Do Fish Drink Water?

Oceans take up the largest areas of planet earth, as well as house over a million aquatic species. But, how do these animals drink if they live in water? Humans, plants and most animals need to consume freshwater with a maximum concentration of 2% salt in order to survive. If the salt percentage in water is too high, the body will expel a greater amount of liquids, which could lead to severe dehydration or even death. So, if saltwater animals do not have access to fresh water, how do they stay hydrated?

Sea fish drink water constantly. To survive and adapt to the marine environment, the animals that inhabit its waters require a balance between the water salinity and that present in their own bodies. Simple and ancient organisms such as anemones, sponges or sea urchins have equal amounts of saline in their bodies to that found in water. However, this is not the case with all fish species, such as teleosts.

Teleosts are the largest infraclass of fish, characterized for having bone vertebrae, a tail, scales and a swim bladder (gas bladder). Most fish species belong to the Teleost infraclass. Another one of their most relevant characteristics is that the concentration of salt in their bodies is lower than that existing in the sea. These fish then drink water to regulate salt levels in the body and avoid dehydration, a completely normal phenomenon whereby they lose water through the skin.

This essential regulation mechanism is called "osmosis" and serves to retain adsorbed salts when drinking. Additionally, these fish urinate in small quantities, with the aim of not losing this retained salt. In this way, when the fish drink water, leftover salts which will not be used in a fish’s body, are secreted through cells present in their gills.

This mechanism is regulated by a structure present in the kidneys, also known as the renal corpuscle, which is responsible for performing necessary filtrations. This then allows fish to take advantage of the components needed, while discarding those that are not.

Urinating is a completely normal and essential organic function in these animals. The amounts and concentrations of the elements, however, usually vary according to the species and the characteristics of the water in which they live.

For more, you may also be interested in our article where we discuss how do fish breathe?

You may also be wondering, do betta fish drink water or do goldfish drink water? As with marine fish, river or freshwater fish also drink water, but in noticeably smaller quantities. In this case, these fish have a higher concentration of salts in the body than that of their environment.

In this case, when the fish drink water, it forces a loss of salt in the body. To control this, they use two mechanisms: the first is found on the outside of the body, whereby the scales and mucous substance that covers the organism limits the access of water to the organism. The second is in the kidneys, since these organs process excess water and expel it in large quantities, but in lower salt concentrations. In fact, freshwater fish are among the animals that urinate the most. In addition, in some cases, these fish avoid drinking entirely.

Now that you know that fish drink water, you may be wondering, what about other marine species, such as dolphins? These marine animals, known as cetaceans, have low amounts of salt in the body, in the same way as terrestrial animals. For this reason, drinking salt water is counterproductive for them. However, dolphins still drink water, although moderately.

But in addition, it is essential to note that dolphin feeding contains foods rich in salt, so how do they avoid a high salt concentrations in the body? Dolphins have a reniculate kidney, which is an organ responsible for processing salts, almost expelling them entirely through urine. Because of this, dolphin urine may have salt concentrations higher than in seawater. The same goes for other sea mammals, such as the sea lion.

For more, read about how do dolphins breathe underwater.

As for sharks, their organisms have salt concentrations very similar to those present in their environment. Do whales drink water and/ do sharks drink water? Only in small quantities. Most of the time, sharks drink water unconsciously while feeding, and excess salt secretes through the saline gland, located in a shark’s rectum.

All sharks are saltwater animals, but the bull shark is able to migrate to the freshwater rivers at certain times of the year. When this happens, their body filters lower amounts of salt and more urea, so they are able to withstand the low saline lacking in fresh water. This, in addition to being able to absorb a greater amount of water without the need to recover these salts.

Now that you know that fish drink water, how do they sleep, if they sleep at all?

Fish take short breaks, of a few minutes, in which their activity ceases or is significantly reduced. They do not respond to mild stimuli and reduce their metabolic rate and heart rate. Sometimes they take refuge near rocks, corals or algae to be protected. These are just some of the characteristics that explain how fish sleep, but there are many more.

For more, read, how do fish sleep?

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