A multi-scale analysis of settlement density and pattern dynamics of the barnacle Semibalanus balanoides

Abstract

Settlement pattern and density of Semibalanus balanoides (L.) in the Clyde estuary were studied on wood, concrete, steel and sandstone rock during 1994. The position of each newly metamorphosed juvenile barnacle was mapped to the nearest 1 mm over the course of 75 tides. The total number of settled barnacles on the surfaces varied between 0 and 24.09 cm-2. The tide number at which 50 % of the total number of barnacles had settled on each surface varied between tide 23 and tide 44. A new measure describing surface roughness in relation to S. balanoides settlement using data on cyprid morphology and behaviour was devised; this index was termed Potential Settling Sites (PSS). The number of PSS for 2-dimensional profiles of each of the surfaces, using accurate replicas and a magnified photo mosaic, was measured. For the surfaces the total number of settlers was best related to the Index PSS(min) [Settler Density = (5.68 x PS(min)) - 4.16; p < 0.01]. Two other measures of surface roughness, ISO number, a standard engineering measure of surface relief, and fractal dimensions, were not related to settler density. Settlement pattern was found to vary with settlement density and spatial scale of analysis. A multi-scale analysis using Morisita's index suggested that S. balanoides were generally settling aggregatedly at low densities (< 8 cm-2), with strongest aggregations at approximately 30 mm block sizes; it is proposed that this was related to the scale of the close exploration behaviour of the cyprid. Settlement at higher densities (> 8 cm 2) tended to be random. Aggregated settlement at low densities could be related to the requirement to find a mate in close proximity, whereas avoidance of aggregated settlement by S. balanoides at higher densities could potentially be an intraspecific competition avoidance technique. Nearest neighbour analysis at low settlement densities (0.5 to 1.75 cm-2) found that cyprids settled aggregatedly to at least the first nearest neighbour; it is proposed that this is related to small- scale gregarious settlement behaviour.