Ecology of tropical hermit crabs at Quirimba Island, Mozambique: niche width and resource allocation

Intertidal-zone hermit crab populations in the Quirimba Archipelago occur i n high densities and many species overlap considerably in terms of spatial distribution and types of shell used. The suites of shells used by 5 mid- t o supra-littoral hermit crab species were studied (using multivariate analy ses) across 3 shore zones, 7 islands and several levels of mollusc fishing pressure. Potentially high intra- and inter-specific competition for resour ces in the Quirimba Archipelago hermit crab assemblages is alleviated by a degree of macrohabitat and shore zone separation but largely by significant partitioning of shell resources. The suite of shells used by each species was distinct, even if certain types were in common. The highest influence o n shell usage was tidal height followed by shore-zone area and hermit crab identity. Where the spatial distributions are tighter (in the supralittoral ), the resource (shell) partitioning is greater. Fishing pressure (and asso ciated abundance of target-species shells) was generally unimportant, but 1 species, Coenobita cavipes, used harvested shells opportunistically in pro portion to their availability. Shannon H' and species-richness values of sh ells used increased with shore zone area in the upper shore zone but not in the supra-littoral zone. It is suggested that this was due to high fishing pressure on certain mollusc species making certain shells abundant for her mit crabs (shell middens were deposited on the supra-littoral zone). The re sults suggest a tight niche mosaic of the many hermit crab species in the Q uirimba Archipelago.