The eye-movements of the mantis shrimp <Emphasis Type="Italic">Odontodactylus scyllarus</Emphasis> (Crustacea: Stomatopoda)

Odontodactylus scyllarus makes discrete spontaneous eye-movements at a maximum rate of 3/s. These movements are unpredictable in direction and timing, and there is no detectable co-ordination between the two eyes. The eye-movements were measured with a computer-aided video method, and from 208 of these the following picture of a typical movement emerges. It has roughly equal horizontal and vertical components of 78 ~ taking the eye-stalk axis about 12 ~ around a great circle, and also a rotational component of about 8 ~ The 3 components can occur independently of each other and are thus separately driven by the brain (Fig. 6). The average duration is 300 ms, and average velocity is 40 ~ s (Fig. 5). Most movements are made in a direction approximately at right angles to the orientation of the specialised central band. It is shown that the slow speed of the eye-movements is compatible with scanning, that is, the uptake of visual information during the movement rather than its exclusion as in conventional saccades. Mantis shrimps also make target-acquiring and tracking eye-movements which tend to be somewhat larger and faster than other spontaneous movements. Rotating a striped drum around the animal induces a typical optokinetic nystagmus whose slow phases are smooth, unlike target tracking which is jerky (Fig. 7). Eye-movements may therefore be conveniently grouped into 3 classes: targetting/tracking, scanning, and optokinetic.