Visual constraints on migration behavior of juvenile French grunts

Retinomotor movements of three size classes of juvenile French grunts, Haemulon flavolineatum, are characterized during the periods of their daily evening and dawn migration. Differences in the extension and withdrawal of the cones correlate with the earlier departure and later return to the daytime schooling sites of the smallest juveniles as compared to the larger juveniles. For each size class changes in the dimensions and retinal distribution of the cone cells and optical properties of the eye are used to estimate differences in photosensitivity under conditions of equal radiant intensities. For a target subtending any given angle, calculation indicates that the largest juveniles should be capable of discrimination at one-thousandth the light intensity of the smallest juveniles. The differences in potential photosensitivity correlate with the different intensities at which they migrate. The likely trigger to initiate their daily migrations probably involves circadian factors at dawn and extension of the single cones from the focal region of the retina prior to the evening migration.