Gastric eversion is a remarkable biological process observed in various animal species. This phenomenon involves an animal turning its stomach inside out, expelling it partly or entirely from its body. While it may seem counterintuitive or even harmful, this unique ability serves several distinct purposes in the natural world.
What is Gastric Eversion?
Gastric eversion refers to the act of an animal expelling its stomach through its mouth, effectively turning the organ inside out. When this occurs, the internal lining of the stomach becomes visible outside the animal’s body. The visual effect is striking, as the soft, often pinkish or reddish stomach tissue hangs outside the animal’s mouth. This process is distinct from typical vomiting, where only the stomach’s contents are expelled. Instead, the entire stomach organ is inverted and pushed outwards.
How Animals Evert Their Stomachs
The physiological mechanism behind gastric eversion involves a coordinated series of muscle contractions and changes in internal pressure. Animals capable of this feat possess specialized musculature in their esophagus and stomach, allowing for this extreme movement. The stomach is often loosely attached, enabling it to be everted or inverted with relative ease. For instance, in frogs, a highly mobile and muscular esophagus facilitates the rapid projection of the stomach from the mouth.
Why Animals Evert Their Stomachs
Animals employ gastric eversion for various reasons, including expelling indigestible items, defense, and external feeding. Many species, such as sharks and frogs, use this action to get rid of materials they cannot digest, like bones, shells, or plastic. This allows for a thorough cleaning of the stomach lining, removing any debris or parasites that might be clinging to it.
For some animals, gastric eversion serves as a defense mechanism. Sea cucumbers, for example, can eject their internal organs, including parts of their digestive tract, to entangle or frighten predators. This self-evisceration can include sticky tendrils of intestines, which may even break off to allow the sea cucumber to escape. In certain frog species, expelling the stomach can deter threats by making the frog appear larger or by secreting unpleasant substances.
Starfish utilize gastric eversion for an entirely different purpose: external digestion. When a starfish encounters prey, such as a clam or mussel, it can push its stomach out through its mouth and directly into the shell opening of its prey. Digestive enzymes are then secreted onto the prey, allowing the starfish to digest the soft tissues externally before retracting its stomach back into its body.
After Eversion: Recovery and Impact
After performing gastric eversion, animals typically retract their stomachs back into their bodies relatively quickly. For instance, a frog can retract its everted stomach in as little as 0.3 seconds. Once the stomach is back in place, these animals can resume normal feeding and digestive functions.
The process generally does not cause long-term harm or significant stress to the animal. Sea cucumbers, which may lose parts of their intestines during defense, can even regenerate these organs within a matter of weeks. This ability highlights the efficiency and remarkable regenerative capacity associated with this unique biological strategy.