Review of Halalaimus (Nematoda, Enoplida, Oxystominidae) and description of two new species from the abyssal nodule-bearing Clarion-Clipperton Zone

This review examines the geographical distribution, morphology, and taxonomy of the genus Halalaimus . It consolidates existing descriptions of the currently 102 valid species, resolves nomenclatural inconsistencies, and introduces two new species: Halalaimus marcosi sp. nov . and Halalaimus pierri sp. nov . Halalaimus marcosi sp. nov . is characterised by its large body, papilliform inner labial sensilla, outer labial setae shorter than cephalic setae spaced one head diameter apart, and a conic-filiform tail with the posterior two-thirds filiform. Halalaimus pierri sp. nov . is distinguished by its very strongly tapered anterior end, indistinct inner labial sensilla, and outer labial setae longer than the cephalic setae. Long-standing challenges in species-level identification, mainly due to incomplete descriptions and confused nomenclature, are addressed through a detailed reassessment that corrects duplicated names, assigns replacement names to misattributed species, and incorporates previously overlooked taxa. Halalaimus borealis Gagarin, 2020 is considered a junior homonym and is renamed H. gagarini nom. nov . This study provides interactive global distribution maps for all Halalaimus species, a table summarising species ranges and habitat information, and a table listing 36 morphometric and morphological traits per species. Updated identification keys are presented, including a new key for deep-sea species, offering a practical tool for nematode taxonomists and identifiers worldwide.