taxonID	type	description	language	source
03A942223A36FFE9FDA9FC64FB034EF3.taxon	type_taxon	Type species Mesacanthion lucifer (Filipjev, 1927) Gerlach & Riemann, 1974	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A36FFE9FDA9FC64FB034EF3.taxon	diagnosis	Diagnosis (after Jeong et al. 2019) Outer labial and cephalic setae situated at middle or anterior end of cephalic capsule. Mandibles welldeveloped, provided with claws, arch-shaped, each consisting of two rod-like columns anteriorly united by a curved bar. Teeth shorter than mandibles. Spicule mostly short, unipartite and symmetrical, sometimes long, bipartite (divided by a seam: M. ditlevseni (Filipjev, 1927) Gerlach & Riemann, 1974) and asymmetrical (anisomorphic and anisometric: M. diplechma (Southern, 1914) Filipjev, 1927). If long, usually gubernaculum present with caudal apophysis. Marine and freshwater.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A36FFECFDB2FA55FD714862.taxon	description	urn: lsid: zoobank. org: act: 62 E 01288 - 0803 - 428 A-BB 5 E-D 51 B 592 D 1591 Figs 2, 5 D	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A36FFECFDB2FA55FD714862.taxon	etymology	Etymology In reference to the proximal part of the gubernaculum that ends in two small pointed teeth, from the Latin word ‘ bifidum’.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A36FFECFDB2FA55FD714862.taxon	materials_examined	Material examined Holotype ARGENTINA • ♂; Chubut, Nuevo Gulf, Puerto Madryn beach; 42 ° 45 ′ S, 65 ° 02 ′ W; 1 Mar. 2003; C. Pastor and V. Lo Russo leg.; high tide; fine sand sediments; CNP-NEM 936. Paratypes ARGENTINA • 1 ♀; same collection data as for holotype; CNP-NEM 937 • 2 ♀♀; Chubut, Nuevo Gulf, Bahía Kaiser; 42 ° 46 ′ S, 64 ° 59 ′ W; 8 m b. s. l.; Oct. 1997; C. Pastor leg.; CNP-NEM 938 to 939.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A36FFECFDB2FA55FD714862.taxon	description	Description Measurements See Table 1. Male (holotype) Large and stout body. Cuticle smooth. Few short (about 9 – 12 µm long) somatic setae scattered along body. Cephalic region set-off with presence of cephalic capsule (17 µm in height). Cephalic capsule with same thickness throughout its rough surface. Anterior edge located at level of cephalic setae. Posterior end with no real incisions forming lobes but with slightly scalloped edge. Three rounded lips. Each lip carries two inner slender labial setae (9 µm long). Six outer labial setae (18 µm long) and four barely shorter cephalic setae (17 µm long) located at anterior of cephalic capsule, arranged in single crown. Immediately posterior to cephalic capsule four short subdorsal and subventral subcephalic setae (about 4 µm) and further posteriorly (about 30 µm after cephalic capsule), arranged in four subdorsal and subventral groups of two or three, larger cervical setae present (15 µm long). Amphideal aperture circular and amphideal fovea pouch-shaped, small (8 % of cbd), laterally located just posterior to capsule end. Metanemes not seen. Funnel-shaped buccal cavity with wide opening widens at level of mandibles. Its armature consists of three mandibles, each composed of two vertical rods (16 µm) united by arcuate bar (7 µm), at top ending as left and right claws. Each mandible has tooth (13 µm). The three teeth equal in size. No ocellus nor pigment spots. Pharynx cylindrical with irregular contours, cardia not visible. Nerve ring lying at about 33 % of pharynx length from anterior end. Excretory-secretory system not visible. Reproductive system diorchic, with opposed and outstretched testes in right position relative to intestine. One precloacal supplement, bar-shaped, located about 3 abd above cloaca. No precloacal setae present. Spicules paired, arcuate, with slight manubrium (1.4 abd). Gubernaculum small (26 % of spicule, 0.4 abd), embracing spicule, two dorso-caudal apophyses present. The gubernaculum divided into two parts. Proximal part with two small pointed teeth, surrounds tips of spicules. Arched dense zone, shaped like ring, separates it from distal triangular part. Tail 3.4 abd long, conical with end part