First records of dense aggregations of suspension-feeder brittle-stars in Northwest Africa: Could they be directly linked to upwelling phenomena?

Abstract

Echinoderms are one of the main benthic invertebrate groups, especially in deep waters. Particularly, ophiuroids constitute a common component of megabenthic communities, widely distributed worldwide, from intertidal zone to hadal depths. Within this group, dense aggregations of different species, mostly belonging to suspension-feeder families such as Ophiothrichidae, Ophiactidae, Amphiuridae, Ophiocomidae and Ophiacanthidae, were described worldwide, both in shallow and deep waters and over different sea-bottom types. These aggregations were related with their social behavior, but also with favorable environmental conditions and sometimes were described associated with other suspension-feeders like deep-waters corals. During ten Spanish and Norwegian surveys carried out in Northwest Africa between 2002 and 2012 on board of R/V Vizconde de Eza and R/V Dr. Fridtjof Nansen, an intensive sampling program focused on the study of benthic communities was accomplished in the framework of EcoAfrik and FAO EAF-Nansen projects. In this work, we described, for first time in this area, the existence of ophiuroid sea-beds, mainly concentrated along the Saharan and Mauritanian slope The most impressive sea-beds, estimated in more than 3,700,000 individuals of Ophiacantha abyssicola by km2, was recorded off Western Sahara between 410 and 520 m depth on muddy-bottoms. In addition, dense aggregations of Ophiotrix maculata were described for first time for this species at 410 m in the Saharan slope on living Lophelia pertusa, and also between 197-240 m depth on rocky bottom in the seamount off Mauritania. The particular oceanographic conditions in the Saharan-Mauritanian coast linked to the permanence of the upwelling conditions throughout the year, probably enhance the development of suspension-feeders communities and also of these ophiuroids sea-beds.