With 71% of the Earth’s surface covered with water, you can bet that our ocean-dwelling friends have some rich and unique characteristics about them. Right from their anatomy to their habitats, fish, the largest group of vertebrates, lead curiously fascinating lives. With some estimates placing their appearance at roughly 420 million years ago, these agile swimmers have been around for a lot longer than humans.
While the deep seas hold many secrets on the lives of these creatures, one of the most common questions, highlighting one of their biggest differences with land-dwelling animals, is “why do fish die when taken out of water?”. Today we help you answer this question with everything you need to know about fish, water, and their fascinating respiratory systems!
How does a fish breathe?
Fish breathe through their gills, an organ important to their respiration. The gills are located on each side of a fish’s head and have hundreds of tiny filament-like structures inside them. These structures are responsible for absorbing the oxygen dissolved in water. When a fish opens its mouth, water enters the gills and the filaments absorb oxygen dissolved in this water.
Can you spot the gills on the side of these fish? Image source: Wikipedia Commons
This oxygen enters the fish’s blood from where it travels throughout its body. At the same time, the carbon dioxide from the fish’s body is also released out into these filament-like structures, similar to how our lungs help us absorb oxygen and give out carbon dioxide. In bony fish, the gills are protected by a structure called the operculum.
Why can’t fish live without water?
Human lungs are designed to absorb oxygen from the air. Breathing underwater would not be possible for our lungs as their structure is not equipped to absorb the oxygen dissolved in water. You might have noticed this when you’ve gone out for a swim, you have to hold your breath when you’re underwater.
A swordfish in water. Image source: Wikimedia Commons
Similarly, a fish’s gills are equipped to only absorb oxygen dissolved in water. When a fish is out of the water, its gills cannot absorb oxygen from the air around it, as a result, the fish suffocates and cannot breathe. A fish out of water faces the same respiratory challenge as a human stuck underwater!
Why is the rate of breathing in aquatic organisms much faster than in terrestrial organisms?
If you’ve ever observed a fish in an aquarium you would have noticed that these critters constantly open and close their mouths. This is how they breathe. However, the rate at which they breathe is often much faster than humans, or other terrestrial organisms for that matter. This is because the concentration of oxygen in water is much lower than the concentration of oxygen in the air. This means that fish or other aquatic organisms need to breathe much faster than terrestrial organisms to get enough oxygen to nourish their body.
A goldfish breathing. Image source: Giphy
For this same reason, the number of filament-like structures in a fish’s gills is far higher than similar filaments present in our lungs. A fish’s gills need to work harder and better to absorb oxygen from water than lungs need to from air. Mind you, aquatic animals use the oxygen dissolved in water, not the oxygen that is part of the water molecule (H20).
Can fishes survive out of water?
A fish’s lungs are particularly designed to breathe underwater making a fish’s survival out of water impossible. However, there are a few special varieties of fish that have physical adaptations that allow them to breathe out of water too.
The mangrove killifish is one such variety. A tiny fish found in the mangroves of South America, this fish has the ability to breathe through its gills and its skin! When the water in a mangrove dries up, the killifish finds a moist spot in the mud or under a leaf and breathes through its skin. Once the water returns it goes back to breathing through its lungs!
A mangrove killifish. Image source: Wikimedia Commons
The mudskipper is another famous example of a fish that can breathe out of water. Similar to the killifish these animals are also found in mangroves. When the water in a mangrove dries up, the mudskipper breathes through its skin and can spend some period of land too as long as it stays moist!
A mudskipper. Image source: Wikimedia Commons
Did you learn something new about fish and their fascinating respiratory systems in this article? Let us know in the comments!
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