Rhythmicity under constant conditions in the rock crab, Cancer irroratus

The existence of activity rhythms in migratory populations of the rock crab, Cancer irroratus, which spend most of the year in deep water, was investigated. Mature individuals from the Mid-Atlantic Bight were tested under laboratory conditions of constant light (LL) and constant dark (DD) after an initial ambient photoperiod was presented. Activity levels were simultaneously monitored for 20 crabs maintained in separate compartments, using an infrared beam-break system. Activity was analyzed using an Enright periodogram.

Activity under LL was at a relatively low level and with no rhythmic periodicity exhibited. In DD, activity was concentrated at approximately 25-h intervals, approximating a tidal period. Under ambient (natural photoperiod) conditions, a 24-h rhythm was present with activity greatest during the scotophase and with peaks at dawn and dusk.

Possible advantages to the presence of a tidal rhythmicity in a deep water species are discussed and include temporal partitioning of the environment, a timekeeping mechanism for the initiation of migration, and an evolutionary or ecological remnant from shallow water populations.