Hermit crabs, humans and Mozambique mangroves

There is a complex interrelationship between upper shore hermit crabs (such as Coenobita sp. and Clibanarius sp.), coastal human populations and mangr ove forests in Mozambique. The abundance, activity, shell selection and beh aviour of three species of hermit crab are related to the level of mangrove cover. With increased density of mangrove trees, the study species of herm it crab changed in abundance, tended to become diurnal, spent more time fee ding and were clustered in larger groups when doing so, and selected longer spired shells. All five of the same variables are also linked to the proxi mity and activity of humans through both direct and indirect actions. Direc t effects included a tendency to nocturnal activity with proximity to human activity; indirect effects included increased and more clumped food suppli es, and shell middens from intertidal harvesting and deforestation. Mangrov es are important to local human populations as well as to hermit crabs, for a wide variety of (similar) reasons. Mangroves provide storm shelter, fish eries and fishery nursery grounds for adjacent human settlements, but they also harbour mosquito populations and their removal provides valuable build ing materials and fuel. Hermit crabs may be useful (indirectly) to coastal human populations by being a source of food to certain commercial species, and by quickly consuming rotting/discarded food and faeces (thereby reducin g disease and pests). They can also cause minor problems to coastal human p opulations because they use shells of (fisheries) target mollusc species an d can be more abundant than the living molluscs, thereby slowing down effec tive hand collection through confusion over identification. The mixture of positive and negative attributes that the three groups impart to each other in the Quirimba Archipelago. northern Mozambique, is discussed.