taxonID	type	description	language	source
C80687886B265C00FF0CFAA6BEAC189E.taxon	description	(Figures 2 and 5 A; Table 1) Measurements. See Table 1.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B265C00FF0CFAA6BEAC189E.taxon	materials_examined	Material examinated. Holotype Male, CNP-NEM N ° 16684, collected on 12 / 2 / 2009 at “ Banco Perdices ” beach (latitude 40 º 46 ’ 52.4 ” S, longitude 64 º 51 ’ 5.5 ” W), during low tide. Collectors: Virginia Lo Russo and Gabriela Villares. Paratypes: Two males, same data as holotype. Collection numbers: CNP-NEM N ° 16673 and 16644. Type locality and habitat. “ Banco Perdices ” beach, collected on 12 / 2 / 2009, during low tide; salinity = 48.5 psu; temperature = 22.9 ºC. Environmental characteristics of surface sediment layer: mean particle size Q 2 (50 %) = 0.17 – 0.18; FF (%) = 2.77 – 14.07; SO index = 0.8; SK index = 0.9 – 1.0. Organic matter content / 100 g sediment = 0.1 – 5 mg.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B265C00FF0CFAA6BEAC189E.taxon	etymology	Etymology. The species epithet paraseparatum refers to its similarity with the species R. separatum Lorenzen, 1975.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B265C00FF0CFAA6BEAC189E.taxon	description	Description. Males: Body slender, tapering anteriorly towards the end of pharyngostome and gradually towards the posterior end. Cuticle ringed all along the entire body, starting after the cheilostome. Annuli tire-like, approximately 1 μm thick, weakly sclerotized in the neck region. Somatic setae (about 7 – 10 μm in length) are spread along the body, with a higher concentration in the neck region. Six short outer labial setae (0.3 – 0.5 cephalic diameter) and, in a separated crown, four longer cephalic setae (slightly more than 1 cephalic diameter) are present. Posterior to these setae, another crown of subcephalic setae of the same length as the cephalic setae is observed. Amphideal fovea is difficult to see, circular in shape (48 % cbd, corresponding body diameter, on average), and located just anterior to the base of the pharyngostome (35 – 39 μm from the anterior end). The long stoma consists of a short cheilostome with similar length and width, lacking armature, and a long pharyngostome. The pharynx is muscular, cylindrical and widens towards its posterior end without forming a true esophageal bulb (occupying 83 % of the cbd). The cardia is small and protrudes into the intestine. Paired and opposed outstretched testes, with the anterior testis on the left and posterior one on the right side of the intestine. Spicules are slender with a small capitulum, L-shaped and equal in size. The distal end of the spicules is surrounded by the gubernaculum, which bears six dorsal sclerotized projections. A precloacal setae is present. In addition, two tiny precloacal papillae were found in the holotype and in one paratype. The tail is conical and ventrally curved. Three caudal glands are present, ending in a short spinneret.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B265C00FF0CFAA6BEAC189E.taxon	diagnosis	Differential diagnosis. Rhynchonema paraseparatum sp. nov. is characterized by a two-part buccal cavity and a weakly sclerotized cuticle with tire-like annuli. Its amphideal fovea is round, inconspicuous, and located at the posterior end of buccal cavity. Males possess a precloacal setae and some specimens present precloacal papillae. The copulatory apparatus consists of L-shaped spicules with a capitulum and a gubernaculum with distal projections. Among the 39 valid species of the genus, R. paraseparatum sp. nov. shares the presence of an annulated cuticle without anterior or posterior projections with ten other species: R. ambianorum Boucher, 1974, R. brevituba Gerlach, 1953, R. deconincki Vitiello, 1967, R. gerlachi Vitiello, 1967, R. hirsutum Hopper, 1961, R. longituba Gerlach, 1953, R. lyngei (Allgén, 1940) Gerlach, 1953, R. moorea Boucher, 1974, R. quemer Boucher, 1974, and R. separatum Lorenzen, 1975. Of these species, only R. ambianorum and R. quemer share additionally the absence of gubernacular apophyses. R. paraseparatum sp. nov. is similar to R. ambianorum in the shape and position of the amphideal fovea, which is located at the end of the pharyngostome. However, the amphideal fovea is larger in the new species. The species also differ in the number of cephalic setae (4 vs. 6), with those in the new species being relatively longer in proportion to the cephalic diameter. Although spicules shape is similar, the spicules of the new species are slightly longer and bear a small capitulum. Both species possess distal projections on the gubernaculum, but their morphology differs. Furthermore, in R. ambianorum, the gubernaculum wraps around the entire spicules, whereas in the new species it only covers half their length. Although both species have an annulated cuticle without projections, the annuli are restricted to the amphids region in R. ambianorum, while in the new species they are visible along the entire body. Similar to R. ambianorum, R. paraseparatum differs