Tidal clustering and dispersal of the hermit crabClibanarius digueti

Abstract

At Puerto Peñasco, Sonora, Mexico, field observations indicate that C. digueti exhibits a tidal rhythm of clustering and dispersal. Upon exposure, up to 700 individuals assemble in tight clusters both above and below boulders and in crevices. Throughout low tide, the hermit crabs remain inactive and in physical contact with each other. At the onset of flood tide and near the end of ebb tide, the hermit crabs disperse on upper boulder and other surfaces, apparently to feed. When water depths exceed 0.7 m, the hermit crabs refuge under boulders and in crevices. Thirteen monthly transects on two different substrates, boulder and shell‐hash beachrock, were used to detail clustering trends. In both habitats, highly significant degrees of clustering are shown. The degree of clustering (s2/x) varies according to the relative abundance of C. digueti. However, similar degrees of clustering between and within habitats and with time are demonstrated using Morisita's Index, a measure independent of numerical abundance. The transect data indicate that the clustering of C. digueti is relatively independent of substrate heterogeneity and seasonal variation. Tidal rhythms of activity, skototaxis, phototaxis, and physical contact are demonstrated under constant laboratory conditions of temperature, light, and water depth. A high degree of correspondence occurs between the rhythms in the laboratory and in nature. Experimental evidence suggests, however, that the high tide response predicted from laboratory behavior may differ in nature due to responses to currents, to the absence of light, and/or to increasing water depth. A model of rhythmic and environmentally‐induced behavior is developed to predict the occurrence, of aggregation and dispersal in C. digueti.