New nematode species from the continental slope of New Zealand (Chromadorea, Microlaimida, and Chromadorida), and unexpected placement of the genus Molgolaimus Ditlevsen, 1921

The current nematode classification comprises three primarily marine basal Chromadorean orders: the Microlaimida Leduc et al., 2018; Desmodorida De Coninck, 1965; and Chromadorida Chitwood, 1933. The phylogenetic placement of several taxa within these orders, however, is unclear due to the paucity of taxonomically informative morphological characters for high-level classification and is yet to be tested by molecular phylogenetic analyses due to the absence of molecular sequences. Here, we describe Molgolaimus kaikouraensis sp. nov. and Aponema pseudotorosum sp. nov. from the continental slope of New Zealand and investigate phylogenetic relationships of these species and that of the rare desmodorid genera Onepunema and Pseudonchus, using SSU phylogenetic analyses for the first time. Whilst our analyses provided support for the current classification of Aponema within the family Microlaimidae and of Pseudonchus within the Desmodorida, we could not confirm relationships of Onepunema. We found no support for the placement of Molgolaimus with either the Desmodorida or Microlaimidae/Microlaimida as in the current and previous classifications. Instead, Molgolaimus was classified with the Chromadorida with moderate and strong support in maximum likelihood and Bayesian analyses, respectively. Congruence analysis suggests that in some cases at least, the structure of the female reproductive system is a more taxonomically informative trait for marine nematode classification than the male reproductive system or cuticle.