Why do fish feel thirsty in water?

Drink water when you are thirsty. This is an instinct in human life.

From the moment babies are born, they feel hungry and thirsty.

Not only humans, but other animals also feel thirsty.

For example, the big yellow dog with its tongue hanging out at the entrance of the village;

A camel panting in the desert;

A leopard lying on the grass, looking at its prey;

They all feel thirsty.

The feeling of thirst is due to the loss of water in the body which needs to be replenished.

So will a fish that soaks in water all day feel "thirsty"?

As we understand it, fish live in water, so they shouldn't feel thirsty...right?

But in fact, fish also get "thirsty".

This is related to the osmotic pressure of the fish's body fluids.

However, not all fish are "thirsty".

We should all know that different species of fish have different growth environments.

Fish can be divided into freshwater fish and marine fish. Freshwater fish will never feel "thirsty" throughout their lives, while marine fish are often "thirsty".

The secret lies in the fact that the main components of fish body fluids are salt and protein.

Different water bodies have different salinity contents.

For fish living in fresh water, the water concentration is much lower than that of the fish's body fluids, and the osmotic pressure is also high.

The characteristic of water is that it flows from places with low osmotic pressure to places with high osmotic pressure, so the water around the fish will continue to penetrate into the fish's body.

In addition, fish can also ingest some water while swallowing bait.

If the excess water is not excreted from the body through the excretory organs, the body will swell and die.

Therefore, freshwater fish not only do not drink water, but need to constantly excrete excess water in the body in the form of urine to ensure the osmotic pressure balance in the body.

The situation with marine fish is quite different.

The reason is that the salt content of seawater is much higher than the salt content in the body tissues of marine fish.

The osmotic pressure of seawater is greater than the osmotic pressure in their bodies.

Under such pressure, water in the bodies of marine bony fish will continuously seep out through the body surface (especially the large surface of the gills).

Therefore, marine bony fish must constantly drink water to replenish lost water.

However, Grandpa Marx told us that if we want to find the truth, we must analyze specific issues specifically, so we cannot kill all the sea fish with one blow.

Marine fish do need to drink water, but not all fish living in seawater are "thirsty".

"Outliers" like cartilaginous fish such as sharks and rays do not drink water.

Because they have their own unique way of survival - storing urea in their blood to adapt themselves to life in seawater.

Urea is indeed a harmful substance to most organisms.

Most organisms would quickly excrete it from the body, but sharks and rays have a special mucous membrane that prevents urea from seeping out.

Under the action of urea, the osmotic pressure in the blood of sharks and rays is much higher than that of seawater.

Therefore, they use the same method as freshwater fish to extract the water they need for their bodies.

So, some fish do feel thirsty, but it depends on the species.