Rare sponges from marine caves: discovery of Neophrissospongia nana nov sp (Demospongiae, Corallistidae) from Sardinia with an annotated checklist of Mediterranean lithistids

A new record of lithistid demosponges is reported from a western Sardinian karstic cave. The new specimen matches the trait of the genus Neophrissospongia (Corallistidae) for an ectosomal skeleton of radial dichotriaenes, a choanosomal skeleton as a network of dicranoclone desmas, and streptaster/amphiaster microscleres with short spiny rays bearing blunt tips. The cave-dwelling N. nana nov. sp. diverges from the other species of the genus in diagnostic characters such as the large irregular plate-like growth form, the topographic distribution of inhalant and exhalant apertures, and a smaller size of all spicular types. Moreover it displays an additional rare second type of dichotriaenes with smooth cladomes, shared with other genera of Corallistidae but never reported before for the genus Neophrissospongia. In addition N. nana nov. sp. bears style-like sub-ectosomal spicules shared with N. microstylifer from deep water of New Caledonia. As for the latter trait, a present in-depth analysis of N. nolitangere from the Atlantic Ocean contrasts with previous historical records reporting monaxial spicules; as oxeas/anisoxeas. The diagnosis of the genus Neophrissospongia is therefore emended for the growth form and for the micro-traits of dichotriaenes; and monaxial sub-ectosomal spicules. Morphological data indicate that the new species is allied to N. nolitangere and N. microstylifer from Eastern Atlantic and New Caledonian deep water, respectively, and its record confirms the highly disjunct geographic range of the genus Neophrissospongia in the Lusitanian-Macaronesian-Mediterranean area and the western Pacific Ocean supporting the relic condition of the genus in the Mediterranean Sea. This discovery stresses the key status of Mediterranean palaeoendemics as possible remnants of an ancient Tethyan fauna and focuses the need to plan conservation measures for these rare cave-dwelling sponges.