Do sharks drink water?

Do sharks drink water?

In 1834, Samuel Taylor Coleridge wrote of a stranded mariner expressing the desperate irony of “water, water everywhere; nor any drop to drink.” The ocean can be a hard place for people to find drinkable water, and yet there is plenty of life in it.

So, if they can’t drink the water, how do animals get hydrated? And if they can, how do they handle the salt?

Do sharks drink water? Sharks are certainly capable of drinking water but will get most of the water they need from their food. They are also capable of creating an osmotic (salt) gradient in favor of their bodies that allows freshwater to be pulled from the salty brine of the ocean into their bodies via the gills.

This process happens via an elegant property of salt solutions called osmosis.

Osmosis

Anything that dissolves in water can be called a solute.

When water with a lot of solutes (e.g., salt) in it is separated from water with low amounts of solutes by a semipermeable membrane (such as skin or gills), the principles of osmosis dictate that freshwater will be pulled through the membrane into the saltwater, to balance out this unequal gradient.

This is why, sadly, when animals like slugs touch salt, the water in their body gets pulled out through their skin, and they dry up!

Luckily, human skin is less permeable, allowing us to swim in the sea. But the skin of the lungs and gills is very permeable, and fish gills are always in contact with salty water. This presents a challenge to animals who need to balance their water content with the surrounding environment.

Typically, the water inside animals is a little salty (think sweat), but it’s still relatively fresh compared with the high salt concentrations in seawater. This means that in order to live in the sea, there has to be some form of protection against water being pulled out of their bodies. 

Animals living in this salty world have a couple of strategies for this.

They can become osmoconformers and make their bodies as salty as the ocean, or they can be osmoregulators and work hard to remove salt and hold onto freshwater.

Sharks, as is their contrarian nature, are both.

And that’s an interesting story with a few more new terms to learn, so buckle in!

Osmoconformers

In the ocean, osmoconformers have the same amount of salt in their bodies as the surrounding water.

This means the osmotic gradient is zero – osmotic pressure is equal on both sides of the membrane –  and water will stay inside them without being pulled in or out. They have conformed to their environment.

Osmoconformers, like jellyfish and mussels, which have the same concentration of salt inside and outside their bodies, typically use a lot less energy to regulate their biological processes.

This can be an advantage when you’re sitting in the same water all the time but can limit the range of environments you can tolerate; should an osmoconformer want to set up shop somewhere will less or more salt, they may not have the biological systems to handle it.

Osmoregulators

In order to be able to control the salt levels in the body, osmoregulators adopt different methods, actively usually filtering and removing salt from water.

Humans are osmoregulators whose kidneys act as a filter to remove salt or water from the body to maintain a healthy balance.

Sharks are also capable of removing excess salt through a rectal gland to lower their internal sodium levels; this form of osmoregulation is a great way to actively balance an animal’s internal salinity (saltiness) but does take extra energy to maintain.

This trade-off allows sharks to live in a variety of waters, though, and in some species, it’s effective enough for them even to enter brackish river estuaries and lakes!

So, what do sharks drink? Sharks drink seawater! Sharks undoubtedly swallow seawater with every gulp of their food, but they may also drink it occasionally. The active osmoregulatory glands in a shark’s intestine allow it to excrete salt and hold onto the water.

But drinking water is not necessarily the best method for a shark to hydrate itself. There’s a much more complicated and ingenious way to take advantage of the natural properties of water in the ocean. Here’s how it works.

How do sharks get hydrated?

We mentioned that sharks are osmoregulators, meaning they actively maintain a salt concentration inside their bodies different from the concentration of salt outside.

So by the rules of osmosis, there should be osmotic pressure pulling fresher water from inside the shark back into the ocean across the gills when the shark breathes.

But sharks have a very special defense against this. Urea, a waste product of metabolism, has osmolarity similar to the salt in seawater. This means that if you have a high enough concentration of urea inside the body, you can match the salt concentration of the sea, and your water will stay where it is!

Urea in high quantities is bad for protein and healthy bacteria in the body. That’s why we humans get rid of it in our urine.

