All living things on Earth need water. Us, land animals have to drink it to keep it inside of us all the time.
If you have a pet, one of your jobs might be to make sure that your pet has plenty of food and water. Unless, of course, your pet is a fish.
You have to feed your fish but you don’t have to give them water. There’s water all around them. But do fish need to drink that water? Well, some fish do, but some don’t. It all depends on where they live.
You probably already know that your body needs water for lots of reasons. It is so important in fact, that different parts of your body work together to make sure that there’s about the same amount of water inside of it at all times.
If the amount of water gets too low, you get thirsty and you grab a drink. And if the amount of water gets too high, you get rid of the extra when you pee.
Living in water, it is hard to believe that fish need to keep themselves hydrated. Well, fish also need to keep the right amount of water on the inside of their bodies. And the water outside of a fish’s body can affect whether that fish needs to drink water or not.
Fish not just swim in water, they also drink it. Probably not how you might think. Most fish live in one of two kinds of water. Some fish called fresh water fish live in places where there’s no salt in the water, like lakes ponds and rivers. Other fish called salt water fish live in water that has lots of salt in it, like the ocean.
Fresh water fish have an easier time keeping the right amount of water inside of their bodies. That’s because water enters into a freshwater fish’s body right through its gills. Freshwater fish have more salt in their bodies than the water around them.
Gills are the body parts that fish use to breathe. Fish breathe by pulling water into their mouths, then pushing the water over their gills and out of their body.
And in fresh water fish, gills also end up taking in some of the water and the fish ends up with more water in its body.
So fresh water fish don’t really have to drink water through their mouths like we do. These fish actually have to make sure they don’t get too much water and their body has to work really hard to get rid of the extra water as waste.
But this process is not so easy in salt water fish.
Salt is everywhere on our planet. Some animals evolved to live in it while others didn’t. Salt and water together make up most of the water on our planet and it’s a major component of the beginning of life as we know it.
Everything in nature is balanced, including salinity. Essentially, nature wants to make sure there is a balanced level of salt everywhere. If we drink salt water, we could die of dehydration because it would only make us more and more thirsty. But if salt water fish are drinking salt water daily, how do they avoid dehydration?
They have come up with a clever solution. They pump out the excess salt across their gills.
Unlike fresh water fish, salt water fish have less salt in their bodies compared to the water that they live in. So while fresh water fish get a lot of water in through their gills, saltwater fish lose a lot of water through their gills.
As water flows over a salt water fish’s gills, the salt that is in the water outside of the fish’s body, sucks out some of the water that is inside of the fish’s body through its gills. The gills of salt water fish have adapted to filter out salt from their bodies back into the ocean. Without this adaptation, they would die because of the high salinity.
Since water is always moving over a fish’s gills, that means water is always leaving a saltwater fish’s body. And that means that the fish that live in salt water need to drink – a lot. Most animals can’t drink salt water when they’re thirsty, but these fish can. The salt water fish gulp and swallow water almost constantly. Salt water fish have a special system in their bodies that takes salt and puts it back out into the ocean – and everything stays in balance.
So what happens if you move a salt water fish to a sweet or fresh water tank or pond?
Some species of eel, salmon, bass and flounder have adapted themselves to move between fresh and salt water.
This is a great example of how evolution solves a problem along many different paths.
So some fish need to drink water and some don’t… it just depends on where they live.