Abstract.Two species of cerithid prosobranchs live on a sand flat in Bermuda. At low tide during day lightBatillaria minimais found on the surface whereasCerithium lutosumremains buried in the sediment. This correlates with much higher lethal and coma temperatures, as well as much greater resistance to desiccation, of the former species. However, at other diurnal or tidal stages the two species co‐occur in the same layer of sand. Despite greater resistance to physical factors the population ofB. minimaseems to be declining since in 1980 juveniles of this species were rare on Tuckers Town Flat whereas the population ofC. lutosumwas thrivin