BARNWELL, F.H.; Univ. of Minnesota: Daily and tidal rhythms of locomotor activity in the fiddler crab Uca tetragonon in relation to the “solar” tide at Moorea, French Polynesia.

The island of Moorea, FP, lies at the node of a large amphidromic (Gr., running around) tidal system in the south central Pacific. Mean tidal range is less than 0.25 meters, and amplitude of the principal solar semidiurnal (12.0-h) tidal constituent, S₂, exceeds that of the principal lunar semidiurnal (12.4-h) tidal constituent, M₂. This means that high tides occur on a solar schedule (around noon and midnight) although the exact timing is modulated on a semi-monthly basis by the M₂ constituent. Specimens of Uca tetragonon, which is active at low tide, were collected from a shallow tidal flat at Lake Temae and transported to Minnesota where they were placed in activity recorders under a light-dark schedule set to the times of sunrise and sunset at Moorea. Crabs exhibited strong daily rhythms with a major maximum in the afternoon (following the timing of the noon solar high tide in Moorea). The maximum was modulated in intensity on a semimonthly schedule corresponding to that of the tides, but the amplitude of the modulation was low and scarcely detected by periodogram analysis. Exposure of crabs to a 12.4-h artificial tide for eight weeks induced strong persistent tidal rhythms with periodogram peaks that equaled or exceeded the height of those of the daily rhythm. Comparable exposure of crabs to a 12.0-h tide resulted in a strictly daily pattern of activity with maxima following the noon and midnight high tides. Thus, specimens of U. tetragonon from a most unlikely tidal situation possess the capacity to time strong persistent lunar tidal rhythms and to adjust their strength to match the intensity of the M₂ tidal constituent to which they are exposed.