In 2019, Eliud Kipchoge finished a Vienna marathon in one hour, 59 minutes, and 40 seconds. This staggering time broke the two-hour barrier that most runners previously deemed impossible. However, some researchers weren't as surprised. Recent studies investigating if humans could maintain such a high pace for the length of a marathon had found that elite runners can take in twice as much oxygen as non-runners. And it’s likely that this superhuman ability played a role in Eliud’s victory. But when it comes to breathing efficiently, not even the best runners in the world can compete with the average fish.
That’s right— you’re looking at some of the best breathers on Earth; which actually makes sense when you consider how little oxygen there is in their aquatic environments. Fish can breathe in a variety of ways, but the most common is through the use of gills. These branching organs typically come in four pairs, all enclosed in gill chambers. These chambers are protected by opercle, or gill covers, which are just as essential to underwater breathing as the gills themselves.
When most fish take a breath, they first close these covers and take a gulp of water. Then, they open their opercle, creating a pressure differential that pulls water through the gills, which are composed of thread-like filaments spaced evenly along a gill arch. These filaments are covered in countless small blood vessels called capillaries, in addition to tiny extensions known as gill lamellae which further increase the gill’s surface area. When water passes over these capillaries, the membrane is thin enough for the fish’s red blood cells to pull dissolved oxygen from the water into the bloodstream. And just like when we breathe with our lungs, this process releases carbon dioxide, which passes out into the water through the open gill cover.
This technique only works underwater— on the surface, the pressure differential created by opening and closing gill covers isn’t strong enough to pull in sufficient air. But under the surface, it’s remarkably efficient. Inside the lamellae, blood flows in the opposite direction to the water, creating a counter-current system that optimizes gas exchange. In fact, gills can absorb roughly 75% of the oxygen passing through them; that’s twice the percentage of oxygen our lungs extract from a breath of air. Fish also breathe much more frequently than we do. The average adult human breathes 12 to 18 times a minute, while most fish pull water over their gills anywhere from 20 to 80 times a minute. Those numbers would raise even a marathoner’s eyebrows.
Thanks to this fast, frequent, and efficient breathing fish process far more oxygen than we do. It's also why some species can live at great depths. Bodies of water get most of their oxygen from the surface, where O2 dissolves into the water and begins circulating. But further down, there are oxygen minimum zones, with concentrations as low as 0.5 milligrams of oxygen per liter. To get the oxygen they need, fish living at these depths rely on increased gill ventilation and hearts that pump high volumes of oxygenated blood throughout the body.
Even with these tricks, sometimes there's still not enough oxygen to go around, forcing fish to adapt more extreme solutions. For example, the Australian lungfish lives in a habitat where the water level drops precipitously in August and September, making it almost impossible to survive with gills alone. Fortunately, these fish have gills and lungs. Using their long, thin limbs, they can lift their mouths above the surface and take deep breaths of air, allowing them to survive out of the water for several days. And other species of lungfish can survive above ground even longer in cocoons of mud and mucus.
Fortunately, for most fish these extreme adaptations aren’t necessary. After all, 71% of Earth is covered in H2O, giving these aquatic animals plenty of room to flaunt their gills’ skills.
