Fish possess kidneys, which are fundamental organs for maintaining the stability of their internal environment, a process known as homeostasis. Like in all vertebrates, the kidney’s primary function is to act as a complex filtration system for the blood, removing metabolic byproducts and managing the concentrations of water and dissolved salts. The way a fish’s kidney performs these tasks is uniquely adapted to its aquatic surroundings, differing significantly between species living in freshwater versus saltwater. The kidney works closely with the gills to regulate internal chemistry.
The Structure of Fish Kidneys
A fish’s kidneys are typically found as long, dark red organs situated high within the body cavity, running along the underside of the backbone. While they begin as paired structures during development, in many adult fish they fuse together, often appearing as a single, elongated mass. This organ is composed of two main functional regions: an anterior portion and a posterior portion.
The anterior section, often called the head kidney, primarily contains tissue related to the immune system and endocrine glands. The posterior section is the excretory part, where the filtering units called nephrons are concentrated. Each nephron consists of a renal corpuscle—which includes a glomerulus for initial filtration—and a renal tubule for modifying the filtered fluid. These basic components are present in all fish, though their size and number vary drastically depending on the fish’s environment.
How Fish Manage Metabolic Waste
The kidney’s role in waste management in fish differs from that of terrestrial animals due to the aquatic environment. The most abundant nitrogenous waste product from protein metabolism is ammonia, which is highly toxic. Instead of converting this ammonia into less toxic urea or uric acid, most bony fish simply excrete the majority of the ammonia directly into the water through their gills.
This means the kidney is not the primary organ for nitrogenous waste removal, though it still performs an important function. The kidney filters the blood to eliminate other metabolic byproducts, such as creatinine and small amounts of residual urea and ammonia. It also regulates the body’s acid-base balance by secreting compounds into the urine. While the gills handle the bulk of the ammonia, the kidney acts as a fine-tuning system to remove other toxins and maintain overall internal chemical equilibrium.
Maintaining Water and Salt Balance
The most specialized function of the fish kidney is osmoregulation: controlling the internal water and salt concentrations, which varies dramatically based on the surrounding water’s salinity.
Freshwater Fish
Freshwater fish face the constant physiological challenge of being hypertonic, meaning their body fluids are saltier than the water they swim in. Due to osmosis, water continuously diffuses across the semi-permeable membranes of the gills and skin into the fish’s body. If left unchecked, this water influx would cause the body cells to swell.
To counteract this, the kidneys of freshwater fish are highly developed for water expulsion. They possess large, numerous glomeruli, which are efficient filtration units that produce a continuous, high volume of filtrate. The resulting urine is extremely dilute, flushing excess water out of the body while minimizing the loss of valuable salts. The renal tubules actively work to reabsorb as many ions as possible before the urine is excreted.
Saltwater Fish
Marine fish, in contrast, are hypotonic, meaning their internal salt concentration is lower than the surrounding seawater. They face the opposite problem: a constant osmotic tendency to lose water from their bodies to the saltier environment, leading to dehydration. The kidneys of saltwater fish are adapted to conserve water, not expel it.
These kidneys feature smaller and significantly fewer glomeruli, which drastically reduces the rate of blood filtration and subsequent urine production. Some highly evolved marine teleosts have even lost the glomeruli entirely, resulting in an aglomerular kidney that produces almost no urine. The kidney’s main excretory role shifts to eliminating excess divalent ions, such as magnesium and sulfate, which are ingested when the fish drinks seawater to replace lost water. These ions are actively secreted into the long renal tubules, resulting in a very small volume of concentrated urine.