There are still combinations left: two new deep-sea species of Echinoderes (Kinorhyncha: Cyclorhagida) with an unusual middorsal spine pattern

The middorsal spine pattern is a traditional, key taxonomic character within Kinorhyncha. However, as known species increase, many exhibit identical or highly similar arrangements, requiring additional traits for species-level identification. Here, two new deep-sea Echinoderes species, both bearing a middorsal acicular spine on segment 9, are formally described, representing a highly unusual condition for adults of the family. Echinoderes prior sp. nov. is characterized by having middorsal acicular spines on segments 4–9, lateroventral spines on segments 6–9, and tubes in lateroventral position on segment 5, in lateral accessory position on segment 8, and in laterodorsal position on segment 10. Specimens from the Northwest Pacific and Atlantic Oceans with considerable morphological variation in certain tubes and glandular structures are treated as Echinoderes aff. prior . Genetic differentiation within populations of E. prior sp. nov. suggests cryptic lineages, and the postembryonic development of E. aff. prior is also described and discussed. Echinoderes shimadai sp. nov. is described from the Northwest Pacific Ocean based on combined molecular and morphological evidence. This species is distinguished by having middorsal acicular spines on segments 5–9, lateroventral spines on segments 6–9, tubes in lateroventral position on segment 5, in midlateral position on segment 8, and in laterodorsal position on segment 10, and type 2 glandular cell outlets in laterodorsal position on segment 2. In light of these findings, the diagnoses of the order Echinorhagata, the family Echinoderidae, and the genus Echinoderes are amended. Phylogenetic implications of the presence of a middorsal spine on segment 9 in adults are discussed, particularly regarding the restriction of middorsal spines to segments 4–8 as a family autapomorphy. These findings underscore the importance of exploring understudied deep-sea habitats and the value of combining morphological and molecular data to understand character evolution and lineage diversity within Kinorhyncha.