PHASE-SHIFTING AND ENTRAINMENT OF A CIRCADIAN-RHYTHM IN LIMULUS-POLYPHEMUS BY OCULAR AND EXTRAOCULAR PHOTORECEPTORS

Illumination of just the telson of the horseshoe crab Limulus polyphem us produced increasing phase-shifts in the circadian rhythm of the ERG amplitudes of the compound lateral eye with increasing length of expo sure to light, up to 12 h for 10-h exposures, equal to the phase-shift s produced by illumination of the whole animal or of the anterior phot oreceptors alone. Periodic illumination of the anterior photoreceptors or of just the telson entrained the lateral eye rhythm, which reliabl y reentrained when the phase of the illumination was shifted by +/- 6 h. Illumination for 10 h of one compound eye alone or of the median oc elli alone produced small phase-shifts. Periodic illumination of just one compound lateral eye could be arranged to entrain the rhythm of th e other compound eye, but the rhythm did not always re-entrain followi ng a +/- 6 h phase-shift in illumination. The apparent redundancy in c ircadian photoreceptors may be useful to Limulus in dealing with the v arious photic environments which it inhabits.