But sharks need to hold onto urea, so they’ve developed a protective molecule that counteracts the negative effect of urea in the tissues of sharks. It’s called Trimethylamine-N-oxide – but don’t worry about remembering all that; call it – TMAO.

So, as we mentioned, sharks are osmoregulators and osmoconformers. They regulate their sodium levels and increase their urea levels until the solutes in their bodies conform to the osmolarity of the ocean.

But how does all this relate to hydration?

Well, suppose sharks want to take in more water. In that case, being osmoregulators, they can increase their urea concentrations to the point where the osmotic gradient is reversed – that means they have an even higher concentration of solutes inside their body than the saltwater has outside. This allows water to pass through the gill membrane to balance the gradient. Smart, huh!

Remember, water will always push through a membrane towards a higher concentration of solutes. But this doesn’t mean the solutes have to be identical. In sharks, the concentration of solutes is more-or-less the same, but the solutes in the sea are mostly sodium salt, and the solutes inside the shark are mostly urea.

With TMAO, sharks are able to hold onto enough urea to keep their bodies in balance – or isotonic – with the water around them.

Can sharks be dehydrated?

To dehydrate something usually means to remove water from it, but in biology, dehydration can be thought of more as an imbalance of salt and water inside the body.

One way to dehydrate is to lose water – through sweating, for example – but another way is to take on too much salt.

Since salt is everywhere in the ocean, sharks run the risk of dehydrating if they take on too much of it while eating.

The rectal gland in sharks (and rays) is specialized to extract salt from a shark’s blood and excrete it back into the sea. This helps them maintain a level of sodium well below the dangerous threshold. 

Sharks can become dehydrated. One way for sharks to dehydrate is from osmosis pulling water through their gills into the seawater.

Usually, high concentrations of urea in their blood and tissues prevent this from happening. Still, it does take time to adjust this system, so when a shark enters a new environment, there is a risk of suffering from an Imbalance.

Why do sharks have to be in saltwater?

Sharks spend almost all their time in saltwater.

Their bodies have been well adapted to it, and they have sophisticated osmoregulatory mechanisms to handle the salinity and stay hydrated.

But sharks are not well adapted to freshwater. Freshwater animals have the opposite problem – they need to hold onto their salt and not become bloated with water from osmosis! Because of this, they have their own osmoregulation systems that are different from a shark’s.

On top of this, all swimming animals have buoyancy regulation mechanisms too. These are things that help them float or sink when traveling between aquatic or marine depths.

In sharks, fats from the liver work to control how much they float or sink.

In some other fish, there are bladders of air called swim bladders. These are generally more effective at managing buoyancy in freshwater.

The denser a liquid is, the more an animal will float in it. If you’ve ever been to the dead sea, you’ll have seen an extreme example of this. That sea is so salty that it’s almost impossible to sink in it!

Since saltwater is a lot denser than freshwater, a shark’s buoyancy system is calibrated to that. When sharks swim into freshwater, not only do they struggle with osmosis, they tend to sink! Sharks use 50% more energy to stay afloat in freshwater than saltwater.

But some sharks do manage it. At least for a while.

Bull sharks have such good salt retention that they can spend relatively long periods in brackish or even freshwaters. They reduce the amount of urea in their bodies by peeing it out!

But they’re not as specialized for this environment; bull shark females may choose to give birth in brackish or freshwater to protect the young from their natural predators. Actually, the young start off life with a low tolerance to seawater and must develop it as they age.

However, a bull shark’s true home is the sea. It’s been discovered that they cannot survive in freshwater habitats for more than four years – possibly because there just isn’t enough to eat!

Final Thoughts

Sharks must get thirsty, being surrounded by saltwater all the time. Luckily, they have a bunch of specialized regulatory systems that allow them to hold onto their freshwater, take it in through their gills and even gulp down saltwater and excrete the salt afterward!

Sharks are amazingly adapted to life in the ocean and combine elements of osmoregulation with osmoconfromity to travel between different oceanic habitats. Even venturing up into estuaries and rivers in some cases!

These phenomenal adaptations make sharks one of the most specialized creatures in the oceans.