GILL STRUCTURE OF A FISH FROM AN ALKALINE LAKE - EFFECT OF SHORT-TERMEXPOSURE TO NEUTRAL CONDITIONS

The morphology and morphometry of the gills of Oreochromis alcalicus g rahami, a unique ureogenic teleost that lives in the alkaline environm ent of Lake Magadi, Kenya (pH 10, Cco(2) = 180 mmol . L(-1), temperatu re 30-40 degrees C) were examined by transmission electron, scanning e lectron and light microscopy. Fish were examined in normal Lake Magadi water and 2-3 or 24 h after transfer to Lake Magadi water neutralized to pH 7 with HCl (i.e., CO32- replaced with Cl-), a treatment that ca used severe reductions in urea excretion and O-2 uptake, internal acid osis, and ionoregulatory disturbance. In Lake Magadi water, the organi zation of the filament epithelium of the gill was similar to that of s eawater teleosts. Indeed, chloride cells were located at the bottom of pits bordered by overlying pavement cells and flanked by typical acce ssory cells. Total numbers of chloride cells remained unchanged after transfer to pH 7, but after 2-3 h, many were covered by pavement cells , restricting their communication with the external milieu. At 24 h, t his trend was reversed, an observation indicative of a reactivation of chloride cells. Mucous cells were located at maximum density on the t railing edge of the filament; most of them were empty after 24 h at pH 7. The harmonic mean thickness of the lamellar epithelium (blood-to-w ater diffusion pathway) was very small and not altered by acute or lon ger term exposure to pH 7. A model of alterations in ion and acid-base transport accompanying the morphological changes is presented.