Ecology of tropical hermit crabs at Quirimba Island, Mozambique: vertical migration (tree climbing)

The semi-terrestrial hermit crabs Coenobita cavipes and Coenobita rugosus 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 mangrove trees at all states of Light:dark and tidal cycles, although more crabs were present in the study trees at high water than low water. The consecutive period of time spent in trees by C. cavipes individuals varied between half a tidal cycle (<6 h) and 2 complete tidal cycles (24 h). The vertical distance climbed varied between 5 cm and 4 m, but C. cavipes climbed significantly higher than C. rugosus. The height climbed by C. cavipes was irrespective of whether the tide actually reached within 10 m of the tree being climbed. Mean climb height was related to the size of C. cavipes individuals, such that maxima were achieved by those of mid-sized mass. Shell species used also influenced the climb 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. Species of mangrove tree, degree of shade, number of roots and root ground area were all found to be insignificant as influences, but the mean number of hermit crabs per tree increased from 0.43 in those with no trunk or branch holes to 5.08 in those with 5 or 6 holes. There were marked activity and behavioural differences between hermit crabs taking residence in trunk/branch holes and those clinging to branches or leaf bases. Shell exchange in tree hole gatherings (where up to 30 individuals 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 reducing feeding time.