Taxonomy of Stilbonematinae (Nematoda: Desmodoridae): description of two new and three known species and phylogenetic relationships within the family

A taxonomic study of the subfamily Stilbonematinae (Nematoda: Desmodoridae) based on collected specimens from a coral reef in the Caribbean Sea revealed two new species, Laxus parvum sp. nov. and Leptonemella brevipharynx sp. nov. L. parvum is characterized by the small body size (2738?µm), large cephalated spicules (63?µm), wide gubernaculum, and coccoid-shaped ectosymbiotic bacteria. The diagnosis of the genus Laxus is emended and a dichotomous identification key is given for the seven valid species. L. brevipharynx is characterized by the shape of the amphidial fovea, ‘open’ spiral with 1.25 turns, a short pharynx, hook-shaped gubernaculum, and male tail relatively short. Three known sympatric species, Eubostrichus hopperi, Robbea porosum, and Stilbonema brevicolle, were re-described and illustrated based on morphometric features and morphology from light microscope and scanning electronic microscope observations. For each species, relationships are discussed as well as the diagnostic value of morphological features. Phylogenetic relationships amongst desmodorid species were explored based on small subunit rDNA and cytochrome oxidase c subunit 1 partial loci. The three subfamilies within Desmodoridae (Desmodorinae, Spiriniinae, and Stilbonematinae) are polyphyletic; four of the genera of Stilbonematinae proved to be paraphyletic. Convergent evolution would reconcile the presence of glandular sensory organs and ectosymbiosis with the paraphyly of stilbonematins. The cryptic diversity of R.?porosum could be explained by morphological stasis owing to obligate ectosymbiosis with bacteria. The current classification of nine genera is still the most tractable system for Stilbonematinae in spite of the evidence of its paraphyletic nature based on molecular data