If you think it’s impressive when a dog rolls over and plays dead or a lizard regenerates its lost tail, the sea squirt Polycarpa mytiligera has a trick that will really blow your mind. When threatened, it eviscerates itself and squirts three-quarters of its digestive tract out of its body. After shriveling up and playing dead with its guts hanging out, it can regenerate all the organs it lost in just a couple of weeks.
Sea squirts are tube-shaped invertebrates that attach themselves to rocks, docks, coral reefs, and sometimes even hard-bodied animals like crabs, hanging out and filtering their food out of the water. P. mytiligera and its cousins are some of the most abundant sea squirt species in the world, and while zoologists Noa Shenkar and Tal Gordon were studying the animals in the Red Sea, they discovered the squirt’s regenerative abilities by accident. Some of the squirts they touched seemed to throw something at them and then shrink. The researchers thought they’d killed the squirts by handling them and decided to hang on to the bodies. When they checked in on the specimens a few days later, they were quite alive and on the mend.
Searching through older research, Shenkar and Gordon found that the some squirts’ ability to self-eviscerate and survive was already known—one species is even named Styeloides eviscerans—but not well studied and considered abnormal. To see how common the behavior is and how the animals pull it off, the pair started squishing sea squirts living on the underside of a dock in Israel. They gently squeezed 66 squirts and found that nearly half of them spilled their guts in less than a minute. They then tagged some of the animals to keep tabs on them, and over the next few weeks, culled and dissected them to see what was going on inside their bodies.
The squirts eject their guts by rupturing their filtering organ, called the branchial sac, and pushing their stomach and other parts of their “gut loop” out through the siphon that acts like their mouth. The squirts then contract their bodies and shut their siphon tight, making them look dead and shriveled.
For the next couple of days, the squeezed squirts lay like this, but eventually opened their siphons part way and responded slightly when touched. A week after being squeezed, their siphons were fully open and they responded normally to touch.
Meanwhile, their bodies were hard at work regenerating their ejected organs. The squirts that the scientists dissected 12 days after squeezing had completely new guts inside them that contained bits of digested food and even feces, showing that the new organs were up and running just fine. Another week later, the squirts had also rebuilt their branchial sacs.
It’s quite a feat, and while it seems like an extreme reaction to being jostled, it’s an effective defense. When Shenkar and Gordon offered the eviscerated guts from another group of squirts to hungry triggerfish and pufferfish, which hunt around the same reefs the squirts live on, the fish mostly passed on the free meal and those that took a bite quickly spit it out. The scientists think that if the squirts are bitten by a fish, dramatically faking their own death startles the predators, while contracting their bodies and leaving their guts—which the fish don’t find tasty—floating around protects them from further attacks.
Shenkar and Gordon note that while sea squirts and humans couldn’t appear to be more different, we’re both members of the phylum Chordata and share a lot of basic biochemical and cellular processes. They think that studying P. mytiligera’s regenerative powers could eventually lead to new techniques for treating organ injuries in people.