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.