taxonID	type	description	language	source
3620C3174F51FF83FC90B09A6EFCAC4E.taxon	description	ZooBank registration (http: // zoobank. org): urn: lsid: zoobank. org: act: 8697 C 6 A 0 - 3508 – 4833 - 81 DC-D 2 A 86 B 0 B 00 AC. GenBank entries for 18 S rRNA and COI sequences: see Table S 1. Species from Bermuda can be identified by an amphidial fovea with two turns and the bulbus occupies 30 % – 32 % of the pharynx length. In male specimens, the velum makes up 30 % – 40 % of the tail length and the spicula are strongly arcuate (Figure 2 h-j).	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F51FF83FC90B1176816AEC8.taxon	description	ZooBank registration (http: // zoobank. org): urn: lsid: zoobank. org: act A 659918 E-FD 88 - 4584 – 8115 - CD 4 C 396 AE 400. GenBank entries for 18 S rRNA and COI sequences: see Table S 1. Species from Belize, Central America, can be identified by an amphidial fovea with one and a half turns and the bulbus occupies 32 % – 35 % of the pharynx length. The velum of male specimens makes up 43 % – 47 % of the tail length and the spicula are moderately arcuate (Figure 2 a-g).	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F51FF83FC90B4AA6EB2AF4A.taxon	description	ZooBank registration (http: // zoobank. org): urn: lsid: zoobank. org: act: 417691 DB- 5 EE 3 - 49 B 7 - B 838 - 1 BBF 1569235 F. GenBank entries for 18 S rRNA and COI sequences: see Table S 1. We erect a new genus for this clade with the following characters: cephalic capsule without block layer (Figure 2 b, e, i, l, m); amphidial fovea spiral, no sexual dimorphism in shape of fovea (Figure 2 b, g, i, l, m); male tail with velum (Figure 2 c, j, n); symbiotic bacteria coccobacilli, arranged as multilayered coat covering body except for anterior and posterior end (Figure 1 a, b). All Paralaxus sequences form statistically supported clades in phylogenetic analyses of the 18 S as well as the COI datasets (Figure 3 a, b). The specimens for each Paralaxus species clustered together in both gene sets and clearly separated each Paralaxus species from each other (Figure 3 a, b). We describe three new species and report on two additional ones.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F51FF82FC90B2186CAEADC5.taxon	description	ZooBank registration (http: // zoobank. org): urn: lsid: zoobank. org: act: 4 BABABB 3 - 1 DD 4 - 46 F 1 - A 69 C-BC 06 C 1271 D 01. Species from Florida Keys, USA, can be identified by an amphidial fovea with 1 – 1.2 turns and the bulbus occupies 29 % – 33 % of the pharynx length. In male specimens, the velum makes up 35 % – 37 % of tail length and the spicula are weakly arcuate (Figure 2 k-o).	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F50FF82FFBEB3E16ED9AC35.taxon	description	GenBank entries for 18 S rRNA and COI sequences see Table S 1. Representatives of two additional species, Paralaxus sp. “ heron 1 ” and P. sp. “ oahu 1 ” have been found off Heron Island, Great Barrier Reef, Australia and Oahu, Hawaii Islands. Due to the lack of properly preserved material, a formal description is currently not possible. Morphological details and measurements taken from microphotographs of live animals prior to fixation are presented in the Supplementary Information (Table S 2). Detailed Paralaxus species descriptions are presented in the Supplementary Information (Supplementary Results and Figs. S 1 - S 8).	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F52FF8FFFBEB34969B1A8ED.taxon	description	In addition to the nine Paralaxus specimens, we generated and analysed single worm metagenomes from 20 specimens covering six additional stilbonematine genera and extracted both host 18 S and COI gene sequences for all specimens (Table S 1). To analyse the monophyly of the Stilbonematinae, we reconstructed their phylogenetic relationships based on the 18 S gene, as this is the only gene with sufficient taxon sampling. The 18 S gene matrix, with a minimum sequence length of 1,300 bp, consisted of sequences from 78 Stilbonematinae as well as from 52 closely related Desmodorida and Microlaimida, and an outgroup of 24 Chromadorida sequences. The Stilbonematinae formed a highly supported clade, with other non-stilbonematine Desmodoridae and Draconematidae as closest relatives (Figure 3 a). For the analysis of the Stilbonematinae genera, we created a mitochondrial COI tree in addition to the 18 S tree, using 27 metagenome-derived stilbonematine COI sequences and an ascarid as outgroup (Baylisascaris procyonis mitochondrion, JF 951366) (Figure 3 b). Based on both datasets, all Stilbonematinae genera including Paralaxus gen. nov. formed well-supported clades (Figure 3 a, b), thus corroborating our morphological assignments. In both the 18 S and