Changes in pituitary and plasma levels of MSH in teleosts during physiological colour change

Immunoreactive alpha MSH was measured in the plasma of eels, trout, and flounder, following different periods of adaptation to illuminated white or black backgrounds. In both eels and trout, plasma hormone titres changed rapidly in response to the background colour. By contrast, in the flounder, whose pituitary melanotrophs appeared cytologically unresponsive to background change, plasma hormone levels remained similar for fish from black or white tanks following a 3-week adaptation period, but a difference became apparent after nearly 7 weeks adaptation. Similarly, bioactive and immunoreactive levels of MSH in trout pituitary glands usually showed significant changes with black ground colour, whereas flounder pituitary levels were unaltered. It is concluded that MSH is involved in physiological colour change in eels and trout, but probably not in flounders.