from R. quemer in the number and length of cephalic setae and in the relative size of the gubernaculum in relation to the spicules. These two species share a similar general shape and spicules size, although the new species lacks the terminal hook-shaped adaptation present in R. quemer. Another diagnostic difference lies in amphideal fovea morphology - circular in the new species vs. oval with a spiral structure in R. quemer - and its position on a plate of fused annuli in the latter. Besides the cuticle with tire-like annuli, R. paraseparatum sp. nov. shares with R. moorea the presence of distal projections in the gubernaculum and similar De Man indices a and b. The species differs in the shape and size of the spicules. It is L-shaped and 1.5 cloacal diameter in length in the new species vs. almost straight and 1 cloacal diameter in R. moorea. They also differ in the relative size of the amphideal fovea (48 % on average vs. 74 % cbd), the number of somatic setae and the total body length (452 μm on average vs. 860 μm). R. paraseparatum sp. nov. is closely related to R. separatum. In addition to both having a cuticle with tire-like annuli, both species possess six small external labial papillae and two rings of setae of similar length (slightly more than 1 cbd). The buccal cavities are similar in shape and length, and both species have circular amphideal fovea located at a comparable distance from the anterior end. The esophagus terminates slightly expanded, without forming a true bulb and long somatic setae are present along the entire body. They also have similarly shaped spicules and precloacal papillae. However, key differences exist: in the new species, the cephalic setae crown is composed of four setae, followed by six cervical setae, whereas in R. separatum, the configuration is 6 + 4. R. paraseparatum sp. nov. has smaller amphideal fovea (50 % vs. 95 % cbd). The length and abundance of somatic setae are higher in R. separatum. The spicules of R. paraseparatum sp. nov. are longer than that of R. separatum but narrower relative to the cloacal diameter (28 – 30 μm and 1.4 – 1.8 cloacal diameter vs. 24 – 25 μm and 2.5 cloacal diameter, respectively). The new species has a conspicuous gubernaculum with distal dorsally directed projections. In contrast, the presence of the gubernaculum in R. separatum is uncertain, with observed structures possibly belonging to the spicules or to a possible gubernaculum. R. paraseparatum sp. nov. has three caudal glands, whereas R. separatum has only two. When comparing measurements, the new species shows a shorter total body length and lower De Man indices a and c.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B235C08FF0CF988BBF21DEA.taxon	description	(Figures 3 and 5 B, 5 D and 5 E; Table 2) Measurements. See Table 2.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B235C08FF0CF988BBF21DEA.taxon	materials_examined	Material examinated. Holotype Male, CNP-NEM N ° 1663, collected on 19 / 11 / 2013 at “ Belvedere ” beach (latitude 45 ° 58 ’ 25 ” S, longitude 67 ° 33 ’ 04 ” W), subtidal. Collector: Héctor Zaixso. Four males paratypes, collection numbers CNP-NEM N ° 1664 – 1667, and five females paratypes, collection numbers CNP-NEM N ° 1668 – 1672, same data as holotype. Type locality and habitat. “ Belvedere ” beach, 19 / 11 / 2013, subtidal (12 m depth); salinity = 48.5 psu; temperature = 22.9 ºC. Characteristics of surface sediment layer: mean particle size Q 2 (50 %) = 0.17 – 0.18; FF (%) = 2.77 – 14.07; SO index = 0.8; SK index = 0.9 – 1.0. Organic matter content / 100 g sediment = 1.3 – 1.4 mg.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B235C08FF0CF988BBF21DEA.taxon	etymology	Etymology. The species epithet bicuspidatum refers to the presence of two conspicuous tips at the distal part of the gubernaculums. Descriptions. Males: Body elongated, tapering gently toward the anterior end. Cuticle strongly sclerotized, especially at both ends of the body. The annuli (1 μm thick) start approximately 3.5 μm after the end of the cheilostome and extend until the beginning of the spinneret. From the anterior end to slightly beyond the esophagus, annuli project anteriorly. The annuli of the posterior part of the body, from a point slightly before the cloaca to the end, are projected backward. In the middle part of the body, the annuli have no projections, and the cuticle presents 8 longitudinal bands (4 per side). The exact location of the two inversion points could not be determined. Somatic setae are thin and scattered along the body (5 – 8 μm), those in the cervical region being only slightly shorter (4 – 7 μm). Inner labial sensilla papilliform. Outer labial setae less than 1 μm in length. Six very thin cephalic setae, 3 – 4 μm in length (approximately 1.3 head diameter), are present. Amphideal fovea are circular (65 % cbd), located over the end of the pharyngostome, and are situated 7.2 amphideal fovea diameters from the anterior end. There are 19 – 20 annuli anterior to the amphideal fovea. The stoma lacks armature and consist of a cheilostome