Oxygen is crucial to fish survival, so how exactly do these creatures breathe in water?
While humans may inhale oxygen in the air through their mouths and lungs for easy breathing, the breathing process is actually harder for fish.
How Fish Breathe
In order to breathe, fish make use of their gills to pull out oxygen molecules dissolved in the water. However, the oxygen amount in the air is remarkably higher compared to the oxygen amount in water. Because of this, fish have a harder time breathing compared to humans.
Similar to how humans take in air, fish take water into their mouths. The water then filters through the fish's gills, which are organs that have many feathery filaments that consist of protein molecules.
These filaments have the appearance of tiny brush bristles. They also have thousands of small blood vessels, which aid oxygen entry into the bloodstream.
The blood vessel count in fish allows them to have a larger surface for oxygen to pass through. This would help them pull the water's dissolved oxygen and release CO2 into the water.
Fish Gills
The design differences between gills and lungs are the main reason why humans cannot breathe in water. Compared to the lungs, gills are better at pulling oxygen from water.
Roughly 75% of the oxygen that moves through the fish gills gets extracted.
Moreover, since fish expel less energy for survival, they also require less oxygen. However, they do need a certain degree of oxygen.
This would mean that water with low levels of oxygen could be deadly for fish, the same way air with low oxygen could be deadly for humans. The ocean is full of hypoxic and anoxic zones, which are called dead zones at times, that are ocean bits where oxygen is very scarce for fish to survive.
As to why fish cannot breathe air like humans, gills require water for their structures to be maintained and for tissue collapse to be prevented. Similar to how humans can drown in water, fish can also drown in the air.
If fish gills are exposed to air for prolonged periods, they could end up collapsing, causing fish to suffocate. Similar to how humans are suited for terrestrial life, fish are suited for underwater life.
Labyrinth Fish Can Breathe Air
However, there is a type of fish, known as the labyrinth fish, that is able to breathe air. They are named after their labyrinth organs that are lung-like and that consist of various maze-like lamellae compartments.
Such organs help fish species, such as Gourami, Betta, and Paradise fish, to inhale oxygen from the air like humans. Moreover, because they also have gills, they are capable of breathing dissolved oxygen in water.
These creatures can even survive for hours out of water. Several of them also make bubble nests. Males may blow bubbles in order to make elaborate air nests at the water's surface.
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