Understanding what fish excrete, and particularly the color of it, requires a look into the unique biological adaptations of aquatic life. Unlike terrestrial animals, fish live in an environment where water is constantly available, which fundamentally changes how they handle metabolic waste. This difference in biological strategy means that the common knowledge of mammalian excretion does not apply to the vast majority of fish species.
The Colorless Truth of Fish Waste
The simple answer to the question of color is that fish urine is generally colorless or clear. The recognizable yellow hue of human urine comes from a chemical called urobilin, which is a byproduct of the body breaking down old red blood cells. This process involves converting toxic ammonia into less harmful urea, which is then broken down to produce the yellow-pigmented urobilin that is excreted by the kidneys.
Fish follow a distinctly different pathway for dealing with their nitrogenous waste. Instead of converting toxic ammonia into urea, which requires energy, most bony fish simply excrete ammonia directly into the surrounding water. This process is highly energy-efficient because ammonia is extremely soluble in water and can easily diffuse across the gills. Since fish do not rely on the urobilin pathway to eliminate nitrogenous waste, the pigment that causes the yellow color in mammals is absent from their urine.
The Dual Excretory System of Fish
The unique way fish excrete waste is split between two primary mechanisms: the gills and the kidneys. The vast majority of toxic nitrogenous waste, typically around 90%, is expelled from the body through the gills. This ammonia easily passes from the fish’s bloodstream into the water as ammonium ions, making the water itself the main vehicle for nitrogen waste removal.
The kidneys in fish, therefore, serve a different primary function than they do in mammals. Instead of being the main organ for removing nitrogenous waste, the fish kidney’s main role is osmoregulation, which is the control of the body’s water and salt balance. The “pee” that is produced by the kidney is mostly water, salts, and other metabolic byproducts, with only a small portion of the total nitrogenous waste. This fluid is filtered through the kidney tubules and is crucial for maintaining the fish’s internal equilibrium.
How Environment Affects Urine Volume
The aquatic environment a fish inhabits dictates the volume and concentration of the urine produced by its kidneys.
Freshwater Fish
Freshwater fish live in a hypotonic environment, meaning the water around them has a lower salt concentration than their internal fluids. Consequently, water constantly moves into the fish’s body through osmosis, which could cause them to swell. To counteract this constant influx, freshwater fish excrete large volumes of very dilute, watery urine. Their kidneys are adapted to filter blood quickly and reabsorb almost all the necessary salts before the fluid is passed as waste.
Marine Fish
Conversely, marine fish live in a hypertonic environment, where the surrounding saltwater is saltier than their internal fluids, causing them to constantly lose water. Marine fish must actively drink seawater to replace lost water, and their kidneys are adapted to conserve water. They produce very small amounts of highly concentrated urine that contains excess salts and minerals to help maintain their internal balance.