with strongly cuticularized walls, longer than the width, and a longer pharyngostome. The pharynx is muscular and cylindrical, ending in an oval esophageal bulb. The cardia is inconspicuous. Two outstretched testes are present, the anterior one to the left and the posterior one to the right side of the intestine. Spicules are strongly cuticularized, arcuate with a rounded capitulum, and equal in size and shape (approximately 1.7 cloacal diameter). The gubernaculum surrounds the distal part of the spicules and has a ventral plate with two pointed tips and a short dorso-caudal apophysis. A precloacal setae, located just before the cloaca, can be observed. The tail is conical and ventrally curved. Three caudal glands, which are difficult to see, are present. The spinneret is short and barely visible.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B235C08FF0CF988BBF21DEA.taxon	description	Females: Similar to males in most morphological aspects. Amphideal fovea are equal in shape but slightly smaller (50.7 % cbd) and located farther away, at 9.9 amphideal fovea diameters from the anterior end. The reproductive system is monodelphic and prodelphic, with the outstretched ovary situated to the left of the intestine. The vulva is sclerotized and protruding. The caudal region has the same morphology as in males but with a shorter tail.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B235C08FF0CF988BBF21DEA.taxon	diagnosis	Differential diagnosis. Rhynchonema bicuspidatum sp. nov. is characterised by a strongly sclerotized cuticle with a three part arrangement. In the anterior body region, the annuli project forward, in the posterior body region, they project backward; and the midbody lacks projections but features longitudinal bands. Additional diagnostic features include long cephalic setae, the presence of somatic setae, and circular amphideal fovea. An esophageal bulb is present. The spicules are arcuate with a capitulum, and the gubernaculum has a distal ventral plate with pointed tips, as well as a short dorso-caudal apophysis. The new species belongs to the group of Rhynchonema species where cuticular annuli bear projections (either anteriorly or posteriorly). However, R. bicuspidatum sp. nov. is one of the only three species within this group that deviates from the typical cuticular pattern. Specifically, it exhibits anterior annuli with forward-pointing projections, a reversal point approximately at mid-body, and posterior annuli with backward-pointing projections. Within this group, R. bicuspidatum sp. nov. shares the presence of longitudinal cuticular bands with R. amakusanum Aryuthaka, 1989. However, they differ significantly in two key aspects: R. bicuspidatum sp. nov. has four bands per side, and its annuli within the banded region lack projections, whereas in contrast, R. amakusanum has two bands per side, and retains annular projections along its entire body. Additional similarities between the two species include the number and relative length of cephalic setae, as well as the shape and size of their amphideal fovea and spicules. Despite these similarities, they diverge in the length of the buccal cavity, the position of the amphideal fovea, the shape of the gubernaculum, and the number of caudal glands.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B2F5C08FF0CFD76BCD01CAE.taxon	diagnosis	Diagnosis (emended from Fonseca & Bezerra 2013): Xyalidae. Buccal cavity in two parts; anterior short, at level of cephalic setae, posterior tubular, restricted to the anterior part of the cervical region. Amphids circular, placed well posterior to the end of buccal tube. Males with one or two testes. Spicules short and arcuate. Tail conical.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B2F5C08FF0CFD76BCD01CAE.taxon	description	Number of species: 2	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B2F5C08FF0CFD76BCD01CAE.taxon	type_taxon	Type species: P. warwicki Gourbault, 1982	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B2F5C0EFF0CFBB2BC1D1B76.taxon	description	(Figures 4 and 5 C; Table 3) Measurements. See Table 3.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B2F5C0EFF0CFBB2BC1D1B76.taxon	materials_examined	Material examinated. Holotype Male, CNP-NEM N ° 16787, collected on 12 / 2 / 2009 at “ Banco Perdices ” beach (latitude 40 º 46 ’ 52.4 ” S, longitude 64 º 51 ’ 5.5 ” W), during low tide. Collector: Virginia Lo Russo and Gabriela Villares. One male, collection number CNP-NEM N ° 16801 and two juveniles paratypes, collection numbers CNP-NEM N ° 16843 and 16685, same data as holotype. Type locality and habitat. “ Banco Perdices ” beach, collected on 12 / 2 / 2009, during low tide; salinity = 48.5 psu; temperature = 22.9 ºC. Environmental characteristics of surface sediment layer: mean particle size Q 2 (50 %) = 0.17 – 0.18; FF (%) = 2.77 – 14.07; SO index = 0.8; SK index = 0.9 – 1.0. Organic matter content / 100 g sediment = 0.1 – 5 mg.