Activity of the pituitary gland in embryo and larval stages of coho salmon, Oncorhynchus kisutch

Embryos of coho salmon appear to regulate their internal osmotic environment actively before hatching (Weisbart 1968). However, at the time of hatching the pituitary contains only small numbers of differentiated cell types and it is only within the 1st week after hatching that a rudimentary follicular rostral pars distalis containing active eta (prolactin) acidophils becomes apparent. This suggests that prolactin, which in adult salmonids appears to be involved in freshwater osmo(iono)regulation, may not be involved in the same way in the early larval stages.Late-stage coho alevins can withstand ambient salinities up to 65% seawater for a period of 3 to 4 days. Under these conditions the prolactin cells become less active than in similar fish acclimated to distilled water or well water. The data indicate that the endocrine regulation of freshwater osmo(iono)regulation in late alevins and early fry stages may be similar to that found in adults. The evidence for the involvement of other pituitary factors in the hydromineral regulation of coho fry in hypoosmotic or hyperosmotic environs is inconclusive.Morphological studies of the chorion of the egg and of the yolk sac of the embryo and alevin suggest that these organs are not actively involved in hydromineral regulation.