Long-term Fluctuation in a Population of Acanthochitona defilippii (Mollusca: Polyplacophora) on a Boulder Shore in Japan(<Special Number>the 2nd International Chiton Symposium)

Abstract

Population density and size composition of Acanthochitona defilippii were investigated over a six-year period on a boulder shore at Amakusa, Japan. The vertical distribution range was restricted to the mid and low intertidal zone. The density on the shore varied haphazardly in space and time, but the highest density occurred around the mean tide level. The density showed a slightly decreasing trend over the 6 years at the mean tide level. Seasonal changes in density were detected in individuals of the 2-4 mm size class (fifth valve width), with high density in winter and low density in summer; however seasonal change in the density of smaller individuals (<2 mm) was not apparent from the data. Analyses of size frequency histograms showed bimodal or unimodal patterns, but it was difficult to detect seasonal change of size mode caused by the growth of individuals, or seasonal recruitment of small individuals. These seasonal patterns, which do not show clear modal changes in density fluctuation or size composition, may be caused by low recruitment rate, low growth rate, and/or low rate of mortality.