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Watch this sea star bounce to get around

TAMPA, FLORIDA—People break into a run to speed up; sea stars bounce. Until now, researchers thought these marine invertebrates simply crawled along the rocks and sea bottom. But researchers have now discovered this movement is not peculiar at all: At least five species of sea stars bounce when startled or hungry, suggesting this behavior is widespread, they reported here today at the annual meeting of the Society for Integrative and Comparative Biology.

Sea stars never move very fast, but bouncing is akin to a plodding human breaking into a full-tilt sprint, the researchers discovered. The animals rely on scores of tiny hydraulic “feet” that stick out underneath them. Usually, fluid fills and empties the podia at random to slide the starfish forward. For bouncing, the podia get in sync, with each third of them all filling at one time while the rest are swinging forward. To get a sense of how this is done, imagine having three legs and trying to run. One leg steps forward and the other two push to slide forward to keep up and the sea star moves much, much faster, with podia involved in one of every three steps.

The first starfish studied, Protoreaster nodosus, is a little sluggish, as its podia almost completely empty between bounces, causing the sea star to collapse onto the group, so it takes more energy to bounce. But podia in another, Luidia clathrata, stay stiffer, for a quicker recovery and faster bouncing, the team reports. That species is five times faster than the Protoreaster.

Though even Luidia still can’t bounce fast enough to get away from a hungry fish, it may be able to out-bounce predatory snails and cannibalistic sea stars or chase down a slow-moving clam for dinner.