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B2F5C0EFF0CFBB2BC1D1B76.taxon	etymology	Etymology. The species epithet uncinoa refers to the hook-like projections found on the gubernaculum, derived from the Latin word “ uncino ”. Descriptions. Males: Body elongated, tapering gently toward the anterior end. The cuticle is strongly sclerotized. Annuli, approximately 2 μm thick, begin about 4 μm posterior to the cheilostome and extended to the base of the spinneret. The annuli exhibit an inversion point at 58.2 – 63.8 % of the body length; anterior to this part, annuli project forward, and posterior to it, they project backward. Thin somatic setae, about 10 – 12 μm in length, are scattered along the body. Inner labial setae not seen. Outer labial setae are barely visible. Six very thin cephalic setae, 6 – 9 μm long, are present (approximately two head diameters long) at the base of cheilostome. The amphideal fovea are circular, occupying 52 % of the corresponding body diameter, and are located 3.5 – 3.9 amphideal fovea diameters from the base of the pharyngostome. There are 30 – 31 annuli before the amphideal fovea, and at the amphideal level, annuli encircle the amphideal fovea. The stoma lacks armature and consist of a short cheilostome, which is similar in length and width, followed by a longer pharyngostome. The pharyngostome has non-straight walls (as typically in Prorhynchonema and Rhynchonema). The pharynx is muscular and cylindrical, slightly widened at the base. The cardia is prominent, protruding into the intestine. Two outstretched testes are present, with the anterior one located to the left and the posterior one to the right side of the intestine. The spicules are strongly cuticularized, arcuate with small capitulum, and equal in size and shape (about 1.4 cloacal diameter). The gubernaculum surrounds the distal part of the spicules and has upward projections and a dorsal apophysis. A precloacal short setae (3 μm) is present. The tail is conical and ventrally curved. Caudal glands were not seen due to the cuticular thickness.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B2F5C0EFF0CFBB2BC1D1B76.taxon	description	Juveniles: Similar to adults in most morphological aspects. Their buccal cavity is deeper than its width. Somatic setae are very thin, making them difficult to measure, but they appear to be proportionally longer than those found in adults.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
C80687886B2F5C0EFF0CFBB2BC1D1B76.taxon	diagnosis	Differential diagnosis. Prorhynchonema uncinoa sp. nov. is characterised by having a buccal cavity with two well-defined parts. The anterior part has strongly cuticularized walls and it is equal in width and length. The posterior one is elongated, lacking both straight and cuticularized walls. Round amphideal fovea, located well posterior to the end of the buccal cavity, are surrounded by annuli. The copulatory structure consists of arcuate spicules with a capitulum and a gubernaculum with distal hooks and dorso-caudal apophysis. P. uncinoa sp. nov. is similar to P. warwicki Gourbault, 1982 and P. gourbaultae Nicholas & Stewart, 1995 in total length, De Man’s indeces (a, b and c) and length and distribution of somatic setae. P. warwicki shares a similar shape and relative size of amphideal fovea, as well as the manner in which the annuli surround them. Both species also exhibit a similar location of the inversion point and possess a strong cuticle with well-defined annuli. While the shape and size of the spicules and gubernaculum are similar, their hook positions differ (at the distal end of the spicules in P. warwicki and at the gubernaculum in the new species). P. uncinoa sp. nov. differs to P. warwicki in several key aspects: the shape and length of the buccal cavity (occupying 14 annuli vs. 3 annuli), the number of the cephalic setae (6 vs. 10), the number of annuli before the amphideal fovea (30 vs. 10), the number of testicles (2 vs. 1). Additionally, females of P. uncinoa sp. nov. lacks spermatheca, possesses a precloacal setae and shows no ventral cuticular modification at that region. Beyond the previously mentioned characteristics, P. uncinoa sp. nov. is similar to P. gourbaultae by having six cephalic setae, but differs in setae length (more than 1 cephalic diameter vs. 0.5). The amphideal fovea of P. gourbaultae are cryptospiral (vs. round in the new species), smaller (30 vs. 50 % of the cbd), and are located at similar distance to the anterior part, but different number of annuli (55 annuli before amphids vs. 30). The buccal cavity is longer in the new species (27.5 vs. 17 µm). Furthermore P. gourbaultae has a weakly annulated cuticle and an inversion point positioned further anteriorly. The copulatory apparatus also differs: while the spicules are similar in length and general shape, the distal tip varies (spoon-shaped in P. gourbaultae, pointed in P. uncinoa), and the gubernaculum itself has a different shape.	en	Russo, Virginia Lo, Pastor, Catalina T. (2026): Three new species of Xyalidae (Nematoda: Monhysterida) from Argentinean coast. Zootaxa 5757 (4): 324-338, DOI: 10.11646/zootaxa.5757.4.2, URL: https://doi.org/10.11646/zootaxa.5757.4.2
