Are Sea Urchins One Big Compound Eye??

WHAT? How can sea urchins...SEE?
They don't got a head. And they don't got eyes!

....or DO THEY??

There are two terrific papers by Sonke Johnsen at Duke University in the Journal of Experimental Biology! One by Blevins & Johnsen on the tropical sea urchin Echinometra
and a recent one by Yerramilli and Johnsen on the purple sea urchin Strongylocentrotus which explores vision in these two species!
First, How did they test for vision in something that looks like a big, spiny purple ball??

They ran a series of experiments in this clever apparatus!

(modified from Yerramilli & Johnsen 2009)

Basically, the urchin is put into a big tank and light from a light box is emitted from the top.

The number of times and direction of the urchin relative to the tiny dark target were then recorded and calculated. So, there was experimental rigor and statistical analysis of the data to show that a pattern was present rather then random actions.

Based on the number of times the animal responded, whether via moving away or towards the target-it did so regularly and consistently.

They found that Strongylocentrotus purpuratus could perceive/detect items in a 10 degree diameter target range probably using sensory apparatus in the spines and possibly also on the test!! (see green arrows above)

S. purpuratus doesn't do well with smaller targets (e.g., 6.5 degrees) so 10 degrees is probably the lower limit of what this species can perceive.

The degree to which this species can perceive a target is consistent with the degree that light can be perceived by the SPINES in a manner similar to compound eyes! (Light represented below by yellow arrows hitting the spines!)
What's really cool is that the reaction of the animal to light seems largely tied to the spines. Closer examination of the pedicellariae (stalked pincer-like structures on the surface) and the test (i.e., the endoskeleton) revealed no evidence of the ability to register light.

Spacing of the spines can actually affect the ability of the animal to recognize targets...and the higher the density of spines, the greater the "spatial visual resolution"! The visual acuity of S. purpuratus is roughly equal to that of the chambered (Nautilus)and the horseshoe crab (Limulus) but is still pretty weak relative to the vertebrate eye.

So, what does the urchin use this sight for?? There are many possibilities...

FORAGING for food! (S. droebachiensis shown below)

(Images courtesy of Dr. Robert E. Scheibling!)

Also...possibly to escape from predators...

(image courtesy of Allison Gong, UCSC)

Or, for urchins such as Echinometra, they perceive dark holes as places to find SHELTER...
Or MAYBE it helps with this...