It may sound strange to hear that saltwater fish need to stay hydrated. After all, they live in water. However, a review published in Physiology describes how nearly all saltwater fish are thought to have evolved from freshwater ancestors, and many maintain similar levels of salt in their blood as freshwater animals. Living in a saltwater environment means that these fish have to find a way to get rid of excess salts ingested as they breathe and eat, while at the same time ensuring they retain water.

While species of mammals, birds, and reptiles live in marine environments, marine birds are unique because they can produce concentrated urine that contains more salts than their blood and many can get rid of even more through special salt glands in their nose (diagram above) that produce highly concentrated salt solutions. Marine mammals take it one step further and can actually produce urine that is more concentrated than sea water, whereas marine reptiles have salt glands like birds but are not able to produce concentrated urine. Air breathing marine mammals, birds, and reptiles do not absorb the water through their skin and rarely intentionally drink sea water. Instead, they create fresh water through metabolism or obtain it from their diet.

Unlike marine mammals and birds, teleost fish breathe seawater through their gills instead of air and they drink seawater, but they are not able to produce urine that is more concentrated than their blood. So, it is easy to see why they needed to evolve other ways to get rid of excess salts.

“Water follows salt” is a common mantra in basic science courses, like physiology, used to help students understand the concept of osmosis. This mantra describes how water travels from areas that are more dilute to areas where salt levels are higher. If salt levels are higher in the gut than the blood, then water would stay in the gut and have a hard time moving into the blood and vice versa. The intestines of teleost fish are uniquely adapted to concentrate salts into solid particles which allows them to reduce the salt level in their gut so they can better absorb water into their circulation and prevent it from just staying inside the intestines. These fish also excrete a form of calcium carbonate, called ichthyocarbonate, in their feces which is thought to contribute up to 15% of the new carbonate in the ocean each year! Evidence also suggests that rising CO2 levels in the oceans may actually increase calcium carbonate production by fish. 

Calcium carbonate is super important to healthy ocean ecosystems. Marine animals need it to form skeletons and shells, and it is an important regulator of ocean acidity. Who knew that fish poop could be so vital to healthy oceans?

Source:

M Grosell, AM Oehlert. Staying hydrated in seawater. Physiology. 38(4): 178-188, 2023. 

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Categories: Agriculture, Aquaculture, and Livestock, Environment, Extreme Animals, Nature's Solutions, Ocean Life

Tags: American Physiological Society, animals, ecosystem, fish, health, ocean, osmosis, physiology, salt, science, teleost