Sponge Assemblages on the recently declared Menorca Channel Marine Protected Area (Western Mediterranean Sea)

Abstract

The existence of sponge assemblages in continental shelves and slopes has been known for centuries; however, due to sampling limitations associated to the traditional sampling systems, individual sponge species rather than assemblages were mainly reported. The current study characterizes sponge assemblages over a large geographic area and bathymetric range (40–360 m) encompassing the continental shelf and upper slope. Quantitative analysis of 85 video-transects (corresponding to a total length of 64 km), plus collection of representative samples to confirm identifications, allowed us differentiating six assemblages, mainly segregated by substrate type, depth and species composition. On the continental shelf, a semi-sciaphilous axinellid assemblage dominated shallow rocky outcrops. Shallow maërl beds were predominantly dominated by Haliclona mediterranea, whereas the horny sponge Aplysina cavernicola and several haliclonids mostly dominated deep maërl beds and rocky substrates. Soft sediments in the shelf edge hosted a monospecific Thenea muricata assemblage. Incrusting, columnar and fan-shaped sponges dominated in rocky substrates of the shelf edge. Finally, the upper slope area was dominated by Hamacantha falcula and the hexactinellid Tretodyctium cf. tubulosum. Overall, sponge diversity increased towards the end of the continental shelf, with a peak at 70–90 m depth, followed by a severe plummet at the shelf break. The observed assemblages were highly diverse (> 100 identified species), and reached high density values (> 70 ind/m2) over vast extensions. Due to their exceptional conservation status, these sponge assemblages might give an approximate idea of how sponge grounds might have looked like in the Mediterranean Sea before decades of impacts from bottom trawling fishing