We may call someone gutless who's acting afraid. But certain coral-reef dwellers take gutless to a whole other level: they shoot their digestive tracts out of their bodies when they feel threatened. This seems to deter nearby fish from taking a bite. Even more amazing, though, is how quickly the gutless animals grow back their organs. Polycarpa mytiligera is a little tube-shaped creature called an ascidian, or sea squirt. It resides in tropical waters of the Indian and Pacific oceans. With its base glued firmly to a coral reef or other surface, it feeds by filtering particles from the passing current. Some sea squirts look like colorful vases arranged together on a reef. But Polycarpa mytiligera lives alone and lets other bits of sea life grow all over its body for camouflage. Scientists first noticed in the 19th century that some sea squirts have no guts. The animals are known for regenerating missing body parts, so it seemed that they must occasionally lose their digestive organs and grow them back. But how and why remained a mystery. To get some answers, zoologists Noa Shenkar and Tal Gordon of Tel Aviv University dove into the Red Sea. There, they visited Polycarpa mytiligera individuals growing on the bottom of a floating dock. One thing the researchers wanted to know was how commonly sea squirts squirt out their insides. The researchers gently squeezed the sea squirts with their fingers, pinching each individual just once for up to a minute. (They knew this could trigger a gut explosion because "the first few times the evisceration happened by mistake," Shenkar says, as researchers tried to pluck sea squirts from their homes.) Out of 66 squirts, 47% responded by shooting out their innards like a stomped tube of toothpaste. Since the animals often live on reefs where fish like to feed, they may need to defend themselves against an errant chomp. The pressure of something grabbing them is apparently enough to cause alarm. In the image above, the thing that looks like a plant is the camouflaged sea squirt. The noodly object floating above the scientist's index finger is a long piece of the sea squirt's digestive tract. When the sea squirts ejected their guts, they did so about 16 seconds after first being touched, on average. The length of ejected gut averaged about an inch, or half of the sea squirt's digestive tract. This is also roughly half the height of the whole animal.
The material ejected by one sea squirt, including its stomach, intestine, and part of its rectum. Having one's insides on the outside is an emergency situation for most animals, but the sea squirts weren't too bothered. Their entire metabolism depends on the organs they've just sacrificed, Shenkar points out. Yet as the scientists returned to check on the gutless animals, they saw that their bodies stayed tightly pinched together for a day or two, then gradually opened back up. After a week, the animals looked normal from the outside. After 12 days, scientists found entirely new guts in the sea squirts they dissected. These guts already had feces in them, showing the squirts were back to business as usual. So the animals were fine—but how good a defense was their drastic self-evisceration? Shenkar and Gordon tried feeding the ejected guts to several different species of aquarium fish. All of the fish were hungry, but every one refused to eat the sea squirt innards. Any fish that tried a taste spit it right back out. The researchers think that by expelling their bad-tasting innards when fish are nibbling nearby, sea squirts may drive the fish away. Polycarpa mytiligera isn't the only marine creature that can eject its organs and quickly regrow them. Sea cucumbers do the same thing. By studying these animals' powers of regeneration, Shenkar and Gordon write, we may learn more about how to treat organ injuries in humans. Or, at the very least, someone can use sea squirts to make a much less adorable version of Pixar's Inside Out. https://youtu.be/QPhtKogOzd0 Note: This post has been edited from an earlier version to incorporate comments from Noa Shenkar.Images and video: Shenkar & Gordon (2015).
Shenkar N, & Gordon T (2015). Gut-spilling in chordates: evisceration in the tropical ascidian Polycarpa mytiligera. Scientific reports, 5 PMID: 25880620