Entrainment of Seasonal and Nonseasonal Rhythms by the Rock Crab Cancer irroratus

Abstract

ABSTRACT Rock crabs, Cancer irroratus, from the Mid-Atlantic Bight, spend warmer months in deep water, migrating in fall to shallow waters to molt and mate. In doing so, they move from a habitat lacking solar cues to one with various LD ratios. Exposed to LD photoregimes, crabs entrain locomotor activity to a diel (24 h) periodicity. Most activity occurred in the dark period, with increases at artificial dawn and dusk. To examine whether crabs entrain to seasonal and nonseasonal photoperiods, they were tested for rhythmicity under a variety of photoperiods. Locomotor rhythms rapidly entrained to photoperiods with 6-h phase advances or delays and durations of less than or more than 24 h. Responses were strongest when the photoperiods corresponded to ambient photoperiods. It is proposed that C. irroratus possesses a weak endogenous rhythm and that activity is synchronized by exogenous cues, when they become available in the seasonal inshore migration.