Substratum heterogeneity and complexity influence micro-habitat selection of Balanus sp. and Tubularia crocea larvae

Abstract

This exper~mental study examines the importance of active and passive processes during the settlement of the barnacle Balanus sp. and the hydrozoan Tubulana crocea larvae. Three hypotheses were tested: (1) larvae of Balanus sp. and 7 crocea exhibit settlement preferences with respect to scales of topographic heterogeneity (0, 1, 10 and 100 mm V-shaped grooves); (2) these preferences are modified by substratum complexity (combinations of scales of topographic heterogeneity); and (3) these preferences are mediated by larval behaviour directly on or near the substratum (benthic behaviour: after initial contact with the substratum). Artificial PVC panels of different scales of heterogeneity and complexity were submerged 30 cm below the surface during autumn 1992, a t Beaufort, North Carolina, USA. A companson between the final attachment sites of larvae with their initial contact sites on complex hard substrata was used to determinc the role of larval exploration following initial contact. Results indicated that, for Balanus sp. and T crocea larvae, all 3 hypotheses were confirmed. Evidence of actlve behaviour was observed at the spatial scales considered In t h ~ s tudy (entire panels and types of surfaces within panels), suggesting that active selection occurs at smaller and smaller scales. Balanus sp. larvae were rllure abundant on pancls which d ~ d not include the 1 mm scale of heterogeneity while, at the level of types of surfaces within each panel, they settled preferentially in grooves of 1 and 10 mm. T h ~ s uggests that different factors affect the competent 1a1-vae at d~fferent spatial scales. For T. crocea, the larvae selected the most complex type of panels and generally settled on the most exposed surfaces. Differences between initial larval contact sites and flnal attachment sltes confirmed the benthic behaviour hypothesis.