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The sponge fauna of the Western Mediterranean stands as one of the most studied in the world. Yet sampling new habitats and a poorly studied region like the Balearic Islands highlights once again our limited knowledge of this group of animals. This work focused on demosponges of the order Tetractinellida collected in several research surveys (2016–2021) on a variety of ecosystems of the Balearic Islands, including shallow caves, seamounts and trawl fishing grounds, in a broad depth range (0–725 m). Tetractinellid material from the North Atlantic and more than twenty type specimens were also examined and, for some, re-described in this work. All species were barcoded with the traditional molecular markers COI (Folmer fragment) and 28S (C1-C2 or C1-D2 fragment). A total of 36 species were identified, mostly belonging to the family Geodiidae (15 species), thereby bringing the number of tetractinellids recorded in the Balearic Islands from 15 to 39. Eight species from this study are new: Stelletta mortarium sp. nov., Penares cavernensis sp. nov., Penares isabellae sp. nov., Geodia bibilonae sp. nov., Geodia microsphaera sp. nov. and Geodia matrix sp. nov. from the Balearic Islands; Geodia phlegraeioides sp. nov. and Caminus xavierae sp. nov. from the North East Atlantic. Stelletta dichoclada and Erylus corsicus are reported for the first time since their description in Corsica in 1983. Pachastrella ovisternata is documented for the first time in the Mediterranean Sea. Finally, after comparisons of type material, we propose new synonymies: Geodia anceps as a junior synonym of Geodia geodina, Erylus cantabricus as a junior synonym of Erylus discophorus and Spongosorites maximus as a junior synonym of Characella pachastrelloides.

The Balearic Archipelago (western Mediterranean) is an area of great ecological interest due to the combination of complex geomorphology, highly oligotrophic waters and low fishing pressure. Sponges play a key role in benthic habitats, providing structural complexity and significantly contributing to their diversity and biomass. Here, we present an insight into the sponge communities of this archipelago from the analysis of samples collected during several scientific research surveys carried out on bottom trawl fishing grounds around the Balearic Islands and on sedimentary and rocky bottoms of the Mallorca Channel seamounts. Sampling was carried out with experimental bottom trawl, beam trawl, rock dredge and remotely operated vehicle (ROV). We analyzed species presence/absence data using multivariate methods in order to identify assemblages. Once identified, we characterized their biodiversity, biomass and taxonomic composition. A dbRDA analysis was conducted to test the influence of environmental variables and fishing pressure on the sponge communities. Up to 350 species are reported: 220 at bottom trawl fishing grounds and 189 at seamounts. Communities were structured by depth, temperature, currents, substrate and fishing pressure with sponge presence/absense, biomass and diversity also linked to the presence of deep algae beds. Taxonomic composition differed between bottom trawl fishing grounds and the seamounts, where this fishing activity is almost negligible, pointing to different sensitivity to this fishing impact among the different orders, particularly for Tetractinellida, which was much more diverse and abundant at seamounts.