Mechanism of osmoregulatory system (osmoregulation) in Hilsa / Ilish ( tenualosa ilisha)

Degonto Islam

Department of Fisheries and Marine Science,

Noakhali Science and Technology University.

Hilsa experience, tolerant and adapted to the frequent fluctuation of salinity. In estuary, salinity fluctuation is frequently noticeable. So in term of time of long migration to the estuary, the river from ocean Hilsa needs to maintain its body water and electrolytes balance inside its body such  as example osmotic pressure inside the body. It changes it body mechanism according to the salinity of the water and maintenance of  internal balance between water and dissolve materials regardless of environmental condition.

The osmoregulatory mechanism is triggered by the antidiuretic and steroid hormone for maintaining the osmotic pressure. Cortisol, Arginine- Vasotocine play a major role in this mechanism prolactin also play major role to bring about necessary change. Cortisol hormone bring about necessary change when Hilsa are close to sea water in estuarine and prolactin hormone assistance when Hilsa close to fresh water during migration. Cortisol hormone increase uptake of ion and prolactin vice versa.

Osmoregulatory mechanism of Hilsa when salinity close to sea water:

When Hilsa fish stay in estuary with salinity close to sea water drinking rate increased. When deinking may amount to 8-10 ml/kg body weight of fish per hour. If salinity rate increase the rate of drinking higher and the urine production decreased, the urine production rate amounts to 5ml/kg body weight of fish per hour. But it is decreased to 1 ml/kg body weight of fish per hour.

 

Their Gill epithelial increase its permeability so that it can increase the loss of salt to keep ionic balance of the fish, but the ion exchange mechanism of gill epithelial become less effective . Cortisol hormone increase Na⁺K⁺-ATPase activity in the fill tissue, increased Na+ excretion, increased water permeability of the urinary bladder and uptake of ions and water in the gut. Function of Arginine-Vasotocin involbed in the reduction of urine production in sea water. It reduce the blood supply in glomeruli resulting the reduction of urine production.

Osmoregulatory Mechanism when salinity close to freshwater:

Osmoregulatory mechanism close to freshwater are just opposite to the salinity to sea water. The drinking rate may decrease, the urine production increase and the gill epithelium permeability will decrease and lower the losses of salt. The ion exchange mechanism become more effective. The adenophypophyseal hormone prolactine plays the major role. The function of prolactine is to decrease Na+ excretion, decrease Na+-K+-ATPase activity in gill tissue decrease the water re-absorption ability of iodine tubule and urinary bladder.