Several animals can absorb oxygen through their rear end, and they span a surprisingly wide range of species. Sea cucumbers, turtles, dragonfly larvae, and certain fish all use some form of anal or cloacal breathing, either as their primary respiratory method or as a backup when conditions demand it. Researchers have even demonstrated that mammals, including pigs and mice, can absorb oxygen through the intestine, and a first-in-human safety trial was completed in 2025.
Sea Cucumbers: Built to Breathe This Way
Sea cucumbers are the poster animal for anal respiration. They have a dedicated organ called the respiratory tree, a branching structure attached to the cloaca (the single opening that serves as both anus and exit point for waste). The animal rhythmically pumps seawater in and out of this opening, and gas exchange happens across the thin walls of the respiratory tree. This isn’t a backup system. It’s how sea cucumbers breathe full time.
The pumping action creates an unexpected vulnerability. Pearlfish, slender eel-shaped fish that lack scales, exploit this cycle by waiting for a sea cucumber to open up for a breath, then swimming inside to use the body cavity as shelter. In some cases the relationship is harmless to the host, but certain pearlfish species act as parasites, feeding on the sea cucumber’s internal organs from the inside.
Turtles That Overwinter Underwater
Many freshwater turtles spend the winter hibernating at the bottom of ponds and rivers, buried in mud beneath a layer of ice. They can’t surface to breathe, so they rely on cloacal respiration: absorbing dissolved oxygen from the water through highly vascularized sacs near the cloaca called cloacal bursae.
Because turtles are cold-blooded, their metabolism drops dramatically in cold water, and their oxygen needs shrink to match. The small amount of oxygen that diffuses in through the cloaca is enough to keep them alive for months. If oxygen levels get critically low, turtles can switch to anaerobic metabolism (generating energy without oxygen), though this produces lactic acid that builds up in the body and puts a time limit on survival.
One species takes this ability further than any other. The Fitzroy River turtle of Australia can obtain up to 70% of its total oxygen intake through its cloaca, even when it has access to the surface. Only about 30% comes from its lungs. For most other turtle species, cloacal breathing is a winter survival strategy. For the Fitzroy River turtle, it’s the primary way of getting oxygen year-round.
Dragonfly Larvae and Their Rectal Gills
Dragonfly nymphs are aquatic, living underwater for months or even years before emerging as adults. They breathe using a rectal gill, a structure that evolved from the final section of the colon. The nymph draws water into its rectal chamber, extracts oxygen across the gill surfaces, then expels the water. That expulsion doubles as jet propulsion, shooting the nymph forward to escape predators.
As nymphs approach metamorphosis, their breathing strategy shifts. Younger nymphs rely almost entirely on the rectal gill underwater, but older nymphs begin accessing atmospheric air. Some skim the water’s surface to pull air into their rectal chamber, while late-stage nymphs can swallow air bubbles directly into their gill’s internal basket. This progressive shift prepares them for the transition to air-breathing adult dragonflies.
Loaches: Fish That Gulp Air Into the Gut
The pond loach, a small freshwater fish common across East Asia, swallows air at the water’s surface and passes it through its digestive tract. Oxygen is absorbed across the thin walls of the posterior intestine, and the leftover gas exits through the anus. This makes the loach one of a handful of fish that use the intestine as a breathing organ.
What makes loaches unusual is that they perform this intestinal air-breathing even in well-oxygenated water. It’s not purely an emergency response to low oxygen. Under hypoxic conditions (when dissolved oxygen drops), the blood vessels in the intestinal lining expand toward the surface and the tissue thins, reducing the distance gas needs to travel to reach the bloodstream. A related species, the large-scale loach, shares this posterior-intestine breathing ability. Both are considered bimodal breathers, using gills and gut simultaneously.
Mammals Can Do It Too, Sort Of
In 2021, researchers demonstrated that mice and pigs in respiratory failure could absorb oxygen delivered rectally. Both gas-form oxygen and oxygen-rich liquid pumped into the intestine improved survival, blood oxygen levels, and behavior in animals that were otherwise suffocating. The intestinal lining, it turns out, is thin and well-supplied with blood vessels, making it a viable surface for gas exchange when the lungs can’t keep up.
The idea was inspired by the natural ability of loaches and other animals. Researchers developed a technique called enteral ventilation, essentially delivering oxygenated liquid through an enema-like procedure. In the animal models, the approach was repeatable and showed no major complications.
In 2025, a first-in-human safety trial tested this concept in 27 healthy men, using a special oxygen-carrying liquid called perfluorodecalin delivered rectally. The trial confirmed that the procedure was safe, feasible, and well tolerated, clearing the way for future studies in patients who actually need respiratory support. Whether this technique can meaningfully improve oxygen levels in critically ill people is still unproven, but the safety foundation is now established.
Why So Many Species Evolved This Ability
The cloaca and intestinal lining share a key trait: they’re thin, moist, and packed with blood vessels. Those are the same properties that make lungs effective at gas exchange. In aquatic environments where oxygen can be scarce, or where surfacing to breathe is dangerous or impossible, animals that could extract even small amounts of oxygen through an alternative route had a survival advantage.
The strategy evolved independently in unrelated groups. Sea cucumbers, turtles, dragonfly larvae, and loaches arrived at similar solutions through completely separate evolutionary paths. The anatomy differs in each case (respiratory trees, cloacal bursae, rectal gills, intestinal walls), but the underlying principle is the same: any thin, vascularized tissue in contact with oxygenated water or air can serve as a respiratory surface.