ECOLOGY OF TROPICAL HERMIT-CRABS AT QUIRIMBA ISLAND, MOZAMBIQUE - VERTICAL MIGRATION (TREE CLIMBING)

The semi-terrestrial hermit crabs Coenobita cavipes and Coenobita rugo sus both undertake vertical migrations at Quirimba Island, Mozambique, by tree climbing in the region between High Water Neap tide level and Extreme High Water Spring tide level. Both species occurred in mangro ve trees at all states of Light:dark and tidal cycles, although more c rabs were present in the study trees at high water than low water. The consecutive period of time spent in trees by C. cavipes individuals v aried between half a tidal cycle (<6 h) and 2 complete tidal cycles (2 4 h). The vertical distance climbed varied between 5 cm and 4 m, but C . cavipes climbed significantly higher than C. rugosus. The height cli mbed by C. cavipes was irrespective of whether the tide actually reach ed within 10 m of the tree being climbed. Mean climb height was relate d to the size of C. cavipes individuals, such that maxima were achieve d by those of mid-sized mass. Shell species used also influenced the c limb height. High wind speeds (force 3 to 5 or 12 to 33 km h(-1)) had a negative influence on the mean number of hermit crabs climbing. Spec ies of mangrove tree, degree of shade, number of roots and root ground area were all found to be insignificant as influences, but the mean n umber of hermit crabs per tree increased from 0.43 in those with no tr unk or branch holes to 5.08 in those with 5 or 6 holes. There were mar ked activity and behavioural differences between hermit crabs taking r esidence in trunk/branch holes and those clinging to branches or leaf bases. Shell exchange in tree hole gatherings (where up to 30 individu als were present), as opposed to ground based shell exchange, may have advantages in reducing desiccation (there is sometimes additionally a supply of fresh water), reducing the risk of predation and not reduci ng feeding time.