| Summary: | Sponges, phylum Porifera Grant, 1836, comprise a group of benthic filter-feeding organisms, which are one of the oldest living groups of metazoans on Earth. Currently, four classes are recognized within the Phylum, of which class Demospongiae is the largest and most diverse containing approximately 83% of all species. Within this class, lithistids or rock sponges are a polyphyletic group united by the possession of a spicule type called desma. Desmas are articulated hypersilicified spicules, that frequently provide a hard consistency to these sponges. Due to this common trait, lithistid sponges were originally placed into an order, 'Lithistida' Schmidt, 1870. Even though their polyphyletic nature was acknowledged for a long time, only recently, their phylogenetic relationships were clarified using molecular data. This led to major changes in their classification, with the majority of species and families being assigned to the order Tetractinellida Marshall, 1876. Lithistids are typically found in tropical and warm temperate regions at bathyal depths, yet few species can also occur in shallower waters, usually in caves and reef tunnels. Globally, lithistid faunas are well known from the tropical western Atlantic and the south-west Pacific archipelagos of New Caledonia, New Zealand and the seamounts of the Norfolk Ridge. On these areas, lithistids are remarkably diverse and dominate the faunal assemblages. In the Northeast Atlantic (NEA), knowledge of the lithistid fauna was mostly restricted to the Azores archipelago and some species were known to occur on other Macaronesian Islands. However, the majority of those descriptions were historical and knowledge on their diversity and distribution was sparse and scattered in the literature. Moreover, molecular data of lithistid species occurring in the NEA was almost inexistent.
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