Revision of the genus Robbea (Stilbonematinae: Desmodoridae), worldwide abundant marine nematodes with chromophoric Fe–Br inclusions and the description of a new stilbonematine genus

Nematodes are one of the most widespread and abundant animal taxa across aquatic and terrestrial environments. In marine shallow-water, porous sediments, members of the subfamily Stilbonematinae may be found in high numbers. Stilbonematinae are characterized by their coat of symbiotic bacteria, which give the nematodes a white appearance, while the nematodes themselves are usually colourless. We identified several species of the genus Robbea (Desmodoridae: Stilbonematinae) in which live specimens had a conspicuous dark purple coloration of the glandular sense organs (GSOs), highly specialized epidermal glands that are prominent in the Stilbonematinae. The dark inclusions in the GSOs of Robbea contained high concentrations of iron and bromide, regardless of their habitat. Morphological and phylogenetic analyses show that the genus Robbea is paraphyletic and we define the new genus Cyathorobbea gen. nov. for species with sucker-shaped post-pharyngeal supplements in males. Furthermore, we describe five new Robbea species from around the world. Some Robbea species are morphologically very similar and required the use of principal component analysis of morphometric data to untangle the diversity in this taxon. Our work emphasizes the value of live animal observations that enable the identification of important characters and highlights the overlooked diversity within the Stilbonematinae.