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.