Annelid assemblages on artificial and natural hard substrata differ decades after building: A case study on a one-century old seawall

摘要

Although strong differences have been observed between the assemblages on artificial substrata and on natural hard-bottom habitats, little is known about the mechanisms that cause them. The purpose of this study was to examine the differences in Mediterranean low shore annelid assemblages dwelling on an old artificial, vertical hard substratum and a natural, horizontal one. The structure of the Ellisolandia elongata turfs covering the two habitats showed little divergence in terms of length and number of thalli, as well as in available space for epibionts. In this respect, colonization duration might explain the lack of significant dissimilarities in the phytal canopy. In spite of this, statistical analyses showed that the specific composition of the two annelid assemblages differed in a significant manner. Although they shared a set of constant species, their relative abundances were dissimilar and this was especially evident for several species belonging to families Syllidae and Enchytraeidae. Abundance and species richness were significantly higher on natural structures, as expected, but Shannon diversity and equitability indexes were lower. Apparently, the sciophilous condition of vertical surfaces, which do not occur naturally in the area, is the main driver of the differences in species composition and its capital importance remains after colonization processes have occurred.