Fig. 1: ~3cm wide scallop (©Mickey von Dassow).
Scallops have hundreds of beautiful blue eyes around the edge of the shell, each of which can form a decent image using a combination of a lens, a focusing mirror, and retina1 . The eyes are the blue dots in the image at right. A close up of two of them is below left: you can just make out the pupils. Bivalves are not renowned for their intelligence, so their complex eyes are a puzzle.
What do scallops do with such a complex visual system? They appear to use their eyes both to sense potential predators, to find good habitats  and to decide whether the concentration of suspended particles and water flow rates are good for feeding (as determined with a neat experiment involving playing movies of particles flowing by to scallops [3)].
Animals with two eyes can see distance based on comparing the image in the two eyes. Can scallops, with their many eyes, detect object distance?
Fig. 2: Scallop eyes close up, and diagram of proposed experimental setup.
One could test this by seeing if their startle response (when they rapidly close their shell) depends on angular size or absolute size (see  for a related study on urchins). I (M. von Dassow) had a group of students try to do this by flashing power point slides at scallops (diagram at left). The slides flashed on different sized black squares at the scallops, with the monitor positioned at different distances from the scallop. That way they could independently vary the absolute and angular size of the square. Then they observed whether the scallop closed when the black square appeared. They found no effect of absolute size, but the tests were fairly limited, and - sadly - the original data set was lost by the end of the term.
A simple model
One can predict how close an object would have to be for the scallop to be able to detect differences in distance. Assume, as shown in the diagram at right, that an object sits at a distance x from the scallop, and another object sits at a distance b*x. For these two objects to be differentiated, the angle θ has to be greater than or equal to the angular resolution of the eye (A~0.035 radians according to [3)]). If the scallops half-width is r, then then the minimum value of b at which the scallop could resolve the difference is given by: $b=(r/x)*tan(arctan(x/r)+A)$. This expression goes to infinity at $x=r*tan(π/2-A)$, 43 cm for an ~3cm scallop as shown in the photo. Below that distance, the scallop ought to be able to resolve changes in distance (limited by the expression for b).