Effects of the tube-building polychaete Lanice conchilega (Pallas) on benthic macrofauna and nematodes in an intertidal sandflat

Biogenic habitat structures play a major role for distribution patterns of benthic fauna. Tube-dwelling polychaetes provide one of the main features in the otherwise relatively low structured sandy intertidal. The aim of this study was to investigate the effect of the tube-dwelling sand-mason Lanice conchilega on macrofauna and nematodes in an intertidal sandflat in the Wadden Sea area off the German coast. Field studies and experiments were carried out on the associated macrobenthic fauna of natural L. conchilega aggregates and of artificial tubes. Furthermore, the distribution of nematode species at increasing distances along transects arising from individual L. conchilega tubes, was investigated. Natural as well as artificial tubes had positive effects on numbers of macrofaunal species and individuals. Yet, juvenile Mya arenaria were the only species appearing in higher numbers in both L. conchilega aggregates and artificial tube fields. They used the tube structure to attach themselves with their byssus threads. The polychaete Malmgreniella lunulata only occurred in natural L. conchilega aggregates and was often found inside the polychaetes' tubes. It is possibly a commensal associated to L. conchilega. In artificial tube fields, opportunistic spionids and Capitella capitata settled in higher numbers. This might be due to colonisation of sediment accumulating in the tube fields, utilisation of the tubes as an attachment surface or refuge from predation. However, two predatory polychaetes, Eteone longa and Nereis diversicolor, were also found in higher abundance in natural or artificial tube fields. Nematode abundances were considerably higher next to individual L. conchilega tubes at a distance of 0.5 cm. This distribution was mainly caused by a group of threadlike species with a thin cuticle, which are thought to be adapted to epidermal uptake of dissolved organic matter. It is suggested that there might be a linkage between predatory polychaetes and the distribution pattern found for nematodes around the tubes of L. conchilega.