Pituitary responses to calcium deprivation in a euryhaline teleost, Fundulus heteroclitus, adapted to artificial sea water

Male killifish adapted to calcium-free sea water developed hypocalcemia. In the pituitary the PAS-positive cells of the pars intermedia (PIPAS cells) displayed a strong reaction, increasing in numbers and becoming enlarged and more active. A very minor response was shown by a small minority of the prolactin cells which became marginally activated. No other adenohypophysial cell type responded to calcium deprivation, and no response was detected in the neurohypophysis. It is suggested that the PIPAS cell may secrete a hypercalcemic hormone which plays a major role in calcium regulation in sea water.