Dear Copper,

All living things need water. It’s how life works on Earth. But do animals that swim in water drink water?

I talked about that with my friend Nora Hickey. She’s a fish veterinarian at Washington State University.

She told me it depends on the kind of fish. Saltwater fish constantly drink water. Freshwater fish drink hardly any water.

It’s all about the salts inside their bodies. Those salts make their muscles and nervous systems work.

“If you look at blood from saltwater fish and freshwater fish, they have about the same number of salts in their bodies,” Hickey said.

Saltwater fish and freshwater fish use different ways to keep those salt numbers the same.

It has to do with osmosis. That’s the way water moves across a thin barrier to make things equal on each side.

In this case, one side of the barrier is the water inside the fish. The other side is the water outside the fish.

The water inside a freshwater fish is saltier than the water it swims in. So, water constantly moves into their bodies through their skin and gills.

All that water flowing in could dilute the fish’s body salts. That would be bad for their muscles and nervous system. So, freshwater fish constantly pee to remove extra water. They don’t drink much water because they don’t need more.

Saltwater fish are the opposite. Their insides are less salty than the water they swim in. So, water constantly flows out of saltwater fish through their skin and gills. They replace the water by drinking constantly.

Of course, they’re drinking salt water. So, they need to get rid of the extra salt. They mostly push out the extra salt in their poop. They hardly pee at all because they need to save the water in their bodies.

A shark with two remora attached to its body swims in the ocean.

So, what about sharks?

“Sharks are like a combination of freshwater fish and saltwater fish,” Hickey said.

Bony fish have swim bladders. It’s like a bag of gas inside them. It helps the fish control how much they float or sink—called buoyancy. Sharks don’t have that. They use the shape of their fins, extra fat in their livers and urea in their blood for buoyancy.

Urea and salts aren’t the same things. But they’re both dissolved in the water inside the animals’ bodies. So, they influence whether water flows into the animal or out of the animal through osmosis.

Thanks to that urea, sharks have less water inside than outside. So, water constantly flows into them across their skin and gills. That’s just like freshwater fish—but even more dramatic. Sharks can take in 167% of their body weight in water every hour. That means a 10-pound shark could have nearly 17 pounds of water flow into its body every single hour. Sharks don’t drink, but they do pee constantly.

All that water coming in is saltwater. So, sharks need ways to remove extra salt. That’s like saltwater fish. Sharks have a special gland near the anus that pushes extra salt out of their bodies.

The next time you dip your toes into a river or ocean, remember that freshwater is full of fish pee—and saltwater is full of shark pee. It’s good to know what kind of water you’re in.


Dr. Universe