Convergent evolution of mouthparts morphology between Siphonostomatoida and a new genus of deep-sea Aegisthidae Giesbrecht, 1893 (Copepoda: Harpacticoida)

A great diversity of the basal harpacticoid family Aegisthidae Giesbrecht, 1893 is found in the deep-sea benthic and hyperbenthic environments. Some members of this family have developed adaptations to dig into the sediment, while others have colonized hyperbenthic waters; however, no aegisthid associated with a vertebrate or an invertebrate host has yet been reported. During a sampling campaign aimed at evaluating the abyssal copepod fauna of the polymetallic province in the North East Pacific, we discovered a striking new genus of Aegisthidae which exhibits convergent evolution of mouthpart morphology with that of siphonostomatoid copepods. Siphonis gen. nov. has a labrum modified into a long oral cone similar to that found in ectoparasitic species of the primitive siphonostomatoid family, Asterocheridae Giesbrecht, 1899. The mandible, maxillule and maxilla of Siphonis gen. nov. are also modified into siphonostomatoid-like structures; however, some ground pattern characters of the family Aegisthidae are retained. The new genus is discussed and compared with members of the derived taxon Palinarthra, Seifried, 2003 which comprises all those harpacticoid families with an incipient oral cone and a slightly elongated mandibular gnathobase. Two species of the new genus are described herein and were verified by molecular evidence. Siphonis gen. nov. is placed within the subfamily Cerviniopseinae Brotskaya, 1963 and represents a third major shift in the lifestyle of aegisthid copepods, from benthic and hyperbenthic to associated forms (probably with sponges or cnidarians).