The Animals That Breathe Through Their Anus: A Biological Marvel
When we think of breathing, the immediate images are lungs or gills. Yet, in the animal kingdom, survival dictates rules that defy imagination. Among the most bewildering adaptations is anal breathing, a strategy where the rectum serves as a backup respiratory organ. Here is a list of the main animals involved and the fascinating science behind this phenomenon.
The Champions of Enteral Respiration
Contrary to popular belief, this is not an isolated curiosity. This ability is found in several aquatic species, primarily as a way to cope with low-oxygen environments.
· Freshwater Turtles: These are the superstars of this breathing mode. Species like the Fitzroy River turtle in Australia or the painted turtle of North America can stay submerged for months under ice or in stagnant water. They use highly vascularized sacs in their cloaca, called cloacal bursae. By pumping water through muscular contractions, they extract oxygen directly into the blood across the thin walls of these sacs.
· Sea Cucumbers: These echinoderms possess a unique respiratory system linked to their digestive tract. They draw in and expel water through their anal orifice to supply branched organs, the respiratory trees or water lungs, located inside their body. There, gas exchange occurs with the coelomic fluid.
· Loaches and Certain Catfish: The weather loach is known for swallowing air at the surface and expelling it through the anus. However, more in-depth research on a small Southeast Asian catfish has demonstrated true enteral respiration. When water is hypoxic, these fish swallow air, and oxygen is absorbed by the highly vascularized intestinal epithelium, with residual gas evacuated through the anus.
· Dragonfly Larvae: These fearsome aquatic predators have rectal respiration. They actively pump water into a branchial chamber located in their rectum, where internal gills capture oxygen. The forceful expulsion of this water also serves as jet propulsion to escape danger.
How and Why Such an Adaptation?
"Enteral respiration" is not a primary system, but a backup survival mechanism of remarkable efficiency. The principle is simple: the intestinal and rectal mucosa in these species is exceptionally rich in blood capillaries. It can therefore serve as a gas exchange surface.
This system activates under extreme conditions. For a painted turtle hibernating under the ice of a pond, where the water is completely deprived of oxygen, the lungs fill with water and cease to function. Cloacal respiration then becomes vital, allowing it to slow its metabolism to a minimum while capturing what little dissolved oxygen remains.
A Revolutionary Lead for Human Medicine?
This strange faculty doesn't just interest biologists. It has opened an unexpected avenue of medical research. In 2021, Japanese scientists demonstrated that mammals (mice and pigs in a laboratory setting) could, in acute respiratory distress, absorb oxygen administered rectally via an enema of perfluorocarbons—liquids capable of dissolving large quantities of oxygen.
This experimental treatment, a form of "liquid enteral ventilation," significantly corrected the animals' hypoxemia. Researchers see it as a potential solution to compensate for ventilator shortages during major health crises, like the Covid-19 pandemic. Nature, once again, offers a model of adaptation that could save human lives.
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