Intraspecific variation in foraging behaviour: influence of shore height on temporal organization of activity in the chiton Acanthopleura japonica

Abstract

The chiton Acanthopleura japonica has a wide vertical distribution on moderately exposed Hong Kong shores, and it shows limited seasonal vertical migration. Observations revealed that chitons were active while awash on ebbing or flooding tides during both day- and night-time, but they were inactive in refuges at their resting shore heights when conditions were unfavourable. The most intense activity was always associated with awash periods, but the timing of activity peaks and percentage activity varied between seasons, tidal conditions and the shore height individuals'occupied. Activity periods were longer in summer than winter, but varied from day to day, depending on weather conditions such as wind speed and hours of sunshine. Duration of activity also varied with shore height; chitons living in the high-shore remaining active for nearly twice as long as those lower on the shore. Laboratory studies showed that activity Was controlled by a circalunidian endogenous rhythm, but can be overridden by exogenous, environmental factors, such as wave splash. Therefore, the temporal organization of activity in A. japonica is controlled by both exogenous and endogenous mechanisms, of which the timing and duration of the awash phase of the tide appears to be the dominant controlling factor. To optimize foraging success, individuals exhibit a continuum of foraging strategies determined by season and the shore height they occupy, resulting in intraspecific variability in the duration and timing of activity.