Stomach Eversion in Five Animals

Five Animals that Evert Their Stomachs

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Crown of thorns starfish (Photo: Wiki Commons).

It's Stomach Turning!

I didn't really plan on writing about stomach eversion until I began researching how owls form pellets and I ran across some interesting information about frogs and their ability to burp up their entire stomachs. I started to follow this thread of research and thought I'd share with you information about stomach eversion in five different animals that I found absolutely fascinating.  You'll probably never really use this information at polite cocktail gatherings but will make for great "one up" gross stories while drinking beer with friends.

So what exactly is stomach eversion? Eversion is the verb of evert, and in this case it refers to turning ones internal organs inside out, and in today's blog we're focusing on stomach eversion in five different animals that turn their stomachs inside out and why they do it.

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American bullfrogs can evert their stomachs (Photo: Wiki Commons).

Frogs (Getting rid of toxins)

I'm going to start with frogs, because this is where I started and became fascinated with the research. Species such as green frogs, the American bullfrog, and even the African clawed frog are all known to evert their stomachs. Stomach eversion is usually either a response to eating something toxic (sometimes undigestable) or it can be used as a defense strategy.  Researchers from the University of Halifax, Nova Scotia found that tadpoles don't have the ability to evert their stomachs, only adult frogs that have undergone metamorphosis and that are able to eat a bolus (big glob) of food. Apparently the researchers gave different emetics (barf inducing foods) to tadpoles and then frogs to see who yacked. When the adult frogs hurl, their stomach comes out of their mouths, turning inside out like little wee pockets, and dump the contents of their meal out. The frogs also have a hard-wired neurological response to this bump-n-dump and they lift an arm and brush off their stomach to clear it of food. After this obligatory brushing they swallow their stomach back down and carry on as normal. Awesome eh?

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Dart frog everting stomach (Phot: Auzre 89 on Dendroboard.com)

So why do frogs do stomach eversion? Usually it's not because researchers are feeding them barfy foods, but because they've eaten toxic insects and they need to do the "lateral cookie toss" to get rid of the toxins.

Sharks (Getting rid of undigestable stuff)

Many different types of sharks have been observed doing stomach eversion, this includes Caribbean reef sharks, lemon sharks, shortfin mako sharks, and tiger sharks . Much like in frogs, the stomach eversion in sharks helps to remove undigestable food and materials such as plastic, bones, and even parasites. Below is a video that takes you through the process, first in regular speed and then in slow motion. Notice that the stomach eversion in this video, of this shortfin mako, is in response to swallowing a fishing lure and line.

Rays (Stomach cleansing, no it's not a fad diet)

Rays are closely related to sharks so it's no surprise that they do the "technicolor yodel" too (I can't believe all the synonyms for barf I found online).  Specifically, researchers studied the thornback ray (Raja clavata), which can rinse its stomach by doing the "gastric eversion boogie" just like sharks. I don't have a good video of this, but you can read about it in The Journal Nature online. I couldn't find much more research on ray stomach rinsing but I'm sure some poor grad students somewhere is working on it.

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Thornback ray (Photo: Wiki Commons)

Starfish (Eating out)

Now here's one of my favorites, because I often teach about starfish anatomy and physiology to my students. Starfish don't have much going for them in the brain department, but they are amazingly fierce walking stomachs. Each arm on a starfish is "armed" with thousands of tiny tube feet that move and attach through hydraulic pressure and they can work together to make a pretty powerful pull when challenged with a closed clam shell. When it encounters a clam the sea star surrounds the clam with its arms, sometimes moving it with its tube feet towards its mouth, and then it pulls like WWE wrestler, using its tube feet, on either side of the clam shell (see video below).

Eventually the clam gets tired of keeping its shell closed (its internal adductor muscles give up), and it opens a tiny crack. This crack is enough for the starfish to then evert its stomach through its mouth and into the shell. The starfish then secretes digestive enzymes, partially digests the clam, and slurps it all up (Note: not all starfish do this, some eat their prey alive...not sure which is a better way to go if I was an animal being eaten by a starfish...but then again you have to be pretty slow and dumb to be caught by a starfish).

I like to gross students out by saying starfish feeding would be sort of like if you put your PB and J on the lunch room table, took your stomach out and laid it on top of the bread, chatted with your friends while you digested, and then put your stomach back in. Taking someone for a dinner date would be entirely different too.

Sea Cucumbers (Keep from being eaten)

Are you getting the idea that stomach eversion isn't just for "flashing the hash," "decorating the dunny," or "barking at the ants?" It can have other uses too and the sea cucumber is the ultimate example of this. Sea cucumbers are echinoderms (in the same family as starfish and sea urchins). They are not underwater vegetables! Sea cucumbers are found in oceans around the world, and they are covered with leathery skin. They can range in size from a few inches to nearly a foot and a half. Sea cucumbers are scavengers and feed on algae, waste, and invertebrates. To me they look like tongues (or detached and deflated male reproductive bits) that have taken to crawling around on the ocean floor. To be fair there are a few that get quite large and that are quite colorful and distinctive.

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Sea cucumber (Photo: Wiki Commons).

Sea cucumbers remind me of those water-snake toys you get as a kid, and the ones I've handled feel about like that too.

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Water-snake toys (Photo: Amazon.com)

Looks aside, sea cucumbers have the ability, when threatened by a predator, to spew their toxic internal organs (including the stomach) from their butt. The organs detach, entangle the predator, and allow the sea cucumber to crawl away. The name for spewing internal organs as a defense is called "autotomy." Depending on the threat their organs don't always detach but they can. If they do detach then eventually the guts will regenerate, so don't feel too sorry for the sea cucumber.

Why Don't Humans Evert Their Stomachs?

The emetic response, or "barfing", "hurling", "vomiting", "ralphing", "bending and sending", and "barking like a seal" (there are also 333 more synonyms) occurs in many species, including humans. Fortunately for humans we have an esophageal sphincter (such a great word) that allows food back "up" for vomiting but doesn't allow the stomach to invert or go with it, which is a good thing. Throwing up after a eating bad food would only be made worse if you had to evert your stomach and clean it too. That picture is now in my mind...ick.

No matter how you think of it, stomach eversion is pretty gross but highly functional from cleaning out toxins and eating clams to entangling predators. Regardless of how gross it seems to us it has an important role in nature. Barf on my animal friends, barf on.

 
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About Infinite Spider

I am currently the Education Program Coordinator at the Smithsonian Environmental Research Center. I have also been a curriculum developer for the Smithsonian Science Education Center and a contract curriculum writer for the Discovery Channel. In my spare time I am a blogger here at "The Infinite Spider" (Infinitespider.com), a science and nature blog for naturalists and outdoor educators. I love rowing crew, birding, hiking, kayaking, and being outdoors. My undergraduate degrees are in Environmental Science and Philosophy, and my graduate degree is in Biology. Currently I live in Annapolis, MD.