The East Usambara tree-hole crab (Brachyura : Potamoidea : Potamonautidae) - a striking example of crustacean adaptation in closed canopy forest, Tanzania

An undescribed species of freshwater crab with a remarkable life history was studied over 2 years in areas of natural forest in the East Usambara Mountains, north-east Tanzania. It was found occupying the water-filled boles (tree holes) of several tree species at altitudes between 150 and 900 m asl, a single tree species Myrianthusholstii accounted for 26% of all trees identified as such. There was no apparent association with other freshwater bodies. The 'tree-hole crab' is largely nocturnal but will also forage by day during periods of heavy rainfall, leaving its tree hole in search of leaf-litter molluscs. Once located it carries pieces of the snail shell back to the tree hole and releases these into the water filled bole which is typically acidic. This releases calcium ions (Ca2+) into solution from the snail shell resulting in a more neutral solution (mean pH = 6.6, standard deviation (s) = 0.46, n = 25). This act provides a source of dissolved calcium for the development of exoskeletons following ecdysis. Significant differences were found between depth (P = 0.01) and height (P = 0.05) of occupied and unoccupied water-filled tree holes. This is the first record of a crab exhibiting such behaviour in East Africa.