The distribution of hermit crabs and their gastropod shells on shallow reefs off St. John, US Virgin Islands
Bulletin of Marine Science
Using the coral reefs of St. John, US Virgin Islands, as an experimental system, we tested the hypothesis that the distribution of hermit crabs (Calcinus tibicen Herbst, 1791 and Pagurus brevidactylus Stimpson, 1859) is associated positively with the distribution of the cerithid gastropods shells they favor as homes. Cerithium spp. were surveyed on shallow (<6 m) reefs along 10 km of the south shore of St. John, in 2011 (6 sites) and 2012 (7 sites); they were categorized as live shells (LS), empty shells (ES), or occupied shells (OS, containing hermit crabs). In both years, densities of LS and ES differed significantly among sites and were higher in the east compared to the west; densities of OS did not vary among sites in 2011, but in 2012 were marginally more abundant to the west. Densities of OS were not correlated with the density of LS in either year. The distribution of cerithids is consistent with the hypothesized origin in a patch-depletion model for larval supply, in which recruitment declines along the shore as larvae are transported farther from their eastern sources. In contrast, the distribution of hermit crabs was more uniform along the shore, suggesting dispersal in the pelagic realm or on the benthos was more active than for cerithids. Hermit crabs did not occur simply where live cerithids were found, but instead favored locations where suitable shells (i.e., OS + ES of sufficient size) were common, and potentially dispersed to these locations.