A model on the distribution and abundance of the tube-building polychaete Lanice conchilega (Pallas, 1766) in the intertidal of the Wadden Sea

In the intertidal of the Wadden Sea, assemblages of the tube-building polychaete Lanice conchilega show - on the scale of tens to hundreds of meters - marked differences in density, ranging from 10 to 10.000 tubes/m². These different densities emerge within three to four years after severe winters where all individuals of L. conchilega are killed in the intertidal. In some areas the distribution of L. conchilega is characterized by a small-scale mosaic of mounds with dense aggregations of L. conchilega tubes and depressions with almost no tubes. The typical scale of this mosaic pattern is one to three meters. The causes of both the regional and the local patterns were unknown up to now. Therefore we performed field experiments which showed that larvae of L. conchilega mainly settled at the tubes of adult worms and build their tube attached to the tubes of the adults. The success of initial larval settlement, that is the number of recruits per adult tube, increased with the density of adult tubes. We present a simple simulation model which tries to answer the question if the findings of our experiments are sufficient to explain the distribution patterns of L. conchilega. A first model based on a linear relationship between the density of tubes and the settlement of larvae does not reproduce the wide range of observed densities within four years after defaunation during a severe winter. Only a model where the near-bottom flow regime is taken into account phenomenologically is capable to reproduce the full range and dynamic of the observed densities. As a result of the model, we formulate testable hypothesis about the causes of the regional distribution pattern of L. conchilega. Finally, we use the model to present a graphical model which explains the emergence of the small-scale mosaic pattern.