Distribution of suspension-feeder brittle stars in the Canary Current upwelling ecosystem (Northwest Africa)

Abstract

Brittle stars constitute a common and highly diverse component of epibenthic communities, and they can form dense aggregations on the seabed of all oceans worldwide. Nevertheless, there is limited information on the diversity and distribution of these echinoderms in Northwest Africa. Between 2004 and 2012, 10 multidisciplinary Spanish and Norwegian surveys were conducted off Northwest Africa—between the Strait of Gibraltar and Sierra Leone border—from a depth of 19 to 1888 m. During these surveys, we sampled 1298 stations, and about one million individuals and 124 kg of brittle stars were collected from 502 stations. Forty-two species belonging to eight ophiuroid families considered as suspension feeders were collected from 296 stations and represented 93.4% of the total abundance and 75% of the total ophiuroid-specific richness. Ophiacanthidae was the richest and most abundant family, with 15 species and estimated densities up to almost 4 × 105 by 0.1 square kilometre. The other families found abundantly were Amphiuridae, Ophiotrichidae and Ophiactidae, represented by six, five and four species each, respectively. Ophiotrichidae, Ophiacanthidae and Gorgonocephalidae presented the largest biomass among the suspension-feeder families (35.5%, 33.8% and 28.2%, respectively), and Ophiotrichidae is a well-represented family along the Northwest African coast. The most remarkable concentrations and highest diversity values were detected on the upper slope (depth, 400–600 m) off Western Sahara. Between 24°N and 22°N, we collected huge concentrations of Ophiacantha abyssicola and up to six species per station. This area has permanent and intense upwelling conditions and is out of the minimum oxygen zone. We also noted significant densities and richness at Cape Ghir and Cape Blanc, where important filaments of nutrient-rich waters occur, and at Guinea-Bissau, which is affected by nutrient river inputs. The multivariate analysis clearly separated the brittle stars into two main assemblages, across latitudinal and bathymetrical gradients: northern latitude (Morocco and Northern Sahara) from the area south of 24°N and from the continental shelf–upper slope to the deep slope.