Osmoregulation in Freshwater Fish

Fish live their lives completely surrounded by water on all sides. It surrounds them externally in their habitat and also comprises much of their body mass. Fish must therefore strike some sort of balance between these two separate, and very different, water environments that determine so much about their health. This balance is met through the processes of osmosis and osmoregulation.

Osmosis is a concept which involves the passive movement of water through a membrane which is permeable to the water, but not to the solutes dissolved in the water. With osmosis, the water moves from an area of lower solute concentration to an area of higher solute concentration.

Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. In freshwater fish, the inside of a fish’s body has a higher concentration of salt than the external environment. Consequently, there is a tendency to lose salt and absorb water. To combat this, freshwater fish have very efficient kidneys that excrete water quickly. They also reabsorb salt from their urine before it is ejected to minimize losses and actively take salt from their environment using special cells in the gills.

This constant expenditure of energy to maintain an osmotic balance is a reason why proper nutrition and low stress levels are important for healthy fish. Damage to the kidneys through bacterial infection or other means is often deadly as these organs extract large amounts of water which continually diffuse into the fish's body.

A sudden change in osmotic pressure can put great stress on the osmoregulatory system of a fish. Fish under acute stress produce catecholamines which then increase circulation to the gills to improve oxygen uptake. Simultaneously, the influx of water through the gills rises dramatically. This stress-related diuresis can quickly result in serious electrolyte imbalances due to the loss of chloride and other ions in the urine. Concurrently, hyperglycemia needed for a quick energy boost in a fight-or-flight situation is caused by catecholamine-mediated glycogenolysis of liver stores. If the stressful challenge persists, plasma cortisol levels increase and sustain the hyperglycemia via liver gluconeogenesis.

Freshwater fish need to have low-stress levels and proper nutrition. Fish under great stress are not able to regulate any osmotic pressure differences easily. This is the reason why acclimation should be done in the right way. When putting them in a salt bath, the concentration of salt should be increased gradually.

Important to understand osmosis and how it affects fish. It is a vital component of their well-being. The ability to control problems, safely ship and receive fish, and treat diseases will be enhanced if you pay attention to this aspect of their lives.

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