Dynamics (from 2010-2011 to 2014) of Sabellaria alveolata reefs on the western coast of Cotentin (English Channel, France)

The polychaeta Sabellaria alveolata (Linnaeus, 1767), commonly known as the honeycomb worm, is a gregarious polychaete present on the European coasts from Scotland to Portugal. It is an important reef-building species which enhances topographic complexity in colonized areas. In Europe, the most extensive reef formation is found in France in the Bay of Mont Saint-Michel. Nevertheless, since 2006, Sabellaria bio-constructions (platforms and reefs) have developed on hard substrates along the west coast of Cotentin and in the north of the Bay of Mont-Saint-Michel. The aim of this study is to compare the area covered by bio-constructions in 2010-2011 and in 2014 along 60 km of the west coast of Cotentin, focusing on the temporal changes at four target sites: Champeaux, Lingreville, Blainville-sur-Mer, and Saint-Germain-sur-Ay. The results show a very slight decrease of colonized surface-area between these two periods at the scale of the whole study area, but a significant decrease at two target sites, with considerable degradation of the bio-constructions which appear to be temporary on the west Cotentin coast. Our study also reveals the presence of flourishing permanent bio-constructions at Champeaux, which exhibit the highest densities (>40,000 ind.m−2). The winter size structure of S. alveolata populations on the target sites and during a winter-summer survey at Blainville-sur-Mer reflects the absence of settlement in 2013 and 2014, and the absence of older individuals able to survive over several years. Our results suggest a haphazard transport of pelagic larvae from the S. alveolata permanent reefs in the Bay of Mont Saint-Michel, which leads to the spread of temporary reefs northwards. The evolution of these constructions can be very rapid (<10 years); it is suggested that cold winters and high-energy hydrodynamic conditions generated by strong storms have contributed to a rapid degradation of the reefs at the beginning of the 2010s.