Are there separate circatidal and circadian clocks in the shore crab Carcinus maenas

Shore crabs Carcinus rnaenas collected in winter normally exhibit circadan but not circatidal rhythmicity.When introduced into 20 % seawater (7 ppt), a circatidal rhythmic activity pattern is induced phased to the time of introduction.There is no apparent change in the phase of the extant circadian rhythm which, after treatment, acts to modulate the expression of the circatidal rhythm, the peaks of which are greatest during 'expected' night.These findings suggest that there are separate mechanisms for the control of circadian and circatidal rhythmicity in C. rnaenas.They permit a new appraisal of a key question in rhythm biology as to whether circatidal and circadian rhythmicity can be considered as expressions of the same physioloqcal mechanism.