Effect of habitat architecture on mobile benthic macrofauna associated with patches of barnacles and ascidians

Abstract

2 White Sea Biological Station, Zoological Institute RAN, Universitetskaya nab. 1, 199034 St. Petersburg, Russia ABSTRACT: The physical structure of a habitat strongly affects species composition and diversity in benthic assemblages. In the shallow subtidal zone in the White Sea in northwestern Russia, barnacles Balanus crenatus are often found in clusters on empty bivalve shells or small stones, sometimes over- grown by solitary ascidians (mainly Styela spp. and Molgula spp.). These epibenthic patches are surrounded with muddy sediment. Sediment also fills the space between barnacle shells within the patches. The assemblages of mobile macrofauna associated with epibenthic patches and surrounding unstructured sediment are different in species composition and abundance. We hypothesised that epibenthic patches at least partially affect mobile benthic organisms with their architectural proper- ties as complex cavity-loaded structures. In a field experiment we assessed the strength of this effect, comparing unmanipulated natural epibenthic patches (E), unmanipulated patches of bare sediment (S), and patches of bare sediment with artificial imitation structures (mimics) added (M). Mimics were made of polyvinyl chloride (PVC) tubes and exposed for 1 and 2 yr (half-buried in sediment; 50 tubes × 8 to 9 sets × 2 replications). After 1 or 2 yr of exposure, the assemblages in E and M were much more similar to each other than to S (analysis of similarities, non-metric multidimensional scaling). Most species that were constantly abundant in E responded positively to the treatments. The bivalve Musculus discors and the sigalionid polychaete Pholoe minuta dominated numerically both in E and M, whereas maldanid Rhodine loveni and spionid-like Apistobranchus tullbergi dominated in S. Like many other large aggregated benthic suspension feeders, patched barnacles shape the associated assemblage of mobile benthic fauna with their architectural effect.