Do Fish Have Kidneys? What Do They Use Them For?

Are you a fish enthusiast? Have you ever wondered what the fish’s anatomy would look like? Do you think they have kidneys? And if they do, what is it used for? No problem if there are many more questions running through your mind, you have your answers in this article. Read on!

Fish have kidneys! It is one of the essential organs you will find in their body cavity. They help in the excretion and regulation of water balance in the fish.

This regulation ensures  that the salt concentration in the fish’s body is different from its environment. Freshwater contains a lesser salt concentration than the fish’s body, while saltwater contains more salt concentration.

Do All Fish Have Kidneys?

Yes all fish have kidneys. The kidney is an essential organ in fish. Like in other vertebrates, the kidney is the primary excretory organ in fish. Also, all fish have kidneys and cannot do without them.

The kidney does a great job in the excretory system of fish. It also helps get rid of excess water in the fish body. While removing the water from the fish’s body regulates and balances salt concentration in its body.

It also helps in getting rid of other harmful waste in the fish’s body. It acts as a filter for getting rid of toxic substances and waste. It also helps return vitamins and other vital substances into the bloodstream. It also helps eliminate other simple waste in the fish’s body.

The kidney is so essential in fish that they cannot live without their kidneys. All fish have their kidneys to help in osmoregulation and some excretory functions. A fish with no kidney will be dead in minutes.

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How Many Kidneys Do Fish Have?

The kidney is an essential organ in pairs, as in other vertebrates. In addition to this, fish have two kidneys that help in the excretion of waste and water balance regulation. Also, you can find the kidney at the backbone’s sides in the body cavity.

Kidneys are highly essential in fish as they perform mighty osmoregulation and excretion of waste. The two kidneys work hand-in-hand to ensure the existence of the fish. Fish also undergo degeneration of kidneys, where there is the addition of nephrons at basal rate.

As the fish grows in size, nephrons get added, thereby increasing the kidney’s size. It also occurs when there is an acute injury on the kidney, where the formation and addition of nephrons become increased.

As fish grow, they form their first kidneys called the pronephros and mesonephros. The pronephros regenerates and becomes the mesonephros, which is the fish’s permanent adult kidney. Furthermore, the kidney is highly functional in fish and performs a great job supporting their existence.

What Does the Kidney Do in Fish?

The kidney in a fish performs two main functions; regulation of water balance and excretion. The regulation of water balance is the kidney’s major function in fish as there are organs that perform the excretory function. Although the kidney performs excretory functions, it does more osmoregulation.

As a regulator of water balance, the kidney interacts with the internal body fluids on their relationship with external water fluids. In addition to this, the kidney ensures the salt and water balances in the fish’s body are appropriate. The kidney in a freshwater fish performs slightly different functions than those in saltwater fish due to the salt concentration.

In freshwater fish, the kidney does an important job of preventing the excess loss of solute in the body. There is more salt concentration in the fish’s body than there is in the water.  Furthermore, the concentration causes more water to get into the fish body, causing the fish to lose much water to the environment while retaining the solute.

In saltwater fish, the kidney removes excess solute in the fish’s body. The fish’s environment has a higher salt concentration, causing the fish’s body to lose water to the environment through osmosis. It makes the fish consume more water to replenish the lost ones.

As it does this, it takes in more salt solute. The kidney helps get rid of this solute by removing the excess salt from the fish’s body as concentrated urine. This way, the kidney performs its osmoregulation and excretory functions as much as it can.

Do Fish Get Thirsty?

You might be surprised that fish do not get thirsty. The water flows from the environment into their body system through osmosis. It allows them to have enough water in their body.

As much as this act can be beneficial, it also has its disadvantages. Having too much water causes them to pee more often to get rid of the excess water in them. They also tend to get harmful substances into their body during the water intake process.

This substance can cause lethal damage to the fish if not removed. The kidney does a good job of getting rid of these harmful substances from the fish’s body. It removes them through urine.

Does a Fish Pee?

Like humans, fish have kidneys that help them make urine. However, if the fish do not pee, the urine gets stuck in their body, causing harm to them. Fish need to pee, and they do pee to get rid of their bodies’ urine.

Some fish pee through a small opening called a pore. The pore is a tiny opening found near the fish’s rear end. In addition to this, some other fish use their skin or gills to eliminate their waste.

Whatever way they do it, fish do pee, and they do it because it is necessary. Peeing in fish might not be very visible as the urine misses with the water when it is out. But, you can rest assured your fish does pee.


Fish have kidneys that perform special functions. These kidneys help keep the fish’s water balance at the appropriate level. In addition to this, the kidney also does a good job in getting rid of waste from the fish’s body.

John Brandon

John has kept fish all his life (since he was about 5). He started with keeping guppies and fell in love with fish keeping almost straight away. That was 40 odd years ago. These days John still keeps fish and currently has two large tanks where he keeps many different types of fish such as Angelfish, Neon Tetras, Goldfish, Guppies and many more.