Settlement of barnacle larvae is governed by Euclidean and not fractal surface characteristics

1. Extensive research has been carried out on the biological, physical and chemical characteristics of surfaces that promote, or prevent, marine fouli ng. The texture of the substratum is probably the most important factor aff ecting settlement of the Acorn Barnacle Semibalanus balanoides in the field . The aim of this study was to examine the relationship between fractal and Euclidean descriptions of substratum surface complexity and settlement of the Acorn Barnacle. 2. Replicate settlement panels with smooth, fine, medium and coarse surface s were manufactured using a precise and accurate technique. Two-dimensional profiles were measured using a laser profilometer, and two surface complex ity indices, potential settling sites (PSS) and the Minkowski fractal dimen sion (MFD) were calculated. PSS is a Euclidean measure of the surface compl exity taking into account the body size and settlement behaviour of the bar nacle larvae, whereas MFD is a fractal dimension related to the complexity of the surface at a variety of scales. 3. In a field experiment, settlement density of S. balanoides was positivel y related to both PSS and MFD. To break the correlation between PSS and MFD , surfaces were modified by sandblasting with either 64-125-mu m or 250-500 -mu m grains. This decreased MFD but did not affect PSS. 4. A further field experiment found that this erosion using small-scale san d particles had no effect on settlement of the barnacle. It was concluded t hat S. balanoides larvae were responding to characteristics of the surface related to PSS and not MFD. 5. Although a correlation was found between species abundance and the fract al complexity of its habitat, further experimentation showed that there was no causality in this relationship. Many surveys, with no experimentation, of aquatic and terrestrial communities have shown relationships between org anisms and their habitat and assumed causality; however, care must be taken in interpretation of such studies.