Can seamounts provide a good habitat for polychaete annelids? Example of the northeastern Atlantic seamounts

Two seamount groups in the northeastern Atlantic were investigated during the 1980s and 1990s: the first was located along the Iberian and African coasts (Galicia, to the north of Portugal; and the Ampere, Gorringe, Josephine and Seine banks near the Madeira-Canary Islands) and the second was located offshore of the southern part of the Azores Islands, included the Atlantis, Hyeres, Irving, Meteor and Plato banks. Among the invertebrates, Annelida, specifically Polychaeta, were studied as surrogates for the biogeographical relationship between coastal and mid-oceanic seamounts in the northeastern Atlantic and the fauna in these areas. The dominant families were Onuphidae (27.46%), Syllidae (18.23%), Eunicidae (15.65%), Amphinomidae (11.45%) and Nereididae (5.61%), representing 78.4% of the total fauna. Data analyses clearly distinguished two seamount groups, one for coastal seamounts and the other offshore. Although the species distinguished and the family composition at the sampled sites were different (i.e., most diversified at the Josephine site and most impoverished at the Irving site), no coastal/offshore faunal impoverishment gradient could be identified. Thus, it seems that seamount environments do not favour any relationship between planktotrophic development and the direct development of polychaetes. Though the number of apparent endemic species was low (<7%), it remained in keeping with other invertebrate groups. Still, while seamounts may well encourage oceanic biodiversity in some zoological groups, this was clearly not the case for the polychaetes. We offer two explanations for this paradox: pelagic productivity and local environmental conditions.