What do scallops do with all those eyes?

Scallops have hundreds of remarkable blue eyes (blue dots around the edge of the shell in the image).

 Their eyes are quite sophisticated, combining a lens and mirror to focus an image on a retina (which sits between the lens and retina) [1]. They appear to be able to focus a sharp enough image to have about a 2° angular resolution [2,3]. For comparison, that's about the size of one's thumb (while, my thumb) held at arm's length. Not great, but quite respectable for a bivalve. 

Clams are not particularly brainy, so what do they do with all these eyes? They can clearly see one coming when they're sitting in a tank. They clam up quickly. But lots of animals have shadow responses without sophisticated eyes. 

Speiser and Johnsen [3] tested clever idea: perhaps these filter feeders use their eyes to judge whether conditions are right for filter feeding. Scallops, like most clams, make a living by filtering nutritious particles out of the water. Perhaps they use their eyes to see when there are enough particles flowing fast enough to be worth expending the energy to pump water through their gills. To test this they played videos of particles flowing to the scallops and watched whether they opened up. 

It turned out that whether they opened depended on whether there were particles, how many, and how fast they were moving in the way they predicted based on the speed and resolution of the eyes. So it looks like this sophisticated visual system might be used to detect flowing particles. Perhaps even more interestingly, what we'd normally see as limitations on the eyes – their slow response and low resolution – might make it unnecessary to have sophisticated neural processing.

I remain curious about other possible functions [4]. Why would they need so many eyes for a particle or a shadow sensor? Could they use their eyes to detect distance as well like we use our binocular vision? Some simple calculations suggest an ~3 cm diameter scallop (such as the one shown) might be able to judge distances out to ~20 cm.

1. Land, M.F., Image formation by a concave reflector in the eye of the scallop, Pecten maximus. The Journal of Physiology, 1965. 179(1): p. 138-153.
2. Speiser, D.I. and S. Johnsen, Comparative morphology of the concave mirror eyes of scallops (Pectinoidea)*. American Malacological Bulletin, 2008. 26(1-2): p. 27-33 DOI: http://dx.doi.org/10.4003/006.026.0204.
3. Speiser, D.I. and S. Johnsen, Scallops visually respond to the size and speed of virtual particles. The Journal of experimental biology, 2008. 211(Pt 13): p. 2066-70 DOI: http://dx.doi.org/10.1242/jeb.017038.
4. Hamilton, P.V. and K.M. Koch, Orientation toward natural and artificial grassbeds by swimming bay scallops, Argopecten irradians (Lamarck, 1819). Journal of Experimental Marine Biology and Ecology, 1996. 199(1): p. 79-88 DOI: http://dx.doi.org/10.1016/0022-0981(95)00191-3.