Physiology of Finfish and Shellfish (2+1)

2.4.11. Factors controlling iono-osmoregulation in amphihaline fish

Till date, 2 hormones are found to be very important for the shift in osmoregulation from seawater to freshwater or vice versa. One is cortisol, a cortico- steroid hormone released from the adrenal cortex. Fish do not have adrenal glands; instead their corticoid tissues are dispersed in the head kidney region known as the interregnal which secretes cortisol. Inter-renalectomy (removal of the inter renal organ) seriously inhibits smoltification. Inter-renalectomised mature eels do not show silver colour and their transfer even to dilute seawater results in death. On the other hand, administration of cortisol allows them to survive in seawater. Cortisol therefore is an essential hormone to enable adaptation of these fishes to marine life. Cortisol also has a role in facilitating adaptation to seawater by increasing absorption of water in the intestine with concomitant selective exclusion of ions and by stimulating reabsorption of water from the urinary bladder.

The other important hormone is prolactin, but its requirement is for freshwater life. Prolactin induces the mucous secretion that covers gill filaments and the body surface, inhibiting salts from going out and water from entering. These are the 2 essential requirements for seawater fish as they live in a hypo-osmotic surrounding. Prolactin also regulates water and ion movement through the intestine and urinary tract. Prolactin therefore is very important in maintaining ion balance in freshwater. Besides cortisol and prolactin, thyroid hormone, growth hormone and insulin have also been shown to be involved in iono-omoregulation of amphihaline fish.