Comparative studies of the development and differentiation of chloride cells in tilapine fish with different reproductive styles.

Using light and electron microscopy and fluorescent probes, we followed the ontogenesis of selected organs in embryos of several species of tilapia (Cichlidae, Pisces) with emphasis on chloride cell differentiation in species with two different reproductive styles: we compared the substrate-brooder Tilapia zillii and the mouth-brooders Oreochromis niloticus, O. aureus, Sarotherodon galilaeus, and Tristramella sacra. In all species a transitory blood network system nurtured by the vena caudalis inferiores supplied the yolk sac and preanal finfold during the advanced stages of embryonic and initial stages of larval development. During these stages chloride cells occurred on the yolk sac, as a part of the abdominal epithelium. The cells and their associated blood plexus remained active here until the gill-lamellae, operculum, and mouth became functional. The chloride cells of their epithelium and blood system then took over, concomitant with a gradual degradation of the transitory blood system on the yolk sac. Ontogenesis of these systems (transitory and permanent) progressed at a higher rate in substrate-brooders than in mouth-brooders and was correlated with the earlier functioning of the gill-operculum system. Thus, at a constant temperature of 26 degrees C, the more exposed T. zillii progeny completed metamorphosis at 7-8 days after fertilization, calculated around 5,000 +/- 80 h/temp, whereas juveniles of more protected mouth-brooders attained a similar stage only 15 +/- 1 days after fertilization and around 9,000 +/- 200 h/temp. This earlier development of chloride cells and other pivotal organs in environmentally exposed progeny of substrate-brooders, as compared to the protected progeny of mouth-brooders, shows that their ontogeny was selected for the optimal survival style under specific etho-ecological conditions.