How does the function of kidneys in saltwater fishes differ from their freshwater fishes?

Kidneys play a crucial role in maintaining the water and salt balance in the body of fishes, but the specific functions of kidneys differ between saltwater and freshwater fishes due to the different osmotic challenges they face.

Saltwater Fishes:

1. Osmoregulation: Saltwater fishes live in a hypertonic environment, meaning the salt concentration is higher outside their bodies than inside. To prevent water loss, their kidneys produce relatively less urine, concentrating it to conserve water and retain essential solutes.

2. Salt Excretion: Kidneys of saltwater fishes are specialized in actively excreting excess salt ingested from seawater. They have specialized cells in the renal tubules that actively transport salt from the blood back into the urine.

3. Ion Transport: Saltwater fishes have mechanisms to regulate the uptake of specific ions, such as sodium and chloride, from the surrounding water to maintain proper ionic balance. The kidneys play a role in this regulation by controlling the reabsorption of these ions.

4. Nitrogenous Waste Excretion: Like freshwater fishes, saltwater fishes also excrete nitrogenous wastes like ammonia, urea, and uric acid. However, they might have higher levels of urea and lower levels of ammonia in their urine compared to freshwater fishes.

Freshwater Fishes:

1. Osmoregulation: Freshwater fishes live in a hypotonic environment, meaning the salt concentration is lower outside their bodies than inside. To prevent water uptake by osmosis, their kidneys produce large amounts of dilute urine, excreting excess water while retaining essential solutes.

2. Ion Absorption: Kidneys of freshwater fishes are adapted to absorb essential ions, such as sodium and chloride, from the water to compensate for the loss of ions through the gills and urine.

3. Nitrogenous Waste Excretion: Freshwater fishes primarily excrete ammonia as the main nitrogenous waste product. Their kidneys play a role in the efficient excretion of ammonia to minimize the toxic effects of this compound.

4. Osmoregulatory Hormones: Some freshwater fishes have specialized osmoregulatory hormones, such as prolactin, involved in regulating water reabsorption in the kidneys to maintain water balance.

In summary, while both saltwater and freshwater fishes rely on their kidneys for osmoregulation and excretion, the specific functions of their kidneys differ due to the distinct osmotic challenges they face in their respective habitats. Saltwater fishes focus on conserving water and excreting excess salt, while freshwater fishes focus on eliminating excess water and absorbing essential ions from their surroundings.