Do Fish Urinate?

Yes they do!  But why?

Well, like most living things, fish too produce waste from their metabolic processes.   Peeing is one way of doing that and is referred to as excretion.  Elimination on the other hand refers to pooping, i.e. the removal of solid waste comprising undigested foods and digestive by-products.  How much they pee depends on the environment they live in and the adaptations it has undergone for survival.

Marine fish drink a lot while freshwater fish pee a lot. Why?

Freshwater Fish and Marine Fish

The difference between these two types of fish lies in the salinity of the fish’s environment and the inside of its body. Since the fish’s skin is so thin, especially around places like the gills, external water constantly tries to invade the fish’s body by osmosis and diffusion.

Image credit: Indiana University-Purdue University Indianapolis Earth Sciences

Maintaining the Balance

Osmosis takes place when two solutions are separated by a partially permeable membrane. A partially permeable membrane lets some particles through but not others. Cell membranes are partially permeable. Water can move freely through them but other particles, such as sugar molecules, cannot.

Osmoregulation is the process of maintaining an internal balance of salt and water in a fish’s body and is carried out with the help of the kidneys.  Do bear in mind that a fish is a collection of fluids floating in a fluid environment, with only a thin skin to separate the two.  

Role of the Kidneys

The kidneys are slightly different from mammals. They are smaller, stretched and do not resemble beans. Generally, within fish, the kidney is located towards the very middle of the fish both lengthwise and height wise.

Freshwater Fish
Body is more saline than the surrounding water and constantly gains water from the environment

• The kidney prevents excess solute loss as they contain more salts within their blood than the water located around their bodies. 

• Water will diffuse into the fish through osmosis causing large amounts of water to build up inside. 

• The kidney will then increase the amount of water that passes out in the urine and actively reabsorb the salts that would pass out to maintain balance. 

• This results in the production and excretion of large amounts of dilute urine - almost completely composed of water.

Marine Fish
Body is less saline than the surrounding water and constantly loses water to the environment.

• The function of the kidney here is almost opposite to that of the freshwater fish as the water and liquid around them is highly concentrated with salts and other solutes. 

• Water in the fish is constantly being 'sucked out' into the surrounding water through osmosis. This is because the fluids inside their bodies are less concentrated than the saltwater around their bodies. 

• The fish must counteract this process through drinking water and the use of their kidneys. 

• By constantly taking in water, the fish is constantly replacing water that is sucked out of it however, this can lead to an extreme intake of salts and buildup of harmful substances within the fish. 

• These salts are removed through the use of the kidneys whereby they help the fish retain water and actively excrete salts to produce a very concentrated urine. This also means that marine fish do not urinate as much as freshwater fish.

Shared by Azni

Learning Specialist, Petrosains