Fish live underwater, but do they ever get thirsty?

Everyone knows that fish live in water, but do they ever drink water? Is this question too easy? The truth is not as simple as that.

I bet that both you and I have never questioned this issue before reading this article. I always thought that every fish needed to drink water until my 4-year-old nephew asked me whether fish needed to drink water like humans.

Of course, as an uncle, I had to find the answer for him. So, do fish drink water?

The short answer is yes. But it depends on which type of fish you are referring to. Essentially, freshwater fish do not drink water because it would dilute their blood too much. On the other hand, saltwater fish drink a lot of water to retain moisture in their bodies.

Fish do drink water.

Do Fish Feel Thirst?

While discussing whether fish drink water, I find myself wondering if fish feel thirsty. For us humans, we only drink when we feel thirsty. Thirst is a signal from our body indicating that we need to replenish our water intake.

However, the issue becomes a bit more complicated for fish.

Fish live their entire lives in water, so how can a creature living in a water environment feel thirsty? Do you feel hungry when your home is filled with food?

However!

The answer is not that simple. Freshwater and saltwater fish have different biological anatomies, leading to different behaviors in drinking water.

Fish do not need to feel thirsty in order to drink water. Drinking water is merely a reflex that occurs without any decision from their body. Therefore, it can be said that fish never actually feel thirsty.

Freshwater Fish Drinking Mechanism

In fact, the salt concentration in freshwater fish blood is much higher than that of their surrounding water. Therefore, if freshwater fish were to drink water, they would face a high risk of blood dilution. That is why freshwater fish do not drink water.

Instead, they rely on osmosis. Freshwater fish absorb water through their gills and skin. Additionally, they excrete much more diluted urine to eliminate excess water from their bodies.

Saltwater Fish Drinking Mechanism

Compared to their surrounding environment, saltwater fish have a much lower blood concentration. Therefore, they constantly face the risk of dehydration.

This is why saltwater fish need to actively drink water. Thanks to a special gill structure, saltwater fish can drink seawater, process it, and excrete all excess salt.

What About Fish That Can Live in Both Salt and Fresh Water?

Fish that can thrive in both environments are not very numerous. A prime example is salmon, which can migrate from saltwater to freshwater.

So, do they need to drink water?

How can they survive in two completely different environments?

Salmon fry are born in freshwater and must undergo three significant transformations before leaving their birthplace to return to the ocean.

First, they will drink a lot of water.

Next, their kidneys will reduce urine output.

And most importantly, the mechanisms in their gills will start to work in reverse, excreting instead of absorbing salt from the water.

When adult salmon return to freshwater to spawn, they spend a few days in an intermediary environment, also known as intertidal zones (coastal areas or places with exposed seabed during low tide). Here, their bodies will reverse the entire transformation process they went through as juveniles to survive in freshwater.

Thus, the answer is that fish capable of living in both environments do drink water.

Do Other Aquatic Creatures Drink Water?

Of course, fish are not the only creatures living in water. What about their aquatic neighbors? Do whales, dolphins, or turtles need to drink water?

We all know that these creatures, like all other types, need water. The question to clarify here is whether they drink water from their surrounding environment.

Do Whales Drink Water?

Whales supplement their bodies with water by drinking from their surroundings or absorbing it from their food.

Whales are classified as aquatic mammals. They need to supplement their bodies with water by drinking from their surroundings or absorbing it from their food. Whales have very large kidneys that can excrete excess salt without losing too much water.

Additionally, an interesting fact about whales is that they do not sweat and do not lose water when breathing. For humans, breathing can result in significant water loss. The reason for this difference is that the air above the ocean is saturated with moisture, so whales do not lose water when breathing.

Do Dolphins Drink Water?

Similar to whales, dolphins are also aquatic mammals. There is a hypothesis that dolphins absorb water from their food. Squid, fish, and octopuses contain a lot of water in their bodies, and dolphins will absorb water from their prey. This method is much more effective than drinking seawater directly.

There is also a hypothesis that when dolphins catch prey, they may also drink a small amount of surrounding water. Furthermore, dolphins have a very advanced filtration system. This remarkable mechanism allows them to easily extract excess salt from urine to help them separate salt from the seawater they consume.

Do Turtles Drink Water?

Turtles are reptiles that can live both on land and in water. However, because turtles spend most of their time on land, their bodies require sufficient hydration. Therefore, it can be concluded that turtles do drink water.

Turtles primarily drink water when they swim or wade in water. Some marine turtle species that primarily live in saltwater need to filter out salt from the water they drink. To achieve this, marine turtles have a special gland located behind their eye sockets, known as the “salt gland.”

When marine turtles drink saltwater, the salt is transferred into their bloodstream and carried through their blood vessels to the salt glands. The salt glands then produce a solution that is twice as salty as the water they drank and excrete it through the corners of their eyes.

Since turtles drink water while swimming, those who keep pet turtles need to regularly clean their tanks and replace the water. In the wild, animal waste is handled by other organisms. However, in a closed captive environment, waste will settle at the bottom of the tank and accumulate. Therefore, caretakers need to clean and replace the water frequently to minimize the risk of illness for their turtles.

