Have you ever been asked a question, opened your mouth quickly to answer, and then stopped because you realize you really don’t know what to say? I have, several times, with one of them being when asked ‘can fish drink water?’
I thought when asked, of course, they do, after all they live in water so why wouldn’t they drink it? Then I had the thought that I’ve never seen a fish pee, which would be the result of them drinking, so maybe they don’t. I then decided I was now slightly confused and decided to look into it.
So, do fish drink water? Salt water fish drink water, while fresh water fish do not even though they do need water to survive. Once water enters a fish’s body, salt water fish mainly urinate it out through their gills, since they release the water through osmosis from their body. Freshwater fish release most of their water from the urinary tract.
But this general answer has many caveats, so read on for more.
Why Do Saltwater Fish Drink, Whilst Freshwater Don’t?
To keep it fairly simple, freshwater fish do not drink because they are always absorbing water through their gills and skin due to a process called osmosis. Saltwater fish, on the other hand, are always losing water through their gills and skin also due to osmosis, so need to drink almost constantly. Osmosis involves a flow of water across the membranes of a fish in order to move solutes (salt) from areas of high concentration to low in an attempt to even the distribution out.
The reason why saltwater fish and freshwater fish react differently to the process of osmosis is all down to the salt. Freshwater fish have bodily fluids and blood that is much saltier than the water they swim in, whilst for saltwater fish the opposite is true. This means that when more salt is introduced through the osmosis process, it can cause the saltwater fish to dehydrate. To avoid this, they take water through their mouths and direct it into the digestive tract, hence, drinking it.
It is worth mentioning here, that like humans, fishes bodies only need a certain concentration of salt to function to the best of their ability. Water cannot be allowed, in fish, to just diffuse freely through their gills. This is because when this happens saltwater fish would shrivel up and die, whilst freshwater fish would explode.
To avoid ‘exploding or shriveling’ fish their gills are equipped with special cells that pump salt in or out of their blood selectively. As mentioned above, in freshwater fish these cells constantly draw water in, and in saltwater fish, they constantly draw salt out. The kidneys of the saltwater fish are also used to help with this process.
Do Goldfish Drink Water?
The Goldfish is the most common species enquired about when it comes to whether fish drink. Believe it or not, it is that often asked that it even has its own joke:
‘We once owned a goldfish that had a urine infection. We knew because the bowl kept overflowing.’
However, the answer, of course, as we now know, is that they don’t, they are not an exception to the freshwater/saltwater rule. There are, nonetheless, some fish that do not fall into saltwater fish drink and freshwater don’t categories. These include:
- The Shark whose blood is naturally very salty and virtually matches the salt content of the sea. This means they are never overloaded on salt and hence have no need to drink.
- The Salmon who lives both in freshwater and saltwater dependent on the time of year. This means that they have to adapt to drinking and not drinking varying upon where they are.
- Not all sea living animals drink salt water. Whales and other sea mammals, for example, get their water from their food. Should they drink sea water, it is believed, that they would suffer from the same dehydration that humans do.
- The Nikpoh Lyrad (Archerfish) drinks water into a chamber of its stomach. It does not, however, need this water to survive; rather to spit at its enemies with tremendous speed.
Also interestingly, but debatable, a freshwater lake in Australia was (allegedly) emptied of all fish during a drought leading to the water level not dropping at the same fast rate as it had been doing. This has led to fish keepers in the area arguing that the freshwater fish in there must have been drinking the water!
So If Some Fish Drink, Do They Also Urinate?
It’s easy for us to see that our fish poop, the little strings hanging from their butts give it away. However, it’s not quite so clear whether our fish urinate or not. After all, they’re in the water and you’re hardly going to see it!
From what we know already, that saltwater fish drink and freshwater don’t, the natural assumption would be that the aforementioned urinate and the latter don’t. However, this would not be the truth as both freshwater fish and saltwater fish urinate.
A recent study by the Virginia-Maryland Regional College of Veterinary Medicine, in fact, claims that fish basically urinate all the time. They do this either through their gills or urinary pores with the latter being filtered via the kidneys.
Freshwater fish mainly use their urinary pore to urinate which is located near their anus. They release large amounts of undiluted urine as they have to get rid of all the water they take on board. Saltwater fish, on the other hand, urinate mainly through the gills. Their urine contains high amounts of salt and low amounts of water meaning it is extremely concentrated.
A Word of Caution
As some fish drink and all fish urinate they can suffer from urinary tract and kidney diseases. Renal dropsy, for example, is common in goldfish and other carp species, which is caused by a parasite, Sphaerospora auratus. Fish affected with this condition display swelling in their abdomen from fluids that accumulate as the kidneys fail. This condition is generally fatal.
For a complete guide to all tropical fish diseases read our latest article covering all aspects of illness and recovery.
Whether or not a fish drinks it is clear to see that they need water for their survival, and not just to obtain oxygen. Saltwater fish need it to, in a manner of speaking, to quench their thirst and prevent dehydration; whilst freshwater fish need it to balance their salt distribution out. Water is vital to a fishes life!
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