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-Cl- ([Cl-] = 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 Cl-36(-) 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.
