Behavioural responses to visual stimuli by the snail Littorina irrorata

Abstract

The visual capabilities of gastropod molluses and most other invertebrates possessing structurally simple eyes are poorly known. We studied vision in untrained marsh periwinkles ( Littorina irrorata) in the laboratory, using oriented movements toward test shapes as the response measure. This intertidal species is active when exposed at low tide, both during the day and at night, and it travels vertically on plant stems with a tidal rhythm. In detection tests, the estimated response threshold for a single vertical bar was 0.9°, while the response threshold for an equal-size horizontal bar was 2.4° or 3.7°, depending on bar position. Snails detected a 5°-wide bar in 4.3 1x of light and a 40°-wide square having about 95% reflectance (‘off-white’) on a white (100% nominal reflectance) background in 2800 1x. Discrimination tests revealed a strong preference for vertical bars over both diagonal and horizontal bars of the same width, but no preferences in several other situations. Various factors suggest that L. irrorata may see better than most other gastropods.