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A new harpacticoid copepod from the Patagonian continental slope (Chile) is described. Isthmiocaris longitelson gen. et sp. nov. is characterised by several peculiar features like the presence of an additional collar-like pseudosomite between cephalothorax and first pedigerous somite, a remarkably long telson, small appendages, pronounced sexual dimorphism in the swimming legs P2 to P5, and long sensilla on the body. Several of these characters are unique within Harpacticoida. There are similarities with species of the canthocamptid ‘‘genera’’ Itunella Brady, 1898 and Psammocamptus Mielke, 1975. Isthmiocaris longitelson gen. et sp. nov. is therefore placed into the ‘‘Canthocamptidae’’ Sars. Its phylogenetic position is briefly discussed. Z U S A M M E N F A S S U N G Vom Patagonischen Kontinentalabhang (Chile) wird eine neue Harpacticoidenart beschrieben. Isthmiocaris longitelson gen. et sp. nov. zeichnet sich durch eine Reihe besonderer Merkmale aus, wie einem kragenartigen zusätzlichen Pseudosomit zwischen Cephalothorax und dem ersten schwimmbeintragenden Segment, einem bemerkenswert langen Telson, kleinen zierlichen Schwimmbeinen, einem ausgeprägten Sexualdimorphismus an den Schwimmbeinen P2 bis P5 und sehr langen Sensillen am Körper. Einige dieser Merkmale sind innerhalb der Harpacticoida einzigartig. Andererseits zeigt das neue Taxon Übereinstimmungen mit Arten der ‘‘Gattungen’’ Itunella Brady, 1898 und Psammocamptus Mielke, 1975. Isthmiocaris longitelson gen. et sp. nov. wird deshalb den ‘‘Canthocamptidae’’ Sars zugeordnet. Seine phylogenetische Stellung wird kurz besprochen. The diversity of harpacticoids is full of surprises. Wherever the ocean floor is sampled, new species turn up that mostly belong to known ‘‘families’’ or ‘‘genera.’’ But quite often lately, new forms have been discovered that do not fit any of the higher taxa known. Such forms have given rise to new taxa, which demonstrate the amazing plasticity of the basic harpacticoid design. Among the new discoveries are the Rotundiclipeidae (Huys, 1988), the Cristacoxidae (Huys, 1990), the Styracothoracidae (Huys, 1993), the Superornatiremidae (Huys, 1996), and the Novocriniidae (Huys and Iliffe, 1998). Conversely, other enigmatic species do not necessarily represent new high level taxa but rather the terminal clades within known ones. They can display extreme morphological modification, which renders recognition of their affinities virtually impossible. The present contribution deals with such an amazing form. It was found in the Magellan Region of South America, which has already yielded other interesting discoveries (George, 1997, 1998, 2001, 2002; George and Schminke, 1998). MATERIALS AND METHODS Five specimens (2 males, 2 females, and 1 male CIV) were found in samples from the Patagonian continental slope. The sediment samples were taken with a multicorer (MUC) from station 40/110 during ANT XIII/4 with RV ‘‘Polarstern’’ in 1996 (Fahrbach and Gerdes, 1997; George, 1999). Samples were treated as described in George (1999). The specimens were cleared up with glycerol. Drawings were made with the aid of a camera lucida on a Leitz-Dialux 20 EB compound microscope equipped with a phase contrast 100-times objective. The morphological terminology has been adopted from Huys and Boxshall (1991). The terminus ‘‘telson’’ is used according to Schminke (1976). The terminology related to phylogenetic systematics is used according to Ax (1984). Abbreviations used in the text: cphth: cephalothorax; A1: antennula; aes: aesthetasc; A2: antenna; md: mandibula; mxl: maxillula; mx: maxilla; mxp: maxilliped; enp: 119 JOURNAL OF CRUSTACEAN BIOLOGY, 23(1): 119–130, 2003

journal of crustacean biology

Isthmiocaris Longitelson Gen. Et Sp. Nov., a Strongly Derived Harpacticoid (Copepoda) from the Magellan Region, and Its Systematic Affinities to Certain “Canthocamptid” Taxa