Eye movements in Daphnia pulex (De Geer)

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Abstract
The various types of eye movement exhibited by the cyclopean eye of Daphnia pulex were studied using high speed motion photography. This rudimentary eye, which consists of only 22 ommatidia, can move through approximately 150° in the sagittal plane and 6o° in the horizontal plane. Four classes of eye movement were found: (1) a high speed tremor at 16 Hz with an amplitude of 3–4°, which resembles physiological nystagmus, (2) a slow rhythmic scanning movement at 4 Hz, and 5–6° amplitude, (3) large fast eye movements similar to saccadic eye movements and (4) optokinetic nystagmus produced by moving striped patterns. Where the fast tremor occurred concurrently with the slow rhythmic scan, a Fourier analysis revealed that the former was the fourth harmonic of the latter.