Scale of observation and distribution of adult conspecifics: their influence in assessing passive and active settlement mechanisms in the barnacle Balanus crenatus (Brugière)

The settlement behaviour of Balanus crenatus (Brugiere) was examined in field experiments in relation to different patterns of distribution of adult conspecifics. The study was carried out in August 1994 in Anse du Petit Mitis, Quebec, Canada. Flat panels supporting random, aggregated, and uniform distributions of adult conspecifics or ceramic mimics were moored vertically on 3 lines. Panels were placed at random 2.5 m below the water surface. Settlement only occurred on panels supporting barnacles. Over 30% of the observed settlement occurred on adult conspecifics. ANOVAs showed no significant effects of distribution on number of settlers at the scale of the panel. Settlement tended to become homogenized over the panel with uniform distributions, while it became aggregated on panels with random and aggregated adult distributions. Mean nearestneighbour distances of spat did not vary among spatial patterns tested. When settling on adults, cyprids settled preferentially on the rostrum, facing current flow. A nested ANOVA carried out on the number of spat per adult on a given panel showed no effect of adult distribution. Differences in spat distribution among replicate panels, barnacle individuals, and positions were observed, suggesting that settlement of B. crenatus was affected by variations of the hydrodynamic environment at the scale of the adult barnacle. Flume experiments using inert particles and the same field panels were carried out to determine the position of initial contact on the panel. Comparison of field and flume results suggests that it is the small-scale behaviour taking place after initial contact on the panel and prior to permanent attachment that determines the patterns of spat distribution.