Ocular Tracking of Rapidly Moving Visual Targets by Stomatopod Crustaceans

Abstract

1. Ocular tracking in two species of stomatopod crustaceans, Squilla empusa (Say) and Gonodactylus oerstedii (Hansen), was studied by presenting animals with small targets moving to their right and left. 2. Squilla empusa showed no indication of ocular movement responses synchronized with the target's motion, whereas Gonodactylus oerstedii often tracked the target through large angular amplitudes. 3. The region of visual fixation in G. oerstedii is probably the ommatidial patches in line with the eyecup axis. This is suggested by the arrangement of ommatidial axes on the eye's circumference, and by the alignment of the eyes and the rotational motions they make as they observe an approaching target. 4. Tracking is irregular, probably because the animal pays attention to the target only intermittently. Targets are most stimulatory as they move nearly in front of an animal. Eye tracking responses become larger, more frequent and more accurate with increasingly anterior target positions. 5. During visual tracking, the eyes perform both smooth and saccadic tracking movements. Eye movements in the size range 7.5°–15° are made to near the position of the target at movement onset, but are less accurate relative to the target's position at the end of the movement. 6. During visual tracking, the two eyes apparently act with complete independence. Movements of one eye are uncorrelated with movements of the other, both for extreme and central locations of the moving target. 7. The existence of ocular independence during smooth pursuit and saccadic tracking in G. oerstedii may be possible because of the redundancy of visual fields existing in each eye, which could permit monocular measurements of distance to a viewed object. If so, each eye is capable of providing a complete description of the location of a target in space.