The menotactic orientation of the prosobranch mollusc Littorina littorea

Abstract

The menotactic orientation of the prosobranch mollusc Littorina littorea to a one stripe pattern (Fig. 2) has been investigated under open-loop and closed-loop conditions. Results: 1) Under open-loop conditions the animals try to compensate for an angular deviation of the pattern from the menotactic angular position by turning movements of, on the average, constant angular velocity (Fig. 4). 2) The angular velocity $\dot \Theta$ depends on this deviation according to the $\dot \Theta$-characteristic (Fig. 5). The orientation behaviour under closed-loop conditions can be directly derived from the $\dot \Theta$-characteristic. Within experimental limits the snail does not show an “optomotor” response. 3) To patterns of wedge-shaped intensity distribution the snails orientate at a small angle relative to the edge of the pattern (Fig. 3). The orientation angle of the animals to this pattern is much less variable than the orientation angle to a single light source. The orientation behaviour shows that the intensity gradient of the pattern is important for the orientation of the animal. 4) A normally distributed noise process is superimposed on the average orientation direction. The variance of this noise process is the same for the orientation under open-loop and under closed-loop conditions (Fig. 7). In both cases the power spectrum of the noise process contains pronounced maxima at equally spaced frequencies (Fig. 8).