Comparative studies of tidal rhythms. I. The characterization of the activity rhythm of the pliant‐pendulum crab,Helice Crassa

Abstract
The locomotor activity of 28 Helice crassa was studied, individually, in constant conditions. The rhythms displayed were the most complicated ever described for a crab. Two basic cyclic patterns were found: 25% of the crabs displayed a unimodal form (the group period averaging 26.5 + 1.4 hours); while a bimodal pattern was shown by 68% of the individuals (mean period = 25.7 + 0.5 hours). These rhythms persisted for varying lengths of time in the laboratory. Period lengths sometimes increased or shortened spontaneously, and sometimes one or both peaks of a rhythm disappeared and reappeared again later.