- 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.
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.
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.
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.
Can a Shore Crab See a Star?
F. E. DOUJAK; Can a Shore Crab See a Star?. J Exp Biol 1 May 1985; 116 (1): 385–393. doi: https://doi.org/10.1242/jeb.116.1.385
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