Eyeless Urchins "See" With Spines

[n2doc]

Matt Kaplan
for National Geographic News
Published February 5, 2010

The spiky body of a sea urchin acts as one big, spine-covered eye, confirms a new study that tested how well urchins can see.

Sea urchins, like their close relatives the sea stars (starfish), don't technically have eyes. Instead, the ball-like invertebrates detect light striking their spines and compare the beams intensities to get a sense of their surroundings.

(Related: "Sea Urchin Genome Reveals Striking Similarities to Humans.")

To explore urchins' visual capabilities, Sonke Johnsen and colleagues at Duke University collected 20 Strongylocentrotus purpuratus sea urchins from the wild and tested their reactions to sets of black disks.

"The urchins were really fussy to deal with. Some just wouldn't move, like deer in headlights … if you can imagine a deer as a spiked ball," Johnsen said.

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http://news.nationalgeographic.com/news/2010/02/100205-sea-urchins-spines-eyes/