IMIS

Hymenodora represents the most scantily studied branch of the pelagic shrimp belonging to the superfamily Oplophoroidea. We present a phylogenetic revision based on 80 morphological characters and six genes. Molecular and morphological phylogenetic analyses, along with molecular distances calculated with the use of six genes, suggest a family-level status of the clade Hymenodoridae including Hymenodora and Sclerodora, which is also supported by morphological characters linked to the mouthparts. We erect and diagnose a new family and provide a key to families of Oplophoroidea. We tested seven morphological characters, found that only one of them provides a confident identification of Hymenodora gracilis and Hymenodora glacialis complexes. We provide an amended key to species of Hymenodoridae. Molecular trees, along with genetic distances based on COI and 16S genetic markers, showed that H. gracilis and H. glacialis represent complexes of cryptic lineages that cannot be identified on the basis of solitary morphological characters; however, when we included seven morphological characters and ran principal component analysis and generalized linear mixed models, we could distinguish all cryptic lineages confidently. Evolutionary traits of Hymenodoridae are linked to adaptations to feeding in the deep sea (mouthparts), strengthening the membranous carapace (carinae, sulci) and passive defence (shape and proportions of the rostrum and carapace).