Accepted by J. Hooper: 4 Nov. 2016; published: 19 Dec. 2016 ZOOTAXA ISSN 1175-5326 (print edition) ISSN 1175-5334 (online edition) Copyright © 2016 Magnolia Press Zootaxa 4208 (4): 349–364 http://www.mapress.com/j/zt/ Article 349 https://doi.org/10.11646/zootaxa.4208.4.3 http://zoobank.org/urn:lsid:zoobank.org:pub:B827BB02-8D58-42E0-AD3A-AFBBCB2F6853 Coping with brackish water: A new species of cave-dwelling Protosuberites (Porifera: Demospongiae: Suberitidae) from the Western Mediterranean and a first contribution to the phylogenetic relationships within the genus PAOLO MELIS 1,3 , ANA RIESGO 2 , SERGIO TABOADA 2 & RENATA MANCONI 1 1 Department of Science for Nature and Environmental Resources (DIPNET), University of Sassari, Via Muroni 25, Sassari, Italy. 2 Department of Life Sciences, The Natural History Museum of London, Cromwell Rd, London, UK. 3 Corresponding author. E-mail: paolo.melis83@gmail.com Abstract We used both morphological and genetic approaches to investigate and to describe a new Mediterranean sponge species of the genus Protosuberites from the estuarine-anchialine Bue Marino Cave of Sardinia (Tyrrhenian Sea). The morpho- traits of the specimens were compared versus congeneric species with the strongest affinities, covering the genus geo- graphic range worldwide. Protosuberites mereui sp. nov. is light yellow, thinly encrusting, devoid of any special ectosomal skeleton, with spicular complement of tylostyles of three size classes, single or arranged in bundles/tufts, with round to suboval heads. The new species is characterized by an exclusive diagnostic trait recorded for any cave-dwelling Protosuberites i.e. suboval and basally plated resting bodies with a foraminal aperture ornate by a collar. Resting bodies were found in the basal spongin plate firmly adhering to the substratum singly or in small groups. Also the rare, small tylostyles with a sinuous shaft and a typical mushroom-like head were never recorded in the Western Mediterranean and Atlantic species of the genus. The phylogenetic reconstruction using maximum likelihood (ML) and Bayesian Inference (BI) analyses (COI, 18S rRNA, and 28S rRNA) recovered a robustly supported sister relationship between the Mediterra- nean P. mereui sp. nov. and Protosuberites sp. ‘Panama’ from the Eastern Pacific Ocean. The genetic distances based on COI sequences between all compared Protosuberites species were always higher than 2%, a value sufficient to confirm that P. mereui sp. nov. is a distinct species within the genus. Morphological and genetic analyses confirm unanimously P. mereui sp. nov. as a new species. Our results contribute to the assessment of biodiversity in anchialine/estuarine caves and increase data on sponge adaptive strategies in these extreme ecosystems. Key words: sponges, integrative taxonomy, Sardinia, Bue Marino Cave, estuarine/anchialine karstic caves Introduction The biodiversity of estuarine caves sensu De Waele & Forti (2003) that match in part the definition of anchialine environments (Holthuys 1973; Sket 1996) is poorly known worldwide, but they appear to be hot spots of endemicity for taxa of marine origin (Humphreys 2006; Humphreys et al. 2009). In particular, the sponge fauna of anchialine caves is very scarcely studied, with a few records in the Mediterranean and Atlantic water (van Soest & Sass 1981; Pansini & Pesce 1998; Manconi et al. 2012; Melis et al. 2013) i.e. the estuarine/anchialine Bue Marino Cave of Sardinia (Western Mediterranean Sea) where the physical-chemical parameters, e.g. water salinity and water movement, vary dramatically around the year. Among these sponges, a species of the genus Protosuberites Swartschewsky, 1905 (order Suberitida) preliminarily identified as Protosuberites cf. epiphytum (Manconi et al. 2012; Melis et al. 2013, Melis 2015) is particularly interesting, since only a single cave-dwelling species of the genus i.e. Protosuberites geracei (van Soest & Sass 1981) is known to date from the Caribbean Sea. The taxonomic status of the genus Protosuberites is problematic. It currently comprises 23 valid species (van Soest et al. 2016) defined as “Suberitidae with encrusting habit and a skeleton differentiated in a choanosomal arrangement of single tylostyles or bundles of tylostyles, and ectosomal brushes of tylostyles” (van Soest 2002, p. 235). Delineation of species within the genus Protosuberites is difficult because they only possess tylostyles of