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Summary
Mugil cephalus and Mugil capito present a typical example of the influence of the outer medium on the biology and sexual behavior of fish. Fry are caught in the sea for stocking and rearing in freshwater ponds. In these specimens, prevented from seaward migration, development of the gonads is arrested (and the fish do not reproduce).
Histological changes and volumetric variations of the pituitary were studied. In freshwater landlocked fish, the rostral pars distalis increases to 42% of the total hypophysis. Hypertrophy of this region is correlated with the role of prolactin in osmoregulatory phenomena in hypotonic medium. Prolactin content of the pituitary was tested by the water drive assay (in Triturus vulgaris). The proximal pars distalis, where gonadotropin cells are localized, is restricted to a fine layer of basophil cells, very poor in LH, tested by Galli-Mainini bioassay. NLT (nucleus lateralis tuberis) is disorganized in fresh-water fish, especially among older specimens. Plasma thyroxine level is low (Blanquet test).
In seawater fish, the rostral pars distalis is reduced (27% to 8%), according to the species. The gonadotropin region increases to 37%, with distinct LH and FSH cells in maturing fish. NLT shows signs of increased activity during the spawning season. Plasma thyroxine level is higher.
There may be antagonism between prolactin and gonadotropin secretion, and the infertility of the fish in freshwater may be considered to be a consequence of prolactin cell hyperplasia.