Visual Thresholds and Spectral Sensitivity of Flatfish Larvae

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Abstract
1. Plaice and sole larvae have a pure-cone retina and no retinomotor responses. 2. Observations on the extinction of phototactic and light-dependent feeding behaviour, using both white and coloured light, enabled visual thresholds and spectral sensitivity to be determined. 3. Visual threshold decreased (that is, sensitivity increased) with age. The values for extinction of negative phototaxis ranged from 10-4-10-6m.c. 4. Sole larvae could feed in the dark from the early post-hatching stage, plaice larvae only at metamorphosis. Thus light intensity thresholds for feeding were only determinable in younger plaice larvae. The values ranged from 10°-10-2m.c. 5. The spectral sensitivity curves were plateau-like and photopic in nature. Thus the larval cones have spectral properties similar to those of adult teleosts. The differences in threshold obtained by the two techniques are probably a measure of the extent of recruitment of high-threshold cones required for image formation in feeding. 6. No evidence of a dermal light sense was found.

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