Stilbonematinae 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 relatively 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 genus Paralaxus (Stilbonematinae) with three species: Paralaxus cocos sp. nov., P. bermudensis sp. nov. and P. columbae sp. nov. We used single worm metagenomes to generate host 18S rRNA and cytochrome c oxidase I (COI) as well as symbiont 16S rRNA gene sequences. Intriguingly, COI alignments and primer matching analyses suggest that the COI is not suitable 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 genus Paralaxus as well as corroborate the status of the other stilbonematine genera. Paralaxus most closely resembles the stilbonematine genus Laxus in overlapping sets of diagnostic features but can be distinguished from Laxus by 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.

中文翻译：

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专性外共生体的形态揭示了 Paralaxus gen。nov.: 海洋茋线虫的一个新的环热带属

Stilbonematinae 是显眼的海洋线虫的一个亚科，其特征在于其角质层上有一层硫氧化细菌外共生体。与大多数线虫一样，蠕虫宿主具有相对简单的解剖结构和很少的分类信息特征，这导致了大量的分类重新分配和同义化。最近结合使用形态学和分子特征的研究有助于改善茋亚科的分类学，但也对几个属的有效性提出了质疑。在这里，我们描述了一个新的环热带分布的属 Paralaxus (Stilbonematinae) 与三个物种：Paralaxus cocos sp。11 月，P. bermudensis sp。十一月 和 P. columbae sp。十一月 我们使用单线虫宏基因组来生成宿主 18S rRNA 和细胞色素 c 氧化酶 I (COI) 以及共生体 16S rRNA 基因序列。有趣的是，COI 比对和引物匹配分析表明，COI 不适用于茋亚科基于 PCR 的条形码方法，因为该属具有高度多样化的碱基组成且没有保守的引物位点。然而，所有三个基因组的系统发育分析证实了形态学分配并支持新属 Paralaxus 的建立，并证实了其他 stilbonematine 属的地位。Paralaxus 在重叠的诊断特征集上与茋属 Laxus 最相似，但可以通过属特异性共生体外壳的形态与 Laxus 区分开来。我们对共生体被毛形态的关键参数作为所有须骨科属的特征的重新分析表明，通过修改描述，包括被毛，