Giant clams (Tridacna spp.) have several hundred small pinhole-type eyes on the exposed mantle. They respond by withdrawing the mantle to movements of dark objects, even if these cast no shadow on the animal as a whole. I investigated this 'sight reaction' using black and white square-wave gratings whose phase abruptly changed so that the white areas became dark and vice versa. Gratings with periods of 13.5 degrees were ineffective, but gratings of 20.7 degrees caused partial retraction of mantles or siphons. This implies an acceptance angle for the best-resolving eyes of between 8.7 degrees and 21.8 degrees. A single black spot was effective if its angular diameter was 13.5 degrees but not 11.7 degrees. The mean threshold for the pure dimming of a large field was a decrease of 12.3%, but responses increased in strength up to a dimming of 35%. Anatomically the eyes are ca. 400 microm deep from aperture to receptors, the aperture has a mean diameter of 90 microm and the receptors are 25 microm across. This gives an angular acceptance angle for single receptors of 16.5 degrees, which is completely consistent with the behavioural measurements.