A complex case of a calcareous sponge Leucosolenia complicata (Montague, 1814): hidden diversity in boreal and Arctic regions with description of a new species.

In this study, we present the first integrative revision of the Boreal and Arctic calcareous sponges of the genus Leucosolenia with a specific focus on its biodiversity in the White Sea. The material for this work included a combination of newly collected specimens from different regions of the North-East Atlantic and the White Sea and historical museum collections. An integrative analysis was implemented based on vast morphological data (light microscopy, scanning and transmission electron microscopy), microbiome observations, ecological data, accompanied by molecular phylogenetic and species delimitation analyses based on three nuclear markers (28S rRNA, 18S rRNA and histone H3). We demonstrate that Leucosolenia complicata, previously reported from Arctic waters, is restricted to the North-East Atlantic, while in the Arctic, the Leucosolenia diversity is represented by at least four species: Leucosolenia corallorrhiza, Leucosolenia variabilis and two new species, one of which is described herein under the name Leucosolenia creepae sp. nov. The molecular phylogeny analysis supports the species identity of these species. In addition to conventional morphological characters (external morphology and spicule types), new informative fine morphological characters (skeleton organisation in different parts of the cormus; spiculation of the oscular crown; cytological structure including specific cell types and morphotypes of symbiotic bacteria) were found, providing a baseline for further revision of this group in other regions