the COI-based analysis, the genus Leptonemella was phylogenetically most closely related to Paralaxus, with high statistical support in the 18 S dataset and acceptable support in the COI-based phylogenies (Figure 3 a, b). Other than that, the branching patterns between genera were largely inconsistent between the COI and 18 S datasets. This could be linked to the remarkably low support values for many internal nodes in the COI dataset compared to the much higher spectrum of support values for the 18 S-based tree topologies. In an extended 18 S and COI data matrix, we also included eight short-length, PCR-amplified sequences of five different Stilbonematinae species with a record of problematic phylogenetic placement — Laxus parvum, Robbea porosum, Stilbonema brevicolle, Catanema exile and Leptonemella brevipharynx (Armenteros, Ruiz-Abierno, et al., 2014; Figs. S 9 and S 10). With our new 18 S- and COI-based datasets, the placements of Leptonemella brevipharynx and Catanema exile sequences were resolved and both clustered with sequences of their respective genera. For the sequences of the other three species, we observed inconsistent grouping with Stilbonematinae genera between the datasets (Figs S 9 and S 10). In the extended 18 S tree, two Stilbonema brevicolle sequences appeared in two different clades, one closely related to Leptonemella, while the other one clustered with a yet undescribed genus-level clade designated as genus ‘ B’ (Zimmermann et al., 2016). In contrast to the 18 S-based placement, the Stilbonema brevicolle sequences grouped together in the extended COI tree and formed a novel sister clade to several genera including Stilbonema, Robbea, Leptonemella and Paralaxus (Figure S 10). In the extended 18 S data matrix, the Laxus parvum sequences clustered within the genus Robbea (Figure S 9). Similarly, in the COI dataset the L. parvum sequence grouped with the only available sequence of the genus Robbea (Figure S 10). The sequences designated as Robbea porosum formed a divergent genus-level clade but the placement of this clade was not consistent for the two marker genes (Figs S 9 and S 10). In the 18 S-based analyses, they were the sister clade to the genus Laxus, while in the COI dataset, they formed the sister clade to all other Stilbonematinae. 3.6 | Symbiont 16 S phylogeny shows genus-level specificity We constructed a matrix from all full-length 16 S ‘ Thiosymbion’ sequences, available in the databases and 21 metagenome-derived symbiont sequences — using five free-living Chromatiaceae as an outgroup. All symbiont sequences from the same host genus clustered in well-supported clades, irrespective of the number of gutless oligochaete symbionts included within those clades (Figure 4). In congruence to the host data, the symbionts of all newly sequenced Paralaxus individuals as well as the two previously published sequences from the Caribbean and Australia (Zimmermann et al., 2016) formed a supported clade in the 16 S phylogeny (Figure 4). 3.7 | COI primers cannot cover the compositional diversity in Stilbonematinae Despite a highly sensitive PCR setup, we could not amplify the COI of any Paralaxus specimen using the widely used primer set by Folmer et al. (1994) (data not shown). To elucidate these PCR-based amplification problems, we analysed all our metagenome-derived full-length stilbonematine nematode COI sequences for compositional patterns. The COI sequences of the 19 stilbonematine nematode species that belonged to seven genera had a highly variable guanine and cytosine (GC) content, ranging from 27.7 % in the genus Robbea to 53.1 % in the genus Leptonemella. We identified substantial priming problems with the Folmer et al. (1994) primer set as well with as a second set (JB 3 F and JB 5 R) that is commonly used for COI amplification (Armenteros, Rojas-Corzo, et al., 2014; Armenteros, Ruiz-Abierno, et al., 2014; Bowles, Blair, & McManus, 1992; Derycke, Vanaverbeke, Rigaux, Backeljau, & Moens, 2010). Across the Stilbonematinae diversity, the Folmer et al. (1994) primers had a range of 3 – 11 mismatches for the 1490 F primer and 1 – 5 mismatches for the 2198 R primer. Similarly, the primers JB 3 F and JB 5 R used in a recent study by Armenteros, Ruiz-Abierno, et al. (2014) showed up to 11 mismatches for each primer (Figure 5). Of all four primers tested, only the JB 5 R perfectly matched and only to three sequences from Robbea hypermnestra (Figure 5). We performed an in silico analysis that mimics PCR conditions of low stringency, which would overcome up to 11 primer mismatches and would cover the whole diversity of Stilbonematinae. Our analysis showed that such permissive conditions would lead to unspecific binding at multiple alternate sites of the COI gene. We tried to design alternative primer sets, but no primer pair, neither for the full-length nor for partial regions, would reliably target the COI gene and would cover all genera of Stilbonematinae. 