Here we describe two new species of the genus Turrispirillina associated with large siliceous sponge reefs on the Karasik Seamount (Gakkel Ridge, Arctic Ocean). Careful analysis of Rose Bengal-stained samples and observations on untreated frozen sponge material revealed both species live exclusively inside siliceous sponges of the genus Geodia. More detailed information on the intra-sponge habitat was obtained from untreated frozen sponge material that showed both Turrispirillina species attach themselves to the large megascelere spicules that stabilize the choanoflagellates-harbouring subcortical crypts situated under the thick sponge cortex. Unstained specimens of both species were very abundant in the sediments surrounding the sponge. The number of Rose Bengal-stained specimens in each sample obviously depended on the penetration of a Geodia sponge, likely also the exact position of penetration with respect to the sponge. As sexual and asexual generations are observed in both taxa and sampling took place in autumn, opportunistic behavior with rapid reproduction following the spring bloom may determine the standing stock of both species. These are the first Arctic Turrispirillina species described with pseudospines.

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