1| INTRODUCTION
Dissolved ions in seawater play a crucial role in maintaining the
osmolality of body fluids of marine invertebrates. Na+and Cl– are the most abundant ions in seawater,
constituting ~85% of total ions, in addition to minor
ions such as SO42–,
Mg2+, Ca2+, and
K+. If dissolved ions are required only for
maintaining the osmolality of body fluids, minor ions should not be
essential and their replacement by Na+ and
Cl– should have no effect on the life of marine
invertebrates. However, the euryhaline prawn Penaeus mondoncannot survive in 0.17% NaCl solution, although it can survive in
diluted artificial seawater of the same salinity (Cawthorne et al.,
1983). In addition, the presence of K+,
Mg2+, and SO42–increases the survival rate of euryhaline prawns Litopenaeus
vannamei and Melicertus latisulcatus (formerly called P.
latisulcatus) (Saoud et al., 2003; Zhu et al., 2004; Davis et al.,
2005; Prangnell and Fotedar, 2005, 2006; Roy et al., 2007). Therefore,
minor ions are required for the survival of marine invertebrates, at
least for some euryhaline crustaceans. However, the mechanisms by which
minor ions affect the survival of marine organisms are unclear.
Most crustacean species inhabiting estuaries are osmoregulators; they
can regulate hemolymph osmotic and ionic concentrations in response to
ambient salinity changes to some extent. Typical osmoregulation in
estuarine crustaceans is hyper-isosmotic, with hyperregulation in low
salinity and isosmotic regulation in salinity close to or higher than
seawater (Charmantier et al., 2009; Lignot and Charmantier, 2015). Such
osmoregulatory pattern allows crustacean species in estuaries to be
euryhaline and tolerate a wider range of ambient salinity. Many studies
have investigated the mechanisms by which euryhaline crabs exposed to
fresh/brackish water maintain their hemolymph osmolality higher compared
to the environment, and chloride cells in posterior gills play an
important role in hemolymph osmotic and ionic regulation (Freire et al.,
2008; Henry et al., 2012). Chloride cells express
Na+/K+ ATPase (NKA) in the
basolateral membrane, which transports Na+ from the
cytoplasm to the hemolymph and maintains intracellular
Na+ concentration relatively low. Another key enzyme
in chloride cells is cytoplasmic carbonic anhydrase (CAc), which
catalyzes the formation of H+ and
HCO3– from H2O and
CO2, and the derived H+ supports
Na+ uptake through activation of the
Na+/H+ exchanger (NHE) located in
the apical membrane. Other molecules, such as V-type
H+ ATPase and
Na+/K+/2Cl–cotransporter (NKCC), are also involved in regulating salinity and ionic
composition of hemolymph (Freire et al., 2008; Charmantier et al., 2009;
Henry et al., 2012; Griffith, 2017). Transcriptome analysis has
identified numerous genes whose expression levels are modified by
changes in ambient salinity (Towle et al., 2011; Lv et al., 2013), and
these genes might also contribute to hemolymph osmotic and ionic
regulation. However, it is unknown at present what factor(s) in
environmental water are used as key signal of ambient salinity, and what
factor(s) induce changes of the gene expressions in the gills.
This study analyzed the effects of Mg2+,
Ca2+, and K+ (hereafter called
“ambient minor cations”) on survival, hemolymph ionic composition, and
gene expression in the gills of three euryhaline crabs: Helice
tridens , Macrophthalmus japonicus , and Chiromantes
dehaani. The different response to ambient minor cations between
euryhaline species could be due to the difference in the
microenvironment of their habitats or phylogeny.