| Summary: | Abstract Keysercythere reticulata sp. nov. and Redekea abyssalis sp. nov., collected from the wood fall submerged in the Kuril-Kamchatka Trench (Northwestern Pacific), are only the second records of the naturally occurring, wood-associated ostracod fauna from a depth of over 5000 m. At the same time, K. reticulata is the second and R. abyssalis is the third representative of their respective genera. While Keysercythere Karanovic and Brandão, 2015 species are free-living, deep-sea taxa, all Redekea de Vos, 1953 live symbiotically on the body surface of wood-boring isopods, Limnoria spp. Since R. abyssalis is the only genus representative found in the deep sea, we hypothesize that its ancestor colonized this ecosystem as a result of the symbiotic relationship. Newly collected material enabled us to update molecular phylogeny of Cythreoidea based on 18S rRNA gene sequences, especially to clarify the current systematics of the families Keysercytheridae, Limnocytheridae, and Paradoxostomatidae. The resulting phylogenetic tree supports a close relationship between Keysercythere and Redekea and a distant relationship between two Limnocytheridae lineages, Timiriaseviinae and Limnocytherinae. Consequently, we propose a transfer of Redekea from Paradoxostomatidae to Keysercytheridae, and erecting of the two limnocytherid subfamilies onto the family level. The phylogenetic analysis also implies a close relationship between the nominal Limnocytherinae genus and Keysercythere+Redekea clade, albeit with a low posterior probability, requiring further studies to clarify this.
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