How do Fish Breathe Underwater: Scientific Concepts, Fun Facts and Key Adaptation Techniques

Have you ever wondered how fish stay alive underwater while we need to come up for air? It's actually simpler than you might think, and pretty fascinating once you understand it. We breathe air through our lungs. Fish do something similar, but instead of lungs, they use gills, and instead of air, they pull oxygen straight from the water. That's the short answer. Here's the longer one.

Table Of Content: 

• The Science of Fish Respiration
• How Gas Exchange Works
• Key Adaptation Techniques
• Fun Facts
• Why Fish Cannot Breathe Air (Usually)

The Science of Fish Respiration

Fish have gills on either side of their head. When a fish opens its mouth, water comes in. The water is pushed over the gills and out through the sides as the mouth closes. Oxygen enters the fish's blood, and carbon dioxide exits as the water flows over its gills. Our lungs perform the same exchange, but it takes place in water rather than air. The operculum is a little flap that covers the gills of most fish. Since sharks lack this, their gill slits are visible on the outside.

How Gas Exchange Works

Each gill contains tiny, narrow folds that are filled with microscopic blood vessels. The gills have a large surface area thanks to these folds, which increases blood-water contact. The innovative thing is that the blood and water move in different directions. This might sound like a small detail, but it makes a massive difference. Because they're flowing against each other, oxygen keeps moving into the blood all the way along the gill surface. Fish can actually pull more oxygen out of water this way than we pull from the air with our lungs. Nature figured out a pretty smart trick there.

Key Adaptation Techniques

Ram Ventilation

Fast fish, such as tuna and mackerel, swim with their mouths open to keep water flowing over their gills. To breathe, they have to keep moving.

Buccal Pumping

Most fish move their lips and gill covers to push water over their gills. This helps them breathe even when they are not swimming or at rest.

Labyrinth Organs

Fish like gouramis and bettas can breathe air through a special organ. Because of this, they can live in water with very little oxygen.

Lungfish and Bimodal Breathing

In addition to their gills, lungfish have a simple lung. They are able to breathe and survive even when the water momentarily dries up.

Cutaneous Respiration

Some fish, including eels and certain loaches, supplement gill breathing by absorbing oxygen through their moist skin, a process called cutaneous respiration.

Specialised Blood Chemistry

Deep-sea fish and those in cold, oxygen-rich waters have haemoglobin with a high oxygen affinity, maximising extraction from their environment.

Fun Facts

• Sharks must keep swimming at all times because they use ram ventilation - if they stop, they suffocate.
• The lungfish can survive for up to four years without water by encasing itself in a mucous cocoon underground.
• Fish gills are so efficient that they can extract oxygen even when levels are as low as 1–2 mg per litre of water.
• The mudskipper is a fish that spends much of its life on land, breathing through its moist skin and the lining of its mouth.
• Electric eels absorb up to 80% of their oxygen through their mouths since their gills are poorly developed.

Why Fish Cannot Breathe Air (Usually)

This one trips people up. Air has more oxygen than water, so shouldn't fish be fine out of water? The problem is their gills. Gills are designed to work when water is holding them open and spread apart. The moment a fish leaves the water, the gills collapse and stick together. Now there's almost no surface area left to absorb oxygen. Despite a wealth of oxygen in the surrounding air, the fish is unable to breathe correctly. Some fish, like walking catfish and lungfish, have special features that let them handle air. But for most fish, being out of water is fatal pretty quickly.