Soft-bottom hydroids: distribution and colonization patterns in the Mauritanian slope

Abstract

Mauritanian continental margins are dominated by muddy bottoms criss-crossed by a complex systems of submarine canyons. This unstable environment, with turbidity currents and wide slumping areas, could appear as an unsuitable habitat for benthic hydroids, regarded as characteristic and abundant representatives of the sessile epifauna on hard bottoms in shallow-water communities. Nevertheless, the intensive sampling programme deployed during the four multidisciplinary Maurit surveys, conducted annually between 2007 and 2010 by the Spanish Institute of Oceanography, has revealed a relatively high specific richness, similar to that obtained in other West African regions. A total of 6169 colonies belonging to 63 species and 19 families were collected in 174 trawl stations more or less uniformly distributed along the entire Mauritanian coast, highlighting the capacity of hydroids to colonize soft-bottoms, linked with two different strategies. The first strategy, displayed by 13 species, involves morphological adaptations of their hydrorhiza, which produced a bundle of interwoven tubules that adhere to muddy or sandy sediments. The second strategy, shared by 48 species, includes epizoism on 11 taxa of invertebrates. Species that develop both strategies were the most abundant and also the most frequent along the Mauritanian slope. Moreover, large hydroid species with modified hydrorhizae were heavily colonized by hydroids, as well as by other epifaunal representatives, contributing effectively to the development of a three-dimensional habitat and enhancing biodiversity. Also remarkable is the eurybathic character of the Mauritanian hydroid fauna, with 58 species (92%) whose bathymetrical ranges exceed 300 m. Species richness and abundances showed similar patterns, with the highest values recorded on the shelf break and upper slope, peaking mainly on the bottoms adjacent to the submarine canyons. This distributional pattern suggests that canyons harbour source populations enabling the colonization of adjacent sea bottoms and maintaining a high diversity in the Mauritanian slope, despite the intensive fishing activity deployed by international trawling fleets. The existence of these source populations in the canyons, the substratum generalist and the eurybathic nature of many hydroid species, could explain the observed hydroid abundance and distribution patterns in this area.