Fish are unusual creatures in that they cannot survive out of water. If you have ever tried to hold your breath, you will most likely have lasted no longer than one minute before needing to surface for air. Fish spend their lives underwater and cannot survive more than two or three minutes out of water. So, how do they get the oxygen they need to survive without air?
Despite living in water, fish still require oxygen for vital bodily functions to occur. To get oxygen, fish take water through their mouths and force it back out through their gills. The gills work like a filter, absorbing oxygen from the water and travelling around the body via the bloodstream.
This is why aquarium experts recommend installing a filter or air stone in your home aquarium. They work by drawing water into the filter, pushing it through a sponge, and forcing the filtered water again.
The motion created in the water creates agitation at the water surface and increases the amount of oxygen available in the water for the fish to thrive.
To better understand how fish breathe underwater, we must look at how the gills function to extract oxygen from the water.
Gills are essentially the marine version of lungs. Fish have eight gills in total, four on either side of the head.
They are a fan-shaped organ that sits just under the gill slit, and they are made up primarily of blood vessels. Oxygen in the water is absorbed into the blood vessels in the gills and passed into the fish’s bloodstream.
The gills also have a secondary function; to remove carbon dioxide. Just like our own bodies, fish are adversely affected by carbon dioxide.
The gills’ size and the skin’s thickness directly impact how much oxygen a fish can absorb. The larger the gill and the thicker the skin, the more efficient the oxygen absorption will be.
Why fish Can’t Breathe air
Now that we know how fish get their oxygen from the water, you may be wondering why they cannot breathe air as we do? Their gills absorb oxygen molecules from the water, just like our human lungs absorb oxygen from the air we breathe.
The answer is simple – a fish’s gills are supported by water and do not function well out of water. Once a fish is on land, the gills are not supported, so the filaments collapse.
This drastically reduces the surface area available for oxygen absorption to take place. This would be similar to a human or other land mammal experiencing a collapsed lung.
Some Fish Do Breathe Air
As with anything in nature, there are exceptions to the rule. A handful of fish species can breathe out of water due to various physiological adaptations.
Mangrove rivulus are small fish with slender bodies. They are classified as amphibious fish due to their ability to survive on land and water. Found around the Caribbean and United States coasts, they feed on smaller fish, crustaceans such as shrimp and crabs, aquatic insects, land insects, snails, and worms.
They can survive on land due to the unique network of blood vessels in their skin. The blood vessels are located so close to the skin’s surface that the fish can easily absorb dissolved oxygen from the air.
Mudskippers are perhaps the most famous air-breathing fish. They are unusual-looking fish, with protruding eyes like a frog, a large, broad head, and a narrow, elongated body. They use their strong pectoral fins to ‘walk’ on land.
They are found on beach mudflats across the Indo-Pacific, including West Africa, Japan, and Australia. When tides retreat, tiny aquatic animals and sprouts of vegetation are exposed. This is what mud skippers come to feed on.
To ensure they do not dry out, they will occasionally roll in the mud, which keeps the skin cool and moist. This increases the oxygen absorption through the skin and into the bloodstream.
The Northern snakehead, often referred to as ‘frankenfish,’ is a predatory creature native to Korea, China, and Russia. They have elongated bodies similar to eels and can survive several days out of water in damp conditions.
While on land, snakeheads use a primitive lung-type organ, or air chamber, located above the gills. They gulp air, which is forced through the air chamber where oxygen is absorbed. This air chamber functions in a similar fashion to a human lung. The absorbed oxygen is then transported around the body via the bloodstream.
They wriggle their bodies in a side-to-side motion to move on land, like snakes and eels. They use a combination of their pectoral fins and strong body muscles to achieve this unusual movement.
They are an invasive species for several reasons. Firstly, they reproduce often, and a mature female can release as many as 150,000 eggs in less than two years.
Secondly, they can tolerate a wide temperature range that would kill most other species. Thirdly, and most impressively, they can survive in poor water conditions, adapting to their new environment.
It is believed that there could be as many as 30 species of fish able to survive both on land and in water. Where conditions are right, amphibious fish can survive for days on land.