Zonation on sandy tropical beaches: a case study using Dotilla intermedia (Brachyura: Ocypodidae)

Abstract

The factors underlying intertidal zonation patterns of fauna on exposed sandy beaches were investigated and focussed on the crab Dotilla intermedia. Dotillids form dense populations on sandy beaches across the Indo-Pacific region and play an important role in processing superficial sediment. Analyses from 2 study sites indicate that the gradient of the beach slope was correlated to the height from the high water mark at which the boundaries of the crab zones occurred, with phys- ical factors associated with the beach gradient influencing the distribution of D. intermedia on the beach. The upper limit of the dotillid zone was controlled by the total water content of the sediment, with D. intermedia absent in areas with total water content less than 15%. Tidal influences defined the lower boundary of the dotillid zone, with crabs requiring an area with a minimum exposure time between tidal immersions (4 to 5 h) to feed on the sediment. Within the dotillid zone, the mid and low shore regions were the most densely populated. There were differences in the distribution of D. inter- media according to crab size, with the low shore area dominated by small crabs, and larger crabs occurring higher up the beach. In terms of the time constraints between successive tidal immersions, the high shore was a more preferential area for large crabs to live in, giving them greater time to feed, construct burrows and engage in other activities, leaving low shore areas open for the settlement of juveniles.