Morphology of obligate ectosymbionts revealsParalaxusgen. nov., a new circumtropical genus of marine stilbonematine nematodes

Abstract

AbstractStilbonematinae are a subfamily of conspicuous marine nematodes, distinguished by a coat of sulphur-oxidizing bacterial ectosymbionts on their cuticle. As most nematodes, the worm hosts have a simple anatomy and few taxonomically informative characters, and this has resulted in numerous taxonomic reassignments and synonymizations. Recent studies using a combination of morphological and molecular traits have helped to improve the taxonomy of Stilbonematinae but also raised questions on the validity of several genera. Here we describe a new circumtropically distributed genusParalaxus(Stilbonematinae) with three species: Paralaxus cocos sp. nov.,P. bermudensissp. nov. andP. columbaesp. nov.. We used single worm metagenomes to generate host 18S rRNA and cytochrome oxidase I (COI) as well as symbiont 16S rRNA gene sequences. Intriguingly, COI alignments and primer matching analyses suggest that the COI is not suitxable for PCR-based barcoding approaches in Stilbonematinae as the genera have a highly diverse base composition and no conserved primer sites. The phylogenetic analyses of all three gene sets however confirm the morphological assignments and support the erection of the new genusParalaxusas well as corroborate the status of the other stilbonematine genera.Paralaxusmost closely resembles the stilbonematine genusLaxusin overlapping sets of diagnostic features but can be distinguished fromLaxusby the morphology of the genus-specific symbiont coat. Our re-analyses of key parameters of the symbiont coat morphology as character for all Stilbonematinae genera show that with amended descriptions, including the coat, highly reliable genus assignments can be obtained.