How Do Fish Breathe Underwater?

Air is about twenty one percent oxygen. Water holds dissolved oxygen at a concentration roughly thirty times lower, and it is also far denser and more viscous, so moving it over a breathing surface costs more energy.

Fish solve this with enormous surface area and a very efficient extraction method. Gills are made of filaments covered in tiny folds called lamellae, packed with blood vessels, giving a huge area for gas exchange in a small space.

This is the key mechanism. Water flows across the gill in one direction while blood flows through it in the opposite direction.

Because of this arrangement, blood always meets water that is slightly richer in oxygen than the blood itself, all the way along the gill. The gradient never reverses, so oxygen keeps transferring the whole way across. Fish can extract up to about eighty percent of the available oxygen, whereas human lungs manage roughly a quarter of what is in inhaled air.

Fish use two main strategies. Most pump water actively by opening the mouth, closing it and forcing water over the gills and out through the gill covers, which lets them breathe while stationary.

Fast swimmers such as tuna and many sharks use ram ventilation instead, simply holding their mouths open and letting forward motion push water through. Some species that rely entirely on this must keep swimming to breathe, which is the origin of the belief that all sharks must never stop moving. Many bottom dwelling sharks pump water and rest quite happily.

A number of fish supplement or replace gill breathing:

• •Lungfish, which have true lungs and can survive dry seasons buried in mud
• •Labyrinth fish such as bettas and gouramis, using a folded air breathing organ
• •Catfish species that absorb oxygen through the gut
• •Mudskippers, which breathe partly through moist skin while out of water

A fish out of water suffocates despite being surrounded by oxygen. Gill filaments are supported by water, and in air they collapse and stick together, destroying the surface area the whole system depends on.

They also dry out quickly, and gas exchange requires a moist surface. This is why fish kept out of water die of what is effectively suffocation rather than dehydration, and why warm or polluted water can be lethal too, since warm water holds less dissolved oxygen.

Oxygen is less soluble in warm water than in cold, so a rise in temperature reduces the oxygen available at exactly the moment fish need more of it, since their metabolism speeds up as they warm.

This double squeeze is why fish kills often follow heatwaves, and why cold water species such as trout and salmon are particularly vulnerable to warming rivers. Nutrient pollution compounds it: algal blooms grow, then die and decompose, and the decomposition consumes oxygen, creating dead zones where fish cannot survive.

Gills are not only for oxygen. Because they are thin, wet and bathed in a constant flow of water, they are also where most fish get rid of their nitrogen waste. Land animals usually convert this waste into urea or uric acid, which costs energy but is less toxic to store. Most fish skip that step and release ammonia, which is highly toxic, straight across the gills. The surrounding water dilutes and carries it away instantly. It is one reason crowded aquarium tanks need filters and water changes.

Gills also help control salt and water balance, and they work in opposite directions in different habitats. A freshwater fish's body is saltier than the water around it, so water constantly seeps in and salts leak out. It produces large amounts of dilute urine and uses special cells in the gills to pull salts back in. A sea fish faces the reverse problem, losing water to the salty sea, so it drinks seawater and uses gill cells to pump the extra salt out. Salmon, which move between rivers and sea, retune these cells as they migrate.

Some small ponds in northern Europe freeze over for months, and beneath the ice the oxygen gradually runs out. Most fish die, but the crucian carp, a close relative of the goldfish, can survive these conditions for weeks or even months. Without oxygen, cells fall back on anaerobic respiration, which releases far less energy from each molecule of glucose and normally produces lactic acid. In most animals, including you, that acid soon builds up to harmful levels. The crucian carp copes by storing large amounts of glycogen in its liver beforehand and slowing its metabolism, while still keeping its brain working.

Its real trick is chemical. Its muscles convert the products of anaerobic respiration into ethanol, the same alcohol found in drinks, using a specially adapted set of enzymes that it shares with goldfish. Ethanol is a small molecule that crosses membranes easily, so the fish simply releases it into the water through its gills instead of letting acid accumulate. The gills themselves can also change shape. In cold water rich in oxygen, the spaces between the lamellae fill with cells, shrinking the exposed surface. When oxygen falls, those cells disappear and the surface available for gas exchange grows several times larger within about a week.

The bony arches that hold up a fish's gills have a long evolutionary history, and part of it is inside you. The earliest fish had no jaws at all. The leading explanation, supported by fossils and by how embryos develop, is that jaws evolved from the foremost of the pharyngeal arches, part of the same series of bars that, further back, supported the gills. Another arch-derived bone, the hyomandibula, came to brace the jaw against the skull in many fish. Sharks, bony fish and every land vertebrate inherited this jawed body plan.

Human embryos still develop a series of pharyngeal arches in the neck region around the fourth week of development, a stage at which they look much like the matching stage of a fish embryo. In fish, these arches go on to support gills. In you, they are reshaped into other structures: parts of the jaw, the small bones of the middle ear and cartilage in the voice box. The stapes, the tiny bone that passes sound to your inner ear, corresponds to the hyomandibula of fish. Evolution tends to repurpose existing structures rather than build new ones from scratch.

Fish breathe by passing water over gills with a huge surface area, using a countercurrent flow that keeps oxygen transferring the whole way across, which lets them extract far more of the available oxygen than lungs manage from air.