The grapsid crab Armases miersii breeds in supratidal rock pools on the northern shore of Jamaica, West Indies. In 5 replicate field experiments, temperature and salinity were monitored during the course of larval development, and the hypothesis was tested that these rock pools represent a food-limited environment for the development of planktotrophic crab larvae. Food limitation should select for and, hence, may explain, ontogenetic traits that have been observed in the early larval stages of this species: abbreviated development, enhanced initial energy reserves, starvation resistance, and potential for partially endotrophic development (facultative lecithotrophy). In a rock pool with only 1 to 2 ppt salinity, in situ growth (measured as change in carbon content per larva) and development (occurrence of moulting to subsequent stages) were retarded, and only the zoea 2 stage was reached within a 6 d observation period. Successful larval development to the megalopa stage w! as observed in salinities ranging from 5 to 24 ppt and temperatures between 24 and 32 degree C. Diurnal temperature variation (up to 7 degree C) and sudden salinity changes (up to 16 ppt) after heavy rain falls did not visibly influence larval growth or development. These field observations show that the larvae of A. miersii are very tolerant of extremely low and variable salinity and, hence, are well adapted to survive and develop under the harsh and unstable physical conditions prevailing in supratidal rock pools. Larval growth in all 5 experiments was significantly reduced and development delayed (only from the zoea 3 stage) under natural (unmanipulated control) conditions compared with sibling larvae that received Artemia sp. nauplii as an additional food source. These results provide evidence that food is actually a limiting factor in supratidal rock pools, where larvae of A. miersii develop. In a series of 5 replicate laboratory experiments, the possible significance of ! cannibalism, especially in late stages, was tested. Canniba! listic activity shows an increasing trend with decreasing endotrophic potential of subsequent larval stages, reaching a maximum in the megalopa. Thus, multiple release of larvae in rock pools may provide younger larvae as a potential food source for older conspecifics, and cannibalism may be a nutritional buffer (comparable with nurse eggs) in a food-limited breeding habitat.