Photic entrainment of circadian activity patterns in the tropical labrid fish Halichoeres chrysus

Yellow wrasses (Halichoeres chrysus) show clear daily activity patterns. Th e fish hide in the substrate at (subjective) night, during the distinct res t phase. Initial entrainment in a 12h:12h light-dark (12:12 LD) cycle (mean period 24.02h, SD 0.27h, n = 16) was followed by a free run (mean period 2 4.42h, SD 1.33h) after transition into constant dim light conditions. Light pulses of a comparable intensity as used in the light part of the LD cycle s did not result in significant phase shifts of the free-running rhythm in constant darkness. Application of much brighter 3h light pulses resulted in a phase-response curve (PRC) for a fish species, with pronounced phase adv ances during late subjective night. The PRCs differed from those mainly obt ained in other vertebrate taxa by the absence of significant phase delays i n the early subjective night. At that circadian phase, significant tonic ef fects of the light pulses caused a shortening of the circadian period lengt h. Entrainment to skeleton photoperiods of 1:11 LD was observed in five of six wrasses exposed, also after a 3h phase advance of this LD cycle. Subseq uently, a 1:11.25 LD cycle resulted in entrainment in four of the six fish. It is suggested that the expression of the circadian system in fish can be interpreted as a functional response to a weak natural zeitgeber, as prese nt in the marine environment. This response allows photic entrainment as de scribed here in the yellow wrasse.