Environmental calcium regulates gill remodeling in a euryhaline teleost fish

Some cyprindid and cyprinidontiform fishes undergo gill remodeling via the proliferation or regression of an interlamellar cell mass (ILCM), resulting in the modification of gill surface area in response to environmental hypoxia or ion levels. We hypothesized that ion-related gill remodeling is regulated by water hardness through the interactions of Ca2+ with tight junctions, predicting that gills will exhibit a lower ILCM and more surface area in a high Ca2+ environment than in a low Ca2+ environment. To test this hypothesis, we acclimated euryhaline mangrove rivulus (Kryptolebias marmoratus) to natural hard water ([Ca2+] = 2.77 mmol/L), low Ca2+ ([Ca2+] = 0.13 mmol/L) freshwater, or high Ca2+ water (5.88 mmol/L). Fish exposed to hard water had a significantly lower ILCM height than fish exposed to low Ca2+ water. The addition of Ca2+ to low Ca2+ water restored gill surface area. Plasma Ca2+ activity was not significantly different between groups. This study provides support for an influence of external Ca2+ on gill remodeling and represents the first evidence of an ionic trigger (Ca2+) for gill remodeling in teleost fishes.