Fish are constantly surrounded by water, but are they thirsty? And how would they drink?
To answer these questions, it is crucial to understand how water, a solvent, interacts with other substances such as salt, which is a solute, across a cell membrane. Through a process called osmosis, water flows across a membrane from areas with low solute concentrations to areas with high solute concentrations until the cell can reach some sort of equilibrium with its external environment.
How much water a fish consumes really depends on the amount of salt in its surrounding habitat. While fish drink some water — salty or fresh, depending on their surroundings — through their mouths, they mostly absorb it through their skin and gills by osmosis.
“You must think of a fish as a leaky boat in the water” Tim Grabowski, a marine biologist at the University of Hawaii, told Live Science. “You have constant movement of water or salts that are in the water between the body of the fish and the external environment.”
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Let’s start with how fish in the ocean stay hydrated. Seawater has approximately 4.7 ounces of dissolved salt per gallon (35 grams per litre), while most fish blood has about 1.2 ounces of salt per gallon (9 grams per litre). This imbalance “will constantly cause the fish to lose water to the external environment and be flooded with salt in its cells and inside its body,” Grabowski said. “A saltwater fish is always thirsty. It drinks all the time.”
These fish need a way to keep the water they are drinking from the ocean but get rid of the salt. To do this, fish have specialized cells in their gills called chloride cells, which essentially act like tiny pumps that actively push salt out of their bodies. To keep as much water as possible, marine fish rarely pee, and when they do, their urine is exceptionally salty.
Freshwater fish face the exact opposite challenge to marine fish when it comes to water, according to Melanie Stiassnycurator of the ichthyology department of the American Museum of Natural History in New York City.
“If you’re a freshwater fish, you have a problem because the water is constantly being pumped in te,” Stiassny told Live Science. Too much water can be a bad thing because it can dilute the body’s salt content, which is critical for blood pressure regulation and support of muscle function. Freshwater fish spend all their time trying to keep water out of their bodies and never drinking it, at least on purpose.
“[Freshwater fish] he might get water accidentally when he’s feeding and stuff, but he never drinks water,” Grabowski said. To combat this constant barrage of fluids, he “pees all the time,” he added. But there’s no need to worry about swimming in a pile of fish pee in lakes or rivers, the urine is mostly just water, Grabowski said.
Similar to ocean fish, freshwater fish also have chloride cells, but their pumps work by drawing salt into their bodies rather than out of them. However, operating these pumps can take a lot of effort.
“[Water] it’s going in passively, but it needs to be vigorously removed,” Stiassny said. “There’s a cost, particularly for saltwater fish that really have to pump out all that salt they’ve brought into their system by having to drink a lot of water.”
There are some fish that follow a completely different regulation for drinking water. For example, sharks maintain high concentrations of urea, a salty byproduct of ammonia, in their bodies. “[Sharks] stop that passive ingress of water because they’ve balanced it with urea and their blood — so they’re basically salty like salt water,” Stiassny said. When they take in seawater, sharks excrete the excess salt through the chloride cells in a gland in their rectum.
Regardless of the mechanism, however, the key to staying hydrated for all fish is finding the perfect salty balance.
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