Fish are extraordinary biological machines built to thrive underwater, and nearly every system in their body works differently from land animals. They extract oxygen from water, control their depth without effort, sense invisible pressure waves, and maintain the right salt balance in their blood whether they live in a river or the ocean. Here’s how all of it comes together.
Breathing Underwater
Fish breathe by pumping water over their gills, which are packed with thin, blood-rich filaments that pull dissolved oxygen out of the water and release carbon dioxide back into it. The key to this system’s efficiency is a design called countercurrent exchange: blood flows through the gill filaments in the opposite direction from the water passing over them. This keeps a consistent difference in oxygen concentration along the entire length of the filament, so oxygen keeps diffusing into the blood at every point. The result is remarkable. Fish and crustaceans can extract up to 90% of the dissolved oxygen from the water flowing over their gills.
For comparison, human lungs extract only about 25% of the oxygen in each breath. Water holds far less oxygen than air does, so fish need that extreme efficiency just to stay alive. They accomplish this while barely appearing to work at it, simply opening and closing their mouths to keep a steady current flowing across the gills.
How Fish Stay at the Right Depth
Most bony fish have a swim bladder, a gas-filled sac inside the body that works like an internal balloon. By adjusting the amount of gas in this organ, a fish can match its overall density to the surrounding water and hover at any depth without constantly swimming. It’s the same principle as a diver’s buoyancy vest, except the fish’s version is built in.
There are two main designs. In some species (like trout and eels), a tube called a pneumatic duct connects the swim bladder to the gut, so the fish can gulp air at the surface to inflate it quickly. In others (like perch and bass), that duct is sealed off. These fish rely on a specialized structure called the gas gland, a patch of tissue laced with capillaries that can secrete gas directly into the bladder. A network of tiny parallel blood vessels surrounding the gas gland acts as another countercurrent exchange system, this time trapping gas to keep the bladder inflated even under high pressure at depth. To release gas and sink, a separate area of the bladder wall absorbs it back into the blood.
Swimming and Muscle Structure
A fish’s main propulsion comes from its body muscles, which are arranged in repeating segments called myotomes that stack along the body like nested cones. Within each segment, muscle fibers follow complex spiral paths from one segment to the next. This three-dimensional architecture acts like a gearing system: when the body bends, fibers across the entire cross-section shorten by roughly the same amount, producing smooth, powerful contractions.
Swimming works by sending a wave of muscle activation from head to tail. Each segment contracts in sequence, creating a backward-traveling wave of bending. As this wave moves down the body and reaches the tail fin, it pushes against the water and generates forward thrust. Different species modify this basic pattern. Eels undulate their whole body, while tuna concentrate most of the bending near the tail for high-speed cruising.
A Heart Built for Water
The fish heart is simpler than a mammal’s. It has four chambers arranged in a line: the venous sinus (a collecting chamber), the atrium, the ventricle, and the bulbus arteriosus (an elastic outflow section). Blood moves in a single loop. The heart pumps it to the gills, where it picks up oxygen, then it flows directly to the rest of the body and returns to the heart. Mammals, by contrast, use a double-loop system with a four-chambered heart that sends blood to the lungs and body separately.
Because fish blood passes through the narrow gill capillaries before reaching the body, blood pressure drops significantly by the time it arrives at the organs. This is one reason fish are generally less active than mammals of the same size. It’s also why the exceptions are so interesting.
Cold Blood, With Exceptions
Most fish are ectothermic, meaning their body temperature matches the surrounding water. But several predatory species have evolved the ability to keep parts of their body warmer than the environment. Tunas, billfishes (like marlin and swordfish), and some sharks, including makos and great whites, all have a form of regional endothermy. They use networks of intertwined arteries and veins called retia mirabilia as heat exchangers. Warm blood leaving the muscles passes its heat to cool blood arriving from the gills, keeping the heat trapped in the tissue instead of losing it to the water.
In lamnid sharks, heat from the red swimming muscles is even redirected to warm the brain and eyes, sharpening vision and reaction time during deep, cold-water dives. This internal heating is one reason these predators can sustain burst speeds and hunt effectively in frigid water that would slow other fish down.
Managing Salt and Water
Fish face a constant battle to keep their internal salt concentration stable, and the strategy flips depending on whether they live in fresh or salt water.
A freshwater fish’s blood is saltier than the water around it. Water floods in through the gills by osmosis, and salt leaks out. To cope, freshwater fish never drink. They produce large volumes of very dilute urine to dump the excess water, and specialized cells in their gills actively grab sodium and chloride ions from the water to replace what’s lost.
A saltwater fish has the opposite problem. The ocean is saltier than its blood, so water is constantly being pulled out of its body while salt pours in. Marine fish drink seawater continuously, absorb the water through their gut, and then rely on chloride cells in the gills to pump the excess salt back out. Their kidneys produce very little urine to conserve water. Some marine species have such reduced kidneys that they lack the filtering structures (glomeruli) found in most vertebrates.
Sensing the Invisible
Fish have a sense that no land animal shares: the lateral line system. Running along each side of the body (often visible as a faint line of tiny pores), it detects movement and pressure changes in the surrounding water. The sensors are clusters of hair cells called neuromasts. Each hair cell has a bundle of tiny projections, a single tall one and several shorter ones, embedded in a jelly-like dome called a cupula. When water movement deflects the cupula, the hair cell bundles bend and generate a nerve signal.
This gives fish a detailed, real-time map of water disturbances around them. They can sense the wake of a nearby fish, detect a predator’s approach in complete darkness, and navigate around obstacles by feeling the way water flows over their own body and bounces off surfaces. Schooling fish rely heavily on this system to coordinate their movements, which is why hundreds of fish can turn in apparent unison without colliding.
Vision Underwater
Fish eyes work differently from yours because water and the outer surface of the eye have nearly the same refractive index. On land, the cornea does most of the focusing, but underwater it becomes optically useless. Fish compensate with a nearly spherical lens that has a gradient of refractive index from its outer edge to its core, bending light sharply enough to focus an image on the retina. Instead of changing the lens’s shape to focus (as your eye does), a fish eye moves the entire lens forward or backward, more like adjusting a camera lens than flexing a contact.
The Slime Layer
That slippery coating on a fish’s skin is not just for reducing drag. Fish mucus is a frontline immune barrier packed with bacteria-killing compounds, including lysozyme (an enzyme that breaks down bacterial cell walls), proteolytic enzymes that digest foreign proteins, antimicrobial peptides, and even antibodies. These defenses are always present in the mucus, providing constant, immediate protection against pathogens in the water. The slime also helps seal wounds, reduces friction during swimming, and in some species plays a role in chemical communication.
How Fish Rest
Fish do sleep, though it looks nothing like what you’d expect. They don’t close their eyes (most species lack eyelids) and they don’t lie down. Instead, fish enter a sleep-like state characterized by reduced movement, a slower response to disturbances, and a specific resting posture, often hovering in place or tucking into a crevice. This state is quickly reversible: a sudden stimulus snaps them back to full alertness. Some species, like parrotfish, secrete a mucus cocoon around themselves at night, possibly to mask their scent from predators while they rest.