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At least 242 epi-and infauna species were found, of which 115 are reported as sponge associates for the first time in the North Atlantic. Sixty species, 19 genera and 1 family are recorded for the first time from the Faroes. Two hundred and twenty-five taxa were found as epifauna on the sponges, three taxa occurred as infauna and 14 taxa both as epi-and infauna.The morphology of the sponges influences the occurrence and composition of the represented fauna. Thus, the largest number of taxa were present on sponges having a spicule 'fur'. A number of species showed preference for the oscular cavities of S. ponderosus and the incurrent furrow of T. levis and T. valdiviae. Only one sponge predator, Hanleya nagelfar, was found. The majority of the associated fauna uses the sponges as a substratum. Three categories of direct effect of interactions between the sponges and the associated fauna are proposed. The results of the present investigation show that demosponges constitute a substratum for a large number of benthic taxa on the outer shelf and upper slope in the northeastern Atlantic, and that the scarcity of earlier reports is a result of insufficient investigation. It is suggested that the fauna associated with sponges in temperate to cold waters is facultative, representing the fauna of the local geographical area. This is in contrast to the large number of apparently obligate associates of Porifera in warm tropical waters.

Geodia species north of 60°N in the Atlantic appeared in the literature for the first time when Bowerbank described Geodia barretti and G. macandrewii in 1858 from western Norway. Since then, a number of species have been based on material from various parts of the region: G. simplex, Isops phlegraei, I. pallida, I. sphaeroides, Synops pyriformis, G. parva, G. normani, G. atlantica, Sidonops mesotriaena (now called G. hentscheli), and G. simplicissima. In addition to these 12 nominal species, four species described from elsewhere are claimed to have been identified in material from the northeast Atlantic, namely G. nodastrella and G. cydonium (and its synonyms Cydonium muelleri and Geodia gigas). In this paper, we revise the boreo‐arctic Geodia species using morphological, molecular, and biogeographical data. We notably compare northwest and northeast Atlantic specimens. Biological data (reproduction, biochemistry, microbiology, epibionts) for each species are also reviewed. Our results show that there are six valid species of boreo‐arctic Atlantic Geodia while other names are synonyms or mis‐identifications. Geodia barretti, G. atlantica, G. macandrewii, and G. hentscheli are well established and widely distributed. The same goes for Geodia phlegraei, but this species shows a striking geographical and bathymetric variation, which led us to recognize two species, G. phlegraei and G. parva (here resurrected). Some Geodia are arctic species (G. hentscheli, G. parva), while others are typically boreal (G. atlantica, G. barretti, G. phlegraei, G. macandrewii). No morphological differences were found between specimens from the northeast and northwest Atlantic, except for G. parva. The Folmer cytochrome oxidase subunit I (COI) fragment is unique for every species and invariable over their whole distribution range, except for G. barretti which had two haplotypes. 18S is unique for four species but cannot discriminate G. phlegraei and G. parva. Two keys to the boreo‐arctic Geodia are included, one based on external morphology, the other based on spicule morphology. © 2013 The Linnean Society of London

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