Eye Movements in the Rock Crab Pachygrapsus Marmoratus Walking Along Straight and Curved Paths

1. Freely walking rock crabs (Pachygrapsus marmoratus Fabricius) were videotaped from above in a quasi-natural environment. The angular position of the long axis of the body and of both eyes in the horizontal plane were studied. 2. Compensatory eye. movements are elicited along both straight and curved paths only during body rotation around the yaw axis. Hence, eye movements reduce the rotational component of the visual image flow during locomotion. 3. Fast saccades, which reset the eyes after a slow compensatory excursion relative to the body, are simultaneously initiated on both sides. They occur either after or before a body turn starts. Therefore, they are partially of central origin. 4. Crabs with one or both eyes blinded are still capable of performing compensatory eye-stalk movements. Thus, nonvisual cues, such as leg proprioceptive afferences or statocyst signals, are also effective in freely walking crabs. However, reducing the number of legs does not substantially impair the movements of the eyes in either seeing or blinded animals. 5. Fixing the seeing eyes to the carapace characteristically alters the pattern of locomotion. Body rotations are jerky in a stationary environment. The tendency to follow a rotating panorama by means of body turns is much higher than in intact animals