Density fluctuations of the ophiuroids Ophiothrix fragilis and Ophiocomina nigra in the Bay of Douarnenez, Brittany, France

n the Bay of Douarnenez, in a reference area surveyed for echinoderms, the relative abundance of epifaunal ophiuroids (Ophiothrix fragilis and Ophiocomina nigra) was estimated by dredging and video surveys. Two surveys were undertaken in 2008 and 2011, following a protocol similar to that used in a long-term survey conducted in the 1980s. The data were then compared between the two periods. The present observations demonstrate a recolonization of this site by these ophiuroids, which had almost disappeared between 1983 and 1988. However, although the mean densities of Ophiothrix fragilis were similar to those found prior to this general ophiuroid decrease, Ophiocomina nigra densities were much greater. This latter species not only increased significantly in density and bottom coverage in the reference area, but also showed strong juvenile recruitments on maerl beds and colonization of other bottoms of the bay at very high densities according to a geostatistical analysis. Further studies would be necessary to establish whether or not there is a cause-and-effect relationship between the increase of green algae in the bay and the development of this opportunistic species. © Marine Biological Association of the United Kingdom 2013.