Most kindergarteners can tell you that an animal eats with its mouth, not its butt.
One species of sea cucumber, however, didn’t appear to get the memo: Scientists have discovered that the giant California sea cucumber (Parastichopus californicus) actually uses its anus as a second mouth.
Scientists already knew that the marine invertebrate, which lives in the shallow ocean waters off the Pacific coast of North America, breathes with its butt.
Because they don’t have lungs, sea cucumbers rely on respiratory trees, a set of long tubes running down either side of the body with a lot of different branches. P. californicus is shaped like a hollow tube, with a mouth at one end and its anus at the other.
The respiratory trees receive oxygen when water is pumped through their anus using the muscles of their cloaca, an opening at the end of the intestinal tract. (Watch a video of sea cucumbers fighting with their guts—literally.)
The 20-inch-long (50-centimeter-long) animal is no slouch: It can pump 3.5 to 4 cups of water per hour through its anus, transferring the oxygen from the water into its respiratory trees, which then oxygenates its cells.
The sheer amount of water flowing into the anus got two invertebrate biologists thinking—since P. californicus sifts plankton and other small particles from water using its tentacles, could it do something similar using its anus?
Though “an animal is not expected to ingest food through its anus”—as William Jaeckle and Richard Strathmann note at the beginning of their study in the March issue of Invertebrate Biology—it turns out the answer is yes.
Their first hint that the sea cucumber anus was doing triple duty came from a structure called the rete mirabile, a set of blood vessels that connect the sea cucumber’s respiratory trees with its gut.
Initially, Jaeckle, of Illinois Wesleyan University, and Strathmann, of the University of Washington, thought that the rete mirabile was used to transfer oxygen from the respiratory trees to the gut. But if P. californicus were obtaining food via its anus, it would likely use the rete mirabile to transfer the food to the gut. (Watch a video of a hairy sea cucumber.)
To test their idea, the team fed several sea cucumbers radioactive algae, which also contained iron particles. The iron and radioactivity proved an easy way to trace the food as it traveled through the sea cucumber’s body. For instance, areas of the body with the highest concentrations of radioactivity would provide clues about which orifices the animal was using to eat.
Not surprisingly, the results showed that the sea cucumbers ate the algae through their actual mouths, which then traveled through their gut. (See pictures of colorful sea creatures.)
However, the researchers also found a high level of radioactivity when they looked at the rete mirabile. The only way that those blood vessels could have such a high concentration of radioactivity is if the animal was transferring food from the respiratory trees to the gut via the rete mirabile.
When the scientists looked at tissue samples from the sea cucumber under the microscope, they found even more hints that P. californicus was using its anus as a second mouth: They found small pieces of algae and iron in the respiratory trees near the anus. (Also see “Why Sea Slugs Dispose of Their Own Penises.”)
In addition, the sea cucumber’s respiratory trees had small, finger-like projections known as microvilli that are normally found in the gut and aid with nutrient absorption. This also indicated that P. californicus was absorbing food using its anus and respiratory trees.
The authors conclude that although they looked only for evidence of bipolar feeding—the more formal and perhaps polite term for eating with your butt—in one species of sea cucumber, many other species are likely to use this method of feeding.
Bottom line? Eating with your butt may not be all that unusual.