1. 1.

    The absolute-intensity threshold of the optokinetic response in the crab Leptograpsus variegatus was measured using a moving continuous monochromatic point source. The results are compared to the photoreceptor responses of the same animal (Doujak, 1984), to determine the photoreceptor signal for the same behavioural threshold stimuli.

  2. 2.

    Optokinetic eye movements demonstrate that at the peak of spectral sensitivity (499 nm), the minimum intensity of light the animal can detect is 4.0 ± 1.5 × 105 photons cm−2 s−1 (mean ± S.D., N = 18) incident on the eye, which is equivalent to a photon flux of about 6 photons facet−1 s−1.

  3. 3.

    Comparison of behavioural and electrophysiological studies shows that at the above behavioural thresholds, the retinula cells respond with a train of discrete membrane depolarizations (bumps). The mean bump rate recorded in retinula cells at the absolute-intensity threshold of the optokinetic response to a moving point source is 22 ± 5 bumps min−1 (mean ± S.D., N = 6).

  4. 4.

    Optokinetic experiments reveal an absolute sensitivity of the crab's apposition eye to a point source that is only about 900 times less sensitive than the human eye: theoretical estimates based on quantum capture efficiency and lens size predict a much larger difference. The experimental findings provide the first definitive proof that an animal possessing a compound eye can see a star, albeit only stars of 0.5 magnitude and brighter.

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