Hidden in the dark: description of Calicotyle carmenae n. sp. (Monogenea: Monocotylidae) from deepwater catsharks (Elasmobranchii: Pentanchidae) off Iceland.

A new species of Calicotyle (Monogenea: Monocotylidae) is described from three deepwater catshark species (Elasmobranchii: Pentanchidae) collected off Iceland. Calicotyle carmenae n. sp. is diagnosed as a species of Calicotyle possessing a haptor with one central and seven peripheral loculi, two hamuli, and several hooklets. It is distinguished from all congeners by the absence of eye spots, having a once-looped male copulatory organ, two U-shaped vaginae, and intercaecal vaginal pores. A molecular phylogenetic analysis based on 28S ribosomal DNA sequences placed Calicotyle carmenae n. sp. within a clade of other Calicotyle species infecting selachians and holocephalans, while revealing clear genetic divergence from these congeners (8.2-11.1%; 74-99 bp). In addition, the morphology and host specificity of C. carmenae and its congeners are discussed. Some inconsistencies and insufficiently described anatomical features in other species of the subfamily Calicotylinae are highlighted, which evidence a need for a revision of this taxon. The study also provides the first record of a species of Calicotyle from pentanchid sharks, highlighting the potential for further discoveries as research expands into different hosts and deeper, underexplored marine ecosystems.