Abstract Individuals of the terrestrial hermit crab Coenobita clypeatus Fabricius, 1787 use different gastropod shells for shelter during their life span. Availability of shells affects population traits, including density and size structure, influencing their fitness and survival. Locality and human presence can induce variation in the diversity and size of gastropod shells, influencing available shells for hermit crabs. We assessed how the diversity, size, and abundance of the occupied species of gastropod shells affect the density and demography of C. clypeatus in two marine protected areas (MPA) with different recreational use in Puerto Rico, Manatí (high) and Mona Island (low). We also evaluated the relationship between three shell traits (length, width, and aperture length) and the length of the crab chela as a size proxy. We did not find differences in the sex ratio. The density and median chela sizes were higher in crabs from Manatí. In Mona, however, they reached larger sizes. The diversity of occupied shell species was higher in Manatí than in Mona. Dimensions of occupied shells differed between genera and between study sites, and they scaled positively with hermit crab size. Differences in shells diversity, sizes, and abundance between MPAs might cause differences in density and demography of C. clypeatus. We only found large shells of Cittarium pica (Linnaeus, 1758), the only species occupied by larger individuals, occupied by hermit crabs in Mona. The low abundance of C. pica shells in Manatí might have affected the size of hermit crabs in this locality. Our in situ findings suggest that hermit crabs need larger shells to reach larger sizes as previously shown, and that location and the degree of human intervention might influence the availability of gastropod shells. Our study is the first attempt to characterize the demography and gastropod shells used by hermit crabs in Puerto Rico.