What Happens When Aquatic Mammals Drink Too Much Salt Water?

This issue is quite complex. If aquatic mammals absorb too much salt, they will begin to experience some dangerous symptoms such as kidney failure, liver failure, and other locomotion problems.

The reason is that aquatic mammals need to filter and eliminate excess salt when drinking seawater, helping them avoid internal organ damage caused by excess salt.

Do Fish “Pee”?

Everyone knows that fish “defecate.” The main evidence is the waste that accumulates at the bottom of the fish tank if you do not clean it regularly. But what about “peeing”?

If you stop cleaning the fish tank, eventually it will start to smell like ammonia. So the answer is quite clear. Yes, fish do “pee.”

However, different fish species have different ways of “going light.” But basically, fish “go light” continuously.

How do fish “go light”?

Most fish species “go light” through their urinary openings or gills.

Most fish species “go light” through their urinary openings or gills. Those that “go light” through gills excrete urine filtered by the kidneys. On the other hand, saltwater fish “go light” through their gills while freshwater fish use their urinary openings.

As mentioned earlier, saltwater fish live in an environment with a higher salt concentration than their bodies. Therefore, saltwater fish absorb a large amount of salt from their surroundings and need to eliminate this excess salt from their bodies. In contrast, freshwater fish need to get rid of excess water in their bodies.

The kidneys of fish

All fish species have two kidneys. The posterior kidneys filter waste through the urinary openings, while the anterior kidneys filter waste through the gills.

For saltwater fish, the kidneys excrete ions and retain water, mostly eliminating waste through the gills. For freshwater fish, their kidneys work in the opposite way compared to saltwater fish. They eliminate water and retain ions.

Fish urine

The main component of fish urine is ammonia. Ammonia exits the fish’s body through the gills, and again, this depends on the type of fish we are discussing.

Furthermore, fish urine contains organic acids like creatine and creatinine, amino acids, and a small amount of urea. Freshwater fish urine is mainly water, while saltwater fish urine consists mostly of salt.

Saltwater fish do not produce much urine, but it is extremely concentrated. In contrast, freshwater fish produce a large amount of urine.

What color is fish urine?

Fish urine is very different from that of humans. Fish urine is usually black or gray.

Types of kidney and urinary tract diseases in fish

Fish can also suffer from urinary tract and kidney diseases.

Like us, fish can also suffer from urinary tract and kidney diseases. These diseases often occur in fish due to a parasite called Renal Dropsy. This type of parasite commonly affects carp and goldfish. Infected fish often exhibit swollen abdomens due to fluid accumulation when the kidneys cease to function.

Another parasite called Sphaerospora angulate also frequently causes kidney failure in carp species. This parasite causes the abdomen and eyes of the host to swell and will not allow them to live beyond six months.

Additionally, farmed juvenile salmon and adult salmon often suffer from kidney hypertrophy. Symptoms of this disease include the fish becoming lethargic and unusually slow.

Fish use urine to show status

According to some studies, fish “go light” on other fish when they want to assert dominance and let others know who is in charge.

What have we learned?

Of course, fish live underwater. Some species inhabit saltwater environments, while others live in freshwater.

Although some fish species can survive for several days out of water, most will die if they cannot return to a water environment afterward. This is because their lungs are structured differently from those of mammals. When mammals breathe, their lungs are filled with air, and specialized cells filter the oxygen from that air. During this process, the lungs become completely dry and do not contain any fluid. In contrast, fish draw water into their mouths and use their gills to filter oxygen from the water.

Moreover, all animals require water to sustain life, so they must drink or absorb water, including those that live underwater. How aquatic species supplement the necessary water depends on the water environment they inhabit and each species’ biological mechanisms.

Saltwater organisms need to maintain a balance of salt concentration in their bodies due to their high-salinity environments. Therefore, their bodies have adapted to excrete large amounts of excess salt. Species living in freshwater do not face this issue but must ensure their salt concentration remains at safe levels. Aquatic mammals have mechanisms to keep their lungs dry while maintaining safe levels of water and salt in their bodies.

Other questions

Can you use tap water for Betta fish (fighting fish)? Yes, you absolutely can. You should not use distilled water as it does not contain enough oxygen and essential minerals for healthy fish survival. In fact, tap water is the best option. You should let the tap water “breathe” for about a day to allow chlorine to evaporate, then let it return to room temperature. Additionally, you can use other water treatment products.

Do fish cry? Fish do not cry like humans. They lack the cerebral cortex and nervous system to produce emotional responses like humans. However, scientists have discovered that they do “cry,” but only through sound.

Do fish get bored? The answer is yes, especially goldfish. Signs of boredom can be mistaken for symptoms of oxygen deficiency, or many assume they are begging for food. Goldfish rarely beg for food, and when oxygen is low, they will swim close to the water’s surface. You can keep another fish or plant some aquatic plants in the tank to make it happier.

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