Relationship of daily and circatidal activity rhythms of the fiddler crab, Uca princeps, to the harmonic structure of semidiurnal and mixed tides

Intertidal organisms may employ circatidal rhythms to track the tidal cycle, but tidal patterns may vary within a species' range and necessitate adaptation to the local tides. Circatidal rhythms were examined in populations of the eastern Pacific fiddler crab Uca princeps (Smith) from four sites with differing tidal characteristics, La Paz (24degrees10'N; 110degrees21'W), San Blas (21degrees33'N; 105degrees18'W) and Manzanillo (19degrees6'N; 104degrees24'W), Mexico (lower amplitude, mixed semidiurnal tides) and Mata de Limon, Costa Rica (9degrees55'N; 84degrees43'W) (high-amplitude, semidiurnal tides). Local tides were characterized by harmonic constants of M-2, S-2, K-1, and O-1, partial tides that largely determine their semidiurnal and diurnal features. Rhythmic structure in continuously recorded locomotor activity of individual crabs held under laboratory conditions was described by cosinor and periodogram methods of time-series analysis. Both daily and circatidal rhythms were found in crabs studied in light-dark cycles set to local conditions at the time of collection. Crabs at all four sites shared a tendency toward bimodality, with a mid-morning activity peak and varying degrees of nocturnal activity. Circatidal rhythms closely matching the period of the 12.42-h M-2 partial tide were consistently present at all sites except Manzanillo. At Mata de Limon, the circatidal rhythm clearly dominated locomotor activity, but was strongly modulated by a daily rhythm in a repeating pattern at a semilunar interval. In contrast, the amplitude of the daily rhythm was higher than that of the circatidal rhythm in crabs from the three mixed tide sites on the Mexican coast, where the tidal pattern is dominated by a diurnal inequality arising from the diurnal K, and O-1 partial tides. These results suggest that populations of U. princeps use both daily and circatidal timing systems to track local forms of the tide generated by their M-2, S-2, K-1, and O-1 geophysical counterparts.