Abstract Hermit crabs are ever alert for more suitable shells to inhabit, but what this may mean for coastal shell middens has rarely been considered. Here, the impact of the most landward-based of hermit crab families, the tropical Coenobitidae, upon archaeological shell-bearing deposits is assessed using a case study: the Neolithic Ugaga site from Fiji. At Ugaga, hermit crabs were found to have removed the majority of shells from the midden and had deposited their old, worn shells in return. The behavioural ecology of genus Coenobita suggests a mutualistic interaction whereby humans make available shell and food resources to hermit crabs, which in turn provide a site cleaning service by consuming human and domestic waste. Diagnostic indicators of terrestrial hermit crab wear patterns on gastropod shells are outlined and the conditions under which extensive ‘hermitting’ of shell midden deposits may occur are investigated. The ability to recognise hermit crab modification of shells is considered not only important for archaeomalacologists analysing tropical shell deposits, but also for field archaeologists wishing to select suitable shell samples for radiocarbon dating.