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Do Fish Drink Seawater? Do They Ever Get Thirsty?

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Water is our planet’s most abundant natural resource. It plays an important role, as all living things rely on water to survive. And without water, life as we know it wouldn’t exist. 

When humans and animals lack the water they need, they get thirsty. And like hunger, thirst is a need that must be satisfied. 

But have you ever wondered if underwater species, such as fish, need water just like we do? And do they even get thirsty and drink water?

Sounds quite bizarre, right? Since fish are always surrounded by water, we might be inclined to think otherwise. But what if I tell you that some fish drink seawater, while others do not?

Now you might be thinking about how that works. But before that, let’s talk about how fish drink water first.

Fish don’t necessarily drink water as how we, humans, drink a glass of water. Instead, fish absorb water through their gills and skin through a process called osmosis.

Osmosis is the movement of water (solvent) from low concentration to high concentration areas. Water moves through selectively permeable membranes of a living cell with small openings that let the water molecules pass through. Osmosis helps balance the concentrations in the two areas.

But do all fish only absorb water through osmosis?

To answer this question, we must consider the fish types and where they live. 

Freshwater fish live in water with a salinity or amount of dissolved salt present in the water, less than 1.05%. Examples of freshwater ecosystems are lakes, rivers, ponds, streams, bogs, springs, and wetlands.

Catfish: An example of freshwater fish

Freshwater fish do not drink water through their mouth to survive. They have a higher salt concentration in their blood and body tissue than in the water surrounding them. As a result, their blood will be overdiluted if they drink water, which could unbalance the salt-water balance in their bodies. Overdiluting is when a liquid is thinned down or weakened by mixing with water or other liquid.

So, freshwater fish consume small amounts of water into their bodies through their skin and gills (osmosis) and then pee the excess water and salt.

On the other hand, saltwater fish actively need to drink water through their mouths to keep hydrated. Saltwater fish, also called marine or sea fish, as the name suggests, live in seawater. 

Clownfish: An example of saltwater fish

These fish need to drink water because the salt concentration in the water they live in is higher than the salt concentration within their bodies. Saltwater fish lose water through osmosis. The water moves from their bodies, through their gills and skin, and out into the sea.

Because of this, saltwater fish run the risk of being dehydrated. That is why they drink seawater directly from their mouth since they’re constantly losing water this way. These fish have kidneys that pump excess salt into their pee to get rid of these bodies. Their gills also pump extra salt out into the sea.

Now let’s answer the other big question: Do fish ever get thirsty?

Since fish basically live in water, they don’t feel thirsty or have a strong urge to drink water. Therefore, they don’t consciously react to seek out and drink water. 

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