Phylogenetic position of the Paramicrolaimidae, description of a new <italic>Paramicrolaimus</italic> species and erection of a new order to accommodate the Microlaimoidea (Nematoda: Chromadorea).

The phylogenetic position of Paramicrolaimidae Lorenzen, 1981, a rare group of marine free-living nematodes, has been the subject of debate due to the unique morphology of the buccal cavity, an unusual combination of other morphological traits, and lack of molecular sequences. Here, <bold><italic>Paramicrolaimus hohonucola</italic> sp. nov.</bold> is described from the continental slope of New Zealand, and the position of the family Paramicrolaimidae is investigated based on analyses of the small subunit (SSU) and D2–D3 region of large subunit (LSU) rDNA genes. This is the first record of Paramicrolaimidae from the Southwest Pacific, and the deepest record of the family to date (347–1514 m depth). Observations using scanning electron microscopy revealed the presence of a transverse mouth opening formed by the fusion of the two ventrosublateral lip lobes and enlarged dorsal lip lobe, resulting in partial dorsoventral symmetry, which is an unusual feature within the phylum. Phylogenetic analyses suggest a close relationship between the Paramicrolaimidae and the family Selachinematidae Cobb, 1915 (order Chromadorida Chitwood, 1933). The Paramicrolaimidae lack cuticle punctations, a key morphological trait of all families currently classified within the Chromadorida; however, in the absence of any clear morphological affinities with other chromadorean orders, we propose that Paramicrolaimidae be placed within Chromadorida based on evidence for a close relationship with Selachinematidae as shown by SSU and LSU phylogenies. Our SSU phylogeny supports the results of previously published analyses showing a close relationship between <italic>Molgolaimus</italic> Ditlevsen, 1921 and the Microlaimidae Micoletzky, 1922. We therefore propose that <italic>Molgolaimus</italic> be removed from Desmodoroidea where it is currently classified and instead placed within Microlaimoidea. We also propose that the family Molgolaimidae Jensen, 1978 be reinstated and moved to accommodate <italic>Molgolaimus</italic> within Microlaimoidea. As in previous SSU phylogenies, our analyses provide no evidence for a close relationship between Desmodoroidea and Microlaimoidea, which currently comprise Desmodorida. Based on this molecular evidence, the clear morphological differences between Microlaimoidea and the closely related Chromadorida, and the lack of a synapomorphy linking Desmodoroidea and Microlaimoidea, we propose the order <bold>Microlaimida ord. nov.</bold> to accommodate the superfamily Microlaimoidea. [ABSTRACT FROM AUTHOR]