Scientists already knew starfish have superpowers. They can regenerate entire lost limbs or organs; some can even regrow a whole body from one arm. And these animals have just revealed another bizarre ability. To two Danish students, it first appeared as the power to really wreck an experiment. At the University of Southern Denmark, students Frederik Ekholm Gaardsted Christensen and Trine Bottos Olsen were asked to tag some starfish. The task was simple: inject the Asterias rubens with microchips, the same kind that veterinarians implant in pet dogs. This would let researchers easily identify individual starfish later on. The technique had already been used successfully in sea urchins. Starfish came to the university from local fishers who had caught them by accident. The students injected the tags into the animals as directed. But within days, those same tags showed up at the bottom of the tank. Somehow, the starfish were expelling the foreign objects from their bodies. When the students witnessed the act, it was a little like a magic trick—one moment a magician's hand is empty, and the next she's holding a bird. The starfish pushed the tags out the ends of their arms, straight through the skin. To find out more, the students and professor Daniel Levitis set up some experiments. First they injected tags into 53 starfish and scanned them each day to see how many of the tags were still there. In less than 3 weeks, all the tags were gone. Then the team tried injecting the starfish with little magnets instead of microchips. Once the magnets were in place, Trine tugged out a strand of her long hair and attached a matching magnet to its end, creating a delicate pendulum. By dangling this pendulum over a starfish and seeing where it was attracted, the researchers could locate the injected magnet inside an animal's body. (The starfish, seemingly more bothered by the magnets than the original tags, shed them much more quickly.) Tracking the magnets' locations over time revealed that although they always emerged from the arm tips, they didn't travel there directly. Instead foreign objects seemed to move randomly within a starfish's body before eventually winding up at the end of an arm. Some items visited more then one arm on their journeys. Finally, a small number of starfish were given ultrasound scans. The researchers could now see that the objects were moving inside the starfish's coelom, the body cavity that extends into its arms. They sometimes noticed a starfish constricting its body right next to the site of the magnet; this seems to be how they push the objects around. The trick is vaguely reminiscent of a splinter popping out of a person's finger—but much, much more impressive, says Daniel Levitis. Our muscles put pressure on a splinter, and it simply squeezes back through its entry wound. But starfish can start with a large object that's "lodged well inside the animal," he says, and move it all the way across the body before expelling it. Levitis plans to keep studying the starfishes' weird power. Already, other scientists have heard about the discovery and contacted him to ask about collaborating. Plenty of questions remain: How large an object can a starfish expel? How does it control the movement of the object? Why use the arm tips? Another question is why this ability exists at all. Levitis thinks it makes sense. "Starfish have rough lives," he says. "They frequently have sizable wounds or lose whole limbs, and they live with all sorts of detritus around them." He guesses that they may frequently end up with foreign objects inside their bodies, such as rocks or shells. "But they are also masters of regeneration and repair," he adds. "The ability to eject hard objects that get inside them is the latest in a long list of amazing healing tricks these animals can do." https://www.youtube.com/watch?v=Y2KrOyugrxs&feature=youtu.be&t=1m30s Image and video: University of Southern Denmark.

Olsen TB, Christensen FE, Lundgreen K, Dunn PH, & Levitis DA (2015). Coelomic Transport and Clearance of Durable Foreign Bodies by Starfish (Asterias rubens). The Biological bulletin, 228 (2), 156-62 PMID: 25920718