Contributions made to chronobiology by studies of fiddler crab rhythms

One of the classic organisms used in chronobiological research is the fiddler crab (genus Uca), an animal unique in that it displays both circadian and tidal (i.e., circalunidian) rhythms. The pioneering work on this animal helped produce the early evidence for many of the standard properties now recognized for all circadian rhythms: near temperature independence of the period, phase lability and setability, the light and temperature sensitivity rhythms expressed by phase response curves, and the persistence of rhythms in organs isolated from a multicellular animal. Importantly, results arising from studies of this crab--and a few other organisms--resulted in the development of the exogenous timing hypothesis. While philosophically sound, the lack of supporting evidence for this hypothesis has resulted in it being discarded by most chronobiologists; but while still in its prime, it drew great interest, and therefore grant support, to the field in general, stimulated a great deal of research that otherwise might not have been performed, and resulted in the discovery of environmental stimuli previously unsuspected to influence organisms. As could be expected, continuing work with this crab, using modern approaches and statistical techniques, has modified earlier findings and interpretations, has revealed new properties, and has resulted in the creation of new hypotheses. The review and update is a synthesis of 45 years of this work.