cylindrical (1 / 5 approximately). Few short caudal setae can be seen. Caudal gland bodies in pre-anal region. Terminal setae not present. Cuticle around spinneret hardly englobed. Female Similar to males in general body shape, anterior sensilla, amphideal fovea and cuticle. Females longer and larger than males. Lips not rounded but pointed. Short somatic setae present all along body (9 – 12 µm) in greater quantity than in males. Reproductive system didelphic amphidelphic, with two antidromously reflexed ovaries, positioned left of intestine. Vulva and vagina conspicuous, with associated musculature. Vulva not sclerotized. Tail conical, ending in 1 / 5 cylindrical part, but this part less differentiated than in males. Caudal gland bodies in pre-anal region. Many short caudal setae. Terminal setae not present. Cuticle around spinneret englobed.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A36FFECFDB2FA55FD714862.taxon	diagnosis	Differential diagnosis Mesacanthion bifidum sp. nov. is characterized by its smooth cuticle, relatively short labial and cephalic setae, onchia of equal size, amphidial fovea pouch-shaped, spicule arcuate, gubernaculum with pointed tips at the proximal end and dorso-caudal apophysis, and tail conical-cylindrical without terminal setae. Following the key of Jeong et al. (2019), our new species is closely related to M. virile. Both species share some characteristics, such as spicules length less than 2 abd and gubernaculum with dorso-caudal apophysis and triangular shape with two parts. The male in M. bifidum sp. nov. has the outer labial setae less than 1 cephalic diameter in length, whereas in M. virile they are about 1.25 cephalic diameters. The position of the precloacal supplementary organ in M. virile is closer to the cloaca (1.1 spicule length distant from the anus) than in M. bifidum sp. nov. (2.7 spicule lengths distant from the anus) and the spicules of both species are quite different. M. virile has an L-shaped spicule with a mid-projection for muscle insertion, whereas M. bifidum sp. nov. has an arcuate spicule without projections. On the other hand, the gubernaculum also has differences in shape. Although in both species the distal part is rather similar, the proximal part is rod-like in M. virile, whereas in M. bifidum sp. nov. two-pointed structures surround the spicule tip in the posterior position.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A33FFE2FDF8FCFAFB454FB9.taxon	description	urn: lsid: zoobank. org: act: CBD 8 B 1 B 5 - E 4 C 4 - 44 A 4 - 8 B 45 - 3180296 C 8 AC 3 Figs 3, 5 A, C	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A33FFE2FDF8FCFAFB454FB9.taxon	etymology	Etymology In reference to the length of the gubernaculum.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A33FFE2FDF8FCFAFB454FB9.taxon	materials_examined	Material examined Holotype ARGENTINA • ♂; Chubut, San Jorge Gulf, Rada Tilly; 45 ° 55 ′ 07 ″ S, 67 ° 32 ′ 79 ″ W; 1 Apr. 2006; C. Pastor leg.; high littoral; fine sand sediments; CNP-NEM 27442. Paratypes ARGENTINA • 2 ♂♂; same collection data as for holotype; CNP-NEM 27259 and 27443.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A33FFE2FDF8FCFAFB454FB9.taxon	description	Description Measurements See Table 1. Male (holotype) Long and slender body. Cuticle slightly striated. Cephalic region set off with presence of delicate cephalic capsule (7 µm in height). Cephalic capsule with same thickness throughout its smooth surface. Anterior edge located above level of cephalic setae. Posterior end of cephalic capsule almost straight. No ocellus nor pigment spots. Three rounded lips, higher than mandibles. Each lip carries two inner labial setae (8.5 µm long). Six outer labial setae (18 µm long), longer than four cephalic setae (13 µm long), located at half the total height of cephalic capsule. Posterior to cephalic capsule one crown of subcephalic setae formed by six pairs of setae with one short (7 µm) and one long seta (20 µm). Then, crown of cervical setae (20 µm behind first) formed by four setae, two in subdorsal position and two in subventral