Activity and Homing Behavior of Two Species of Acanthopleura (Mollusca: Polyplacophora) on a Subtropical Shore in Japan(<Special Number>the 2nd International Chiton Symposium)

Abstract

Moving patterns and homing behavior in Acanthopleura gemmata and A. tenuispinosa were investigated on the rocky shore of Sesoko Island, Okinawa, Japan. In the daytime, both A. gemmata and A. tenuispinosa moved only when washed by sea water, while at night they moved not only when washed by water but also when they were exposed to the air. Almost all chitons rest in a fixed 'home' site in the daytime and during periods when the rocks are submerged. They do not move when strong sunlight heats the rocks in their habitat either. Surface temperatures of dry rocks under such conditions have been measured as high as 64.8 C. When under water, they suffer the risk of predation by fish or other carnivorous invertebrates. Their movement patterns can therefore be explained as avoidance of heat and desiccation of rock surfaces and predation. Homing behavior in these species was observed throughout the period of study. Homing and moving patterns of A. gemmata and A. tenuispinosa were studied to compare daytime and nighttime activity, including when they go out and when they come back 'home'. The nighttime activity was longer than the daytime activity, and activity patterns were slightly different between these two species. The speed of movement was not significantly different between when they go out and when they return. Vying for use of locations as 'home' and cognitive behavior were observed, suggesting that cognition of geographic features is found among chitons.