If you ever end up on Jeopardy! and your clue is "This star-shaped marine creature has five arms," you may want to pause before you offer what seems like the obvious answer. That's because those appendages you see sticking out from the bodies of starfish aren't what they appear to be, according to University of Southampton researchers who studied the animal also known as a sea star for its five distinct points. Instead, "the arms of a starfish are not like our own arms, but more like extensions of the head," Jeff Thompson, co-author of the study published Wednesday in the journal Nature, tells the Guardian.
Scientists have been scratching their heads for some time on how echinoderms—ie, starfish, sea urchins, and other creatures with bodies split into five equal sections—evolved from the "bilaterally symmetric" body shapes, which feature two mirrored sides, that most other animals have. Adding to the confusion is that starfish begin life as larvae with twofold symmetry, then morph into their grown-up versions. Researchers found that genes switched on in the outermost layers of one particular type of starfish, known as Patiria miniata, matched genes in acorn worms and vertebrates that are activated in those creatures' heads.
Therefore, "I would say it's a mostly headlike animal with five projections, with a mouth that faces towards the ground and an anus on the opposite side that faces upwards," Thompson notes, per the Guardian. In an accompanying Nature op-ed, Thurston Lacalli of British Columbia's University of Victoria describes the creature as "a disembodied head walking about the seafloor on its lips—the lips having sprouted a fringe of tube feet, co-opted from their original function of sorting food particles, to do the walking."
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UC Berkeley professor Daniel Rokhsar, another study co-author, tells the Washington Post that we've believed the starfish's head extensions were arms simply because humans are used to thinking about bodies from a self-centered point of view. "We call them arms of the starfish because we're used to thinking of the things that stick out of the body as arms and legs," he notes. "That's our own anthropomorphic perspective." Now scientists need to contend with another mystery: when and how the starfish lost its torso. "Ultimately, this could enable us to uncover the sequence of evolutionary changes that gave rise to the headlike body plan," says Imran Rahman, a paleontologist not involved in the study. (More discoveries stories.)
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