How do fish extract oxygen from the water? With gills. Read this quick article on how gills work as well as other systems used by underwater creatures to get O2.
As it happens, there's plenty of dissolved oxygen in most of the planet's seas, lakes and rivers, though our air-breathing lungs simply can't process it. But the world's water dwellers have evolved several other methods for accessing oxygen in water, experts told Live Science.
An ancient technique
Some animals such as jellyfish absorb the oxygen in water directly through their skin. A gastrovascular cavity inside their bodies serves a dual purpose: digesting food, and moving oxygen and carbon dioxide around, Rebecca Helm, an assistant professor at the University of North Carolina, Asheville, told Live Science.
In fact, Earth's earliest forms of microbial life that used oxygen obtained it the same way as jellies do — through diffusion. This form of respiration likely appeared around 2.8 billion years ago, "sometime after cyanobacteria started pumping oxygen into the atmosphere," according to ocean scientist Juli Berwald, author of "Spineless: The Science of Jellyfish and the Art of Growing a Backbone" (Riverhead Books, 2017).
"Because they only have an outer cell layer and an inner cell layer and their insides are jelly and don't have cells, they don't need as much oxygen as animals that have actual tissues on the inside," Berwald told Live Science in an email.
However, there are also drawbacks to "breathing" through diffusion.
"It's much slower than using a circulatory system to bring oxygen to far reaches of the body. That probably means that there's a limit on how big jellyfish can grow," Berwald added.
Breathing through oxygen diffusion over the body surface is also found in echinoderms — a group of marine animals that includes starfish, sea stars, sea urchins and sea cucumbers.
Sea stars absorb oxygen as water flows over bumps on their skin called papulae, and through grooves in other structures called tube feet, invertebrate zoologist Christopher Mah, a researcher with the Smithsonian National Museum of Natural History in Washington, D.C., told Live Science.
Some types of shallow-water sea cucumbers, however, have a different type of specialized adaptation for breathing: a respiratory "tree" structure located in the body cavity near the anus. As the cucumber's rectal opening sucks water into its body, the respiratory tree extracts oxygen and expels carbon dioxide.
"It literally breathes out of its ass," Mah said.
A "basic blueprint"
In fish, gills have proved to be a successful system for respiration, using a network of blood vessels to draw in oxygen from flowing water and diffuse it through gill membranes, according to the Northeast Fisheries Science Center.
Across most fish, gills have "the same basic blueprint," Solomon David, an assistant professor with the Department of Biological Sciences at Nicholls State University in Louisiana, told Live Science.
"They're made to have this countercurrent exchange of gas — pull oxygen out and release waste," David said. When fish gape their mouths, they create a current of water flowing over their gills. Reddish, highly vascularized tissue sucks out oxygen and expels carbon dioxide, "kind of like capillaries in our alveoli," he said.
However, gills aren't exactly one-size-fits-all. Their structure can vary between species to suit their oxygen needs, according to David. The gills of a fast-swimming tuna, for example, will vary somewhat from those of a fish that's a lie-and-wait predator, such as an alligator gar.
"If you're an active predator that's on the go all the time, you're going to have different gills for higher oxygen demands," David said.
Gill shape can even vary between individuals of the same species, depending on oxygen conditions in the water where they live, he added. Studies have shown that fish can adapt their gill morphology when their watery habitat becomes polluted; over time, their gill filaments become more condensed, to resist the contaminants in the water.
Some aquatic amphibians also have gills — branching structures that extend outward from their heads. This is a larval trait in amphibians that disappears as most species mature, but aquatic salamanders like sirens retain these external gills into adulthood, Kirsten Hecht, an aquatic ecologist with the School of Natural Resources and Environment at the University of Florida, told Live Science in an email.
Lungfish — a group of fish that breathe air as well as water using a modified swim bladder — also have external gills when they're young, "but almost all lungfish species lose them before reaching adulthood," Hecht said.