By Dr. Becker
Of all the burning questions in the world, some you may not have thought of and others, once the query has been posed, make you wonder why you never wondered before. The questions concern the teleost fish, a name that, broken down into the original Greek, denotes “teleios,” meaning “complete,” and “osteon,” meaning “bone.” The website Today I Found Out notes that teleost fish make up about 96 percent of the entire species.1
If you’re yawning already, bear with me, because, according to Wikipedia, this fish bobbed to the proverbial surface during the Triassic period, some 252 million years ago, noted for beginning and ending with “major extinction events.”2 Basic Biology says they’re the most advanced of all fishes, and “found in almost every possible aquatic habitat from the tropics to polar regions around the globe.”3 This may elicit a number of questions, but we’ll start with this one: Do fish get thirsty? The short answer is no, and here’s why:
“The desire to drink is an urge that originates from the hindbrain, whereas in land-based animals it originates in the forebrain. It is generally thought from this that a fish has no real ability to consciously feel thirst as we or many other land-animals would understand it; for them, it’s just a natural reflex …
However, fish do need to take in water, but, as with land animals, too much water can kill, a particular problem when one is surrounded by it! Unsurprisingly, the processes by which this is regulated in fish is quite fascinating, and further answers the question of why most fish must stay in either a saltwater or freshwater environment exclusively.”4
How Do Fish Drink?
Like other animals, fish are required to keep a healthy balance between the water and salt, as well as other minerals, in their bodies to stay alive. They live in water, so getting enough isn’t the issue; it’s getting the right concentration of minerals, such as salt and potassium.
Freshwater fish have higher concentrations of salt than the water around them, called hypertonic, which is a threat of sorts. Their skin doesn’t absorb it, but their gills are literally swimming in it, and this is how they breathe. They’re constantly taking water into their bloodstreams via osmosis through osmotic pressure. To brush up on the term:
“Osmosis is the movement of a solvent (in this case water) across semi-permeable membranes (like cell walls), from areas of lower solute concentration to areas of higher solute concentration. This naturally equalizes the solute concentration on both sides of the membrane.”5
A red blood cell in distilled water will gradually absorb more and more water until it literally blows up. In basic terms, that’s due to osmotic pressure, helped along by the fact that the cell is salty inside and its membrane is thin and semipermeable. If you drop the same red blood cell into water that contains more salt than the cell has inside, the cell will lose so much water it will shrivel up. Placed in water with a similar salt make-up, nothing happens.
In order to regulate the amount of water flowing into its bloodstream, which dilutes the concentration of minerals, including salt, freshwater fish have an almost constant need to urinate. Now you know why you were told to clean the fish tank so often when you were a kid, and why a filter is required.
Freshwater fish have large kidneys containing capillaries known as glomerulus, which help them produce urine more often. Because they need enough salt to maintain optimal levels for their systems, chloride secretory cells in their gills draw salt from the water they’re in so it can enter the bloodstream as their blood is being diluted.
Saltwater Fish Are the Opposite of Freshwater Fish
How saltwater fish drink and urinate is the other way around. Saltwater fish have less salt in their blood than the water they’re in, called hypotonic, so they lose, rather than absorb, the water passing through their gills. This takes place even while the same amount of osmotic pressure is being exerted. That’s why saltwater fish drink through their mouths, but because it’s their hindbrain that controls this function, it’s a reflex, not actual thirst that prompts it.
They don’t actually experience thirst. Further, they don’t urinate much (to conserve water) and their capillary-laced glomeruli are few and far between. But because they drink so much saltwater, they have way more salt than they need. Today I Found Out explains how they swim around that:
“The chloride secretory cells of saltwater fish, rather than absorbing salt from the surrounding water as with freshwater fish, actively release it from the fish’s body into the environment. This all combines to more or less automatically keep the saltwater fish’s salt levels in the optimal range.”6
Amazingly, some fish can swim between saltwater and fresh water with no problem, which means their bodies can function either way, although they do require some time to acclimate in between. An example would be salmon, which start their lives in fresh water. As they transition to saltwater, they automatically begin drinking more and more water.
Meanwhile, they begin urinating less often, and a special enzyme helps facilitate the reversal of the way it functioned previously in the chloride secretory cells of their gills. At that point, they stop absorbing salt from the water surrounding them because their “pump” begins reversing the process, effectively excreting it. It takes a few days, but after their bodies have transitioned, they can swim anywhere in the ocean they want. When they return to a freshwater habitat, the process is reversed once again.
More on Teleost Fish: Some Can Survive on Land
As previously mentioned, teleost fish can go anywhere, from the tropics to the Antarctic. They can also be found in swamps, rivers, lakes and streams and in the deepest oceans where there’s no light at all. This is a function of the ability of some to produce their own light through bioluminescence. According to National Geographic,7 a fish species known as the mangrove rivulus may get overheated and flip onto land to cool itself off. They’re able to do this, experts say, because they have specialized skin that can function as gills.
Then there’s the catfish species that can “walk” across roads and small areas of land, especially after a rainstorm. The prize might go to the West African lungfish: they have both gills and lungs.
They actually secrete mucous that they use as a cocoon in order to burrow in mud, which they’re able to do for years at a time. Some fish leave the water to escape predators or to find food or a mate, and if they’re forced to because the water they’re in is drying up. All in all, fish can be quite adaptable when they have to. What’s amazing is that they can.