Modification of morphology and function of integument mitochondria-rich cells in tilapia larvae (Oreochromis mossambicus) acclimated to ambient chloride levels

Similar to those of the gills of adults, three types of mitochondria-rich (MR) cells with different morphologies of apical surfaces (wavy convex, shallow basin, and deep hole) were identified on the integument of freshwater-acclimated tilapia larvae (Oreochromis mossambicus). The object of this study is to test the hypothesis that these subtype cells may represent MR cells equipped with variable efficiencies in Cl^- uptake. Larvae acclimated to low-Cl^- ([Cl^-] = 0.001-0.007 mM) water developed higher densities of MR cells than those acclimated to high-C1^- ([C1^-] = 7.3-7.9 mM) water. The percentage of wavy-convex-type cells in total MR cells was higher in low-Cl^--acclimated larvae than in high-Cl^--acclimated larvae, which displayed only deep-hole type. In addition, Cl^- influx rates of whole larva measured with ³⁶Cl^- showed a coincident correlation with MR cell densities, that is, low-Cl^- larvae displayed higher Cl^- influx rates than did high-Cl^- larva, suggesting that tilapia larvae develop a higher density of MR cells with larger apical surfaces (wavy-convex type) to boost Cl^- uptake in Cl^--deficient water. The distinct types of apical surfaces may represent different phases of MR cells that possess different efficiencies of Cl^- uptake. Increased apical membrane surface areas of MR cells may provide larvae with rapid regulation of Cl^- before new MR cells differentiate.