Changes in gene expression levels of somatolactin in the pituitary and morphology of gill mitochondria-rich cells in Mozambique tilapia after transfer to acidic freshwater (pH 3.5)

Mozambique tilapia, Oreochromis mossambicus, is easily acclimated to highly acidic water, and thus presents a useful model to unravel endocrine regulation of adaptation to acidic water in fish. We analyzed gene expression of somatolactin (sl), growth hormone (gh) and prolactin (prl), in the pituitary gland and size distribution of mitochondria-rich (MR) cells in the gills after transfer from normal freshwater (FW, pH 7.2) to acidified freshwater (AW, pH 3.5). Plasma osmolality drastically decreased until 2 days after transfer to AW, but had restored to normal after 1 week of acclimation, and this confirmed the excellent acid tolerance of tilapia. Expression levels of sl, gh and prl were all up-regulated during short-term exposure to AW. The expression of sl remained elevated up to 7 days after transfer; the expression of gh and prl was back to initial levels at that time. These findings point to an important and specific role of SL in adaptation to acid water in this tilapia, although temporal contribution of GH and PRL cannot be ruled out. The size distribution of branchial MR cells changed drastically during acclimation to AW. The mean MR cell size was 1.5-fold larger in the fish exposed to AW for 7 days compared to controls in FW. The gills and their MR cells are a likely site of important acid–base regulation, and SL may change ion-transport functions of MR cells to correct plasma osmotic balance disturbed by acid exposure.