, and SO42– increases the survival rate of euryhaline prawns L. vannamei and M. latisulcatus (Saoud et al., 2003; Davis et al., 2005; Prangnell and Fotedar, 2005, 2006; Roy et al., 2007). Furthermore, removal of either Ca2+ or Mg2+ or both, but not K+, from seawater causes lethal damage to the shore crab C. maenas , showing that both divalent cations, but not K+, are required for its survival (Lovett et al., 2006a).
This study revealed that both H. tridens and M. japonicuscan survive even in 8.6 mmol/L NaCl solution but not in 513.3 mmol/L NaCl solution. The high mortality of H. tridens and M. japonicus in 513.3 mmol/L NaCl solution indicates some ambient minor ions besides Na+ and Cl are required for these crabs to survive under isosmotic condition. For complete recovery of the survival rate in 513.3 mmol/L NaCl solution, both K+ and divalent cations, either Mg2+ or Ca2+, are required (Fig. 2). In addition, the required ambient minor ions for both H. tridensand M. japonicus are different compared to other euryhaline crustaceans previously examined: (1) SO42– is not needed for survival ofH. tridens and M. japonicus unlike L. vannamei(Saoud et al., 2003), (2) K+ and either Mg2+ or Ca2+ are essential for bothH. tridens and M. japonicus , although both Mg2+ and Ca2+, but not K+, are required for C. maenas (Lovett et al., 2006a), and (3) while P. mondon cannot survive 48 h in 0.17% NaCl solution (Cawthorne et al., 1983), both H. tridens andM. japonicus can survive in NaCl solution with similar salinity (32.1 mmol/L, corresponding to 0.19%). These data show that the required minor ions for survival differ among species of euryhaline crustaceans, and the osmotic condition itself in which the ambient minor ions are functioning also differ, respectively. This difference might, in turn, reflect the difference between the microenvironments of their habitats and/or phylogeny. On the other hand, C. dehaani has a significantly high survival rate, even in 513.3 mmol/L NaCl solution, in striking contrast to H. tridens and M. japonicas (Fig. 1). C. dehaani is more adaptive to lower salinity compared toH. tridens and M. japonicus (Irawan and Kijima, 1993), which would account for the robustness of this species in 513.3 mmol/L NaCl solution.