For centuries the deep-sea was perceived as an impoverished area barely contributing to the oceans biodiversity. However, the development of increasingly sophisticated remote sensing and survey technologies (e.g. AUVs and ROVs) revealed remarkable biodiversity hotspots among deep-sea ecosystems such as seamounts, hydrothermal vents, cold-water coral reefs and sponge grounds. The conservation and sustainable use of these ecosystems are among the most critical challenges today and have been recognized at the highest level of international policy. Urgent, knowledge-based action is required if we are to halt the expanding human footprint over such vulnerable ecosystems.

Sponges (phylum Porifera) constitute an important and dominant invertebrate group in both hard- and soft-bottom benthic communities of the deep-sea. In some areas sponges are by far the most dominant group forming structurally complex habitats, known as sponge grounds, aggregations, gardens and reefs. They provide key ecosystem services and goods, namely in terms of habitat and nursery provision for other organisms, biogeochemical cycling, climate regulation, and are also at the forefront of blue biotechnology in the field of drug discovery. However, on account of their presumed slow growth, high longevity and largely unknown reproductive and distribution patterns, deep-sea sponge aggregations are considered particularly vulnerable to ongoing (fisheries, oil and gas exploration) and emerging (mining) anthropogenic activities acting upon the deep-seafloor.

Despite their relevance and vulnerability, research into sponge-dominated communities of the deep-sea is scarce when compared with other taxonomic groups. Indeed, apart from the pioneer work of renowned sponge taxonomists such as Topsent, Carter, Arnesen, Lundbeck and Stephens from the late 19th to the mid-20th centuries, the Atlanto-Mediterranean deep-sea sponge fauna has been seldom studied, likely due to the difficulty posed by this group's taxonomic identification and a current shortage of sponge taxonomists. With the general goal of countering this impediment, a group of sponge taxonomists revived the sponge taxonomy workshops, a series started in 1983 (Ireland) and interrupted since 1995 (Brussels). The volume herein presented results from the 1st International Workshop on Taxonomy of Atlanto-Mediterranean Deep-Sea Sponges, held at the University of the Azores (Portugal) in June 2012, that brought together 26 taxonomists of 14 different countries to work side-by-side and discuss the taxonomy, systematics, biodiversity and biogeography of the Atlanto-Mediterranean deep-sea sponge fauna (Figure 1).

Fig. 1. Participants of the 1st International Workshop on Taxonomy of Atlanto-Mediterranean Deep-Sea Sponges, held at the University of the Azores in 2012. Back row (from the left): Gisele Lobo-Hajdu, Magdalena Lukowiak, Julie Reveillaud, Joana R Xavier, Jean Vacelet, Hans Tore Rapp, Eduardo Hajdu, Paco Cárdenas, Claire Goodwin, Rob Van Soest, Megan Best, Pilar Rios, Andreia Cunha, Javier Cristobo. Front row (from the left): Raquel Pereira, Francisca Carvalho, Andrzej Pisera, Francisco Pires, Sigal Shefer, Nicole Boury-Esnault, Iosune Uriz, Dorte Janussen, Shirley Pomponi. Other participants (not in the picture): Maurizio Pansini, Marzia Bo.

These studies cover the most representative deep-sea dwellers such as glass sponges (Class Hexactinellida), demosponges including rock (‘lithistid’ tetractinellids) and carnivorous sponges (Family Cladorhizidae), not forgetting also the less studied calcareous sponges (Class Calcarea). As is often the case when exploring deep-sea habitats, several new or otherwise poorly known species and communities have been discovered and are here fully described and illustrated. These include: new carnivorous sponges found at bathyal and abyssal depths in several areas of the Atlantic (Hestetun et al., Reference Hestetun, Fourt, Vacelet, Boury-Esnault and Rapp2015) including a Lusitanian seamount (Cristobo et al., Reference Cristobo, Rios, Pomponi and Xavier2015); new lithistid sponges from the deep Florida shelf (Pisera & Pomponi, Reference Pisera and Pomponi2015) and the Macaronesian islands (Carvalho et al., Reference Carvalho, Pomponi and Xavier2015); new demosponges from the Northern Mid-Atlantic ridge (Cárdenas & Rapp, Reference Cárdenas and Rapp2015); and sponge-dominated communities found during ROV surveys at the Gulf of St. Eufemia, in the Mediterranean (Bertolino et al., Reference Bertolino, Bo, Canese, Bavestrello and Pansini2015) and on the Great Meteor seamount (Xavier et al., Reference Xavier, Tojeira and Van Soest2015). Some of the studies offer a more comprehensive overview of the sponge diversity in the deeper areas of the Atlanto-Mediterranean region (Bertolino et al., Reference Bertolino, Bo, Canese, Bavestrello and Pansini2015; Boury-Esnault et al., Reference Boury-Esnault, Vacelet, Reiswig, Fourt, Aguilar and Chevaldonné2015; Rapp, Reference Rapp2015) and the role (or the lack thereof) of physical and ecological barriers for its biogeography (e.g. Cárdenas & Rapp, Reference Cárdenas and Rapp2015; van Soest & de Voogd, Reference Van Soest and de Voogd2015). Lastly, the evolutionary history of the subclass Hexasterophora is investigated through a phylogenetic reconstruction based on morphological characters (Henkel et al., Reference Henkel, Borkenhagen and Janussen2015).

With a total of 19 newly described species and many new records reported, this volume is still a rather small contribution to the filling of an enormous knowledge gap on the diversity and distribution of Atlanto-Mediterranean deep-sea sponges.