Monstrilloid copepods have endoparasitic naupliar and postnaupliar stages infecting different groups of marine invertebrates. As adults, they have been recorded as free-swimming, non-feeding planktonic organisms in a wide variety of near-surface coastal and transitional aquatic habitats. The phylogenetic relations of the Monstrilloida, obscured by the lack of antennae and mouthparts, have long been a matter of discussion among copepodologists. Epibenthic samples collected at 2,537 m depth from the Irminger Basin, North Atlantic, yielded an adult male monstrilloid copepod that is unassignable to the Monstrillidae, the only known family of the order Monstrilloida. Herein, we erect a new family of the Monstrilloida to accommodate this individual. We provide a comprehensive morphological description of this specimen, along with a phylogenetic analysis that incorporates three genetic markers (mtCOI, 28S rRNA, and 18S rRNA), which supports the designation of the new family. The most striking character in the new family is the remarkably long, slender antennules that are directed posteriorly, thus diverging from the typical monstrillid pattern with rigid, anteriorly directed antennules. Together with the large furca, these characters are considered traits for their adult planktonic life. Another striking character is the presence of a pair of indistinctly segmented, likely vestigial, appendages flanking the oral protuberance, instead of the absence of oral appendages typical of monstrillids. Also, the presence of biramous male fifth legs has never been observed in other monstrilloids, in which male fifth legs are absent or reduced. The distinctive morphological characters exhibited by this new family, along with its deep-sea benthopelagic occurrence, could provide new elements for re-evaluating the phylogenetic position of the Monstrilloida, as well as insights into their biology and ecology.
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