Variation in abundance and distribution of the chiton Acanthopleura japonica and associated molluscs on a seasonal, tropical, rocky shore

Temporal and spatial distribution of the chiton Acanthopleura japonica and associated molluscs were recorded on a semi-exposed tropical rocky shore in Hong Kong over 24 months. Most molluscs showed a clear vertical distribution pattern which varied with season. Many mobile gastropods, e.g. Monodonta labio, Cellana toreuma, Nerita albicilla and Thais clavigera, were found lower on the shore in summer (June–September), presumably as a response to the hot, desiccating conditions, but extended their range during winter (December–March). Acanthopleura, was distributed throughout the eulittoral and did not show such strong seasonal patterns, possibly due to the ability to use spatial refuges (specifically crevices) as a result of its flexible body-plan. Chitons exposed on horizontal rocks lost more body water during periods of emersion as compared to those in vertical or crevice habitats and, in summer, all animals on horizontal surfaces died. It is suggested that A. japonica uses cracks and crevices, which are not available to hard-shelled molluscs, throughout the year to reduce the impact of heat and desiccation stress during periods of emersion. This ability may explain the slight seasonal changes seen in Acanthopleura as compared to other species, and the dominance of chitons as herbivores on tropical shores.