Mg-calcite, a carbonate mineral, constitutes Ca precipitates produced as a byproduct of osmoregulation in the intestine of seawater-acclimated Japanese eel Anguilla japonica

Marine teleosts are known to produce white feces, which is often referred to as Ca precipitates. Ca precipitates have been suggested to be a product of osmoregulation. In the present study, we examined the physicochemical nature of Ca precipitates, and possible involvement of Ca precipitate formation in hyposmoregulatory processes in seawater-acclimated Japanese eel. Whereas Ca precipitates were not produced in eel acclimated to freshwater, Ca precipitates were seen in eel acclimated to seawater in a salinity-dependent manner. According to X-ray diffraction analysis, Ca precipitates were a mixture of carbonate minerals: Mg-calcite and its amorphia. Quantitative analysis showed that the molar ratio between Ca and Mg was approximately 7:2. Ca precipitate formation was reduced in eel exposed to low-Ca(2+) or low-Mg(2+) seawater, indicating that Ca and Mg in Ca precipitates were derived from seawater.