3.8 | The symbiont coat as a new diagnostic feature for Stilbonematinae genera In addition to several host-based morphological differences, the presence of a multilayered coat distinguishes the new genus Paralaxus from Laxus, which carries a monolayer of symbionts. Extending from this observation, we analysed the structure and morphology of the bacterial coat of all Stilbonematinae genera and found substantial and highly informative differences. We therefore propose to include the description of the bacterial coat given below into the genus trait sets as a new character (see also iconographic overview of the different genera in Figure 6).	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5DFF8FFC90B6D86876AF30.taxon	description	Crescent-shaped bacteria, all cells in contact with host cuticle, double-attached, forming complex monolayer, covering whole body except anterior and posterior tip.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5DFF8FFC90B14D6ED8AE46.taxon	description	Cocci or coccobacilli, all cells in contact with host cuticle, forming simple monolayer, covering whole body except anterior and posterior tip or starting at distance from anterior end.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5DFF8FFC90B0206921AEC8.taxon	description	Rods (bacilli), lying parallel to cuticle, all cells in contact with host cuticle, forming simple monolayer, probably covering whole body.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5DFF8FFC90B36B6828AD80.taxon	description	Filamentous bacteria, all cells in contact with host cuticle, singly attached, forming complex monolayer, covering whole body except anterior and posterior tip.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5DFF8FFC90B0F66876AD1E.taxon	description	Crescent-shaped bacteria, all cells in contact with host cuticle, double-attached, forming complex monolayer, covering whole body except anterior and posterior tip.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5DFF8FFC90B3DD690DACF0.taxon	description	Cocci, coccobacilli or rods (bacilli), all cells in contact with host cuticle, rods standing perpendicular to cuticle surface, forming simple monolayer, covering whole body except anterior and posterior tip or starting at distance from anterior end.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5FFF8DFF58B0F76FBBAD1E.taxon	description	Cocci, only a fraction of cells in contact with host cuticle, forming multilayer embedded in gelatinous matrix, covering whole body except anterior and posterior tip.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5FFF8DFF58B3686FAEAD80.taxon	description	Crescent-shaped bacteria, all cells in contact with host cuticle, double-attached, forming complex monolayer, covering whole body except anterior and posterior tip.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5FFF8DFF58B3E26C3AACD6.taxon	description	Cocci or coccobacilli, only a fraction of cells in contact with host cuticle, forming multilayer embedded in gelatinous matrix, covering whole body except anterior and posterior tip.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5FFF8DFF58B2B0690DAD38.taxon	description	Cocci, coccobacilli or rods (bacilli), all cells in contact with host cuticle, rods standing perpendicular to cuticle surface, forming simple monolayer, covering whole body except anterior and posterior tip or starting at distance from anterior end.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5FFF8DFC90B34569FDADAD.taxon	description	Crescent-shaped bacteria, all cells in contact with host cuticle, single- or double-attached, forming monolayer, starting at distance from anterior end.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5FFF8DFC90B21B6950ACF0.taxon	description	Cocci or rods, all cells in contact with host cuticle, forming simple monolayer covering whole body except for anterior and posterior tip.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
3620C3174F5EFF8CFF5DB1166FBEAFFE.taxon	description	Cocci, only a fraction of cells in contact with host cuticle, forming multilayer embedded in gelatinous matrix, covering whole body except anterior and posterior tip.	en	Scharhauser, Florian, Zimmermann, Judith, Gruber-Vodicka, Harald R., Ott, Jörg A., Leisch, Nikolaus (2020): Morphology of obligate ectosymbionts reveals Paralaxus gen. nov.: A new circumtropical genus of marine stilbonematine nematodes. Zoologica Scripta 49 (3): 379-394, DOI: 10.1111/zsc.12399, URL: https://doi.org/10.1111/zsc.12399
