Daily settlement variability of the barnacle Semibalanus cariosus: Importance of physical factors and density-dependent processes

Abstract

Settlement abundances of barnacles are determined by larval supply and the ratio of settlement to larval supply (K). This ratio can be divided into 2 functions: the ratio of settlement on bare space to larval supply (k1), and the loss of cyprids due to the preempting of rock space by sessile organisms (k2). To examine the 3 functions of settlement (i.e. larval supply, k1 and k2) of the barnacle Semibalanus cariosus, we monitored the daily water column larval supply and the daily settlement density in plots where all benthos were removed and in natural-community plots, which were left undisturbed, during one settling season. In both low- and mid-tidal sites, k1 explained > 75% of day-to-day variability in K. k1 and k2 were density dependent, while daily settlement densities were linearly related to larval supply. k1 varied with tidal height and daily larval supply, but not with other physical variables, k2 varied with daily settlement densities in community-removal plots and was not related to any physical or biological variable. Such density dependence in k1 and k2 may have been caused by 3 processes: (1) inhibition of larval settlement by planktonic larvae, (2) enhancement of settlement by conspecifics (and their chemical attractants) on days with relatively low larval density, and/or (3) preemption of suitable settlement substrata by settlers that attached on days with relatively high larval density.