Ontogenetic changes in prolactin (PRL) production during larval and early juvenile periods of reared black sea bream were examined using an immunocytochemical technique. The onset of PRL production was observed at the end of the yolk-sac stage. Percent PRL (the percentage of hypophysis volume occupied by PRL-cell masses), cited as a quantitative index of PRL pro-duction, increased from about 10% at yolk-sac and preflexion stages to about 17% at early juvenile stage. PRL cells were activated and Percent PRL significantly increased up to 22% in low salinity (5‰) ambient water. These results suggest that PRL has an important osmoregulatory role in marine fish as previously shown in freshwater fish, this being one of the important physiological backgrounds enabling the larvae to inhabit nearshore low-salinity nurseries.
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Kimura et al. (1991) studied this question.