position, and eight setae (7 – 20 µm) in same position as first crown. After nerve ring, another crown composed of groups of 4 – 5 short and scattered setae (about 4 µm) arranged in rows beside one longer seta (20 µm). Rest of body presents few long somatic setae (about 14 µm) until cloaca. Amphideal aperture lentil-shaped, small (14 % of cbd) located just posterior to capsule end. Metanemes not seen. Funnel-shaped buccal cavity with wide opening diminishing its width towards level of mandibles to their base. Mandibles composed of two vertical rods (10 µm) united by delicate arcuate bar (4 µm) at top and ending as claws. Dorsal tooth slightly smaller than two ventrosublateral teeth. All teeth pointed. Pharynx cylindrical with irregular contours. Cardia triangular and going into intestine. Nerve ring lying at about 36 % of pharynx length from anterior end. Secretory-excretory system not visible. Reproductive system diorchic, with opposed and outstretched testes in right position relative to intestine. One small precloacal bar-shaped supplement located about 3 abd above copulatory organ, almost parallel to body wall. Subventral precloacal setae present. Spicules paired, with distal portion arcuate (⅔) and proximal portion straight and dorsally directed (⅓). Gubernaculum without apophysis, barely evident surrounding spicule, 63 % of spicule. Tail 4.6 abd long, conical in first part (⅓) with end part cylindrical (⅔ approximately). Caudal gland bodies in pre-anal region. Few caudal setae of same length as somatic setae and two small (3 µm) terminal setae present. Female Unknown.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A33FFE2FDF8FCFAFB454FB9.taxon	diagnosis	Differential diagnosis Mesacanthion longigubernaculum sp. nov. is characterized by its long and slender body, striated cuticle, relatively long cephalic and cervical setae, onchia of different sizes, amphidial fovea lentil-shaped, spicule arcuate, gubernaculum surrounding the spicule, and tail conical-cylindrical with terminal setae. Following the key of Jeong et al. (2019), M. longigubernaculum sp. nov. is similar to M. longissimesetosum and M. pali. However, M. longigubernaculum sp. nov. differs from M. longissimesetosum in having shorter inner and outer labial setae (8 and 19 vs 12 and 65 – 70 µm, respectively) and cephalic setae (12 vs 65 µm), a higher a index (52.9 vs 30), and a different shape of the gubernaculum. Mesacanthion pali has longer inner and outer labial setae (24 and 84 vs 8 and 19 µm, respectively), lower b and c′ indices (b = 3 vs 4.5; c′ = 2.8 vs 4.4), a longer cephalic capsule (26 vs 16 µm), a gubernaculum with well-developed apophysis and the precloacal organ located at more than 2 spicule lengths from the anus.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A3DFFE6FDDFFA92FB264A1D.taxon	description	urn: lsid: zoobank. org: act: 79 D 5 AC 77 - 6294 - 4397 - A 91 C- 8 DC 83 E 8 F 02 DB Figs 4, 5 B, E	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A3DFFE6FDDFFA92FB264A1D.taxon	etymology	Etymology Named after the locality where this species was found.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A3DFFE6FDDFFA92FB264A1D.taxon	materials_examined	Material examined Holotype ARGENTINA • ♂; Río Negro, San Matías Gulf, San Antonio Oeste; 40 ° 43 ʹ S, 64 ° 58 ′ W; 10 Oct. 2006; C. Pastor leg.; lower littoral; fine sand sediments; CNP-NEM 849. Paratypes ARGENTINA • 1 ♂; same collection data as for holotype; CNP-NEM 848 • 1 ♂; Río Negro, San Matías Gulf, San Antonio Oeste; 40 ° 43 ʹ S, 64 ° 56 ′ W; 17 Jun. 2006; C. Pastor leg.; lower littoral; fine sand sediments; MACN-In 43853.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A3DFFE6FDDFFA92FB264A1D.taxon	description	Description Measurements See Table 1. Male (holotype) Large and stout body. Cuticle slightly striated. Cephalic region set off with presence of cephalic capsule (16 µm in height). Cephalic capsule with same thickness throughout its rough surface. Anterior edge located above level of cephalic setae. Posterior end of cephalic capsule almost straight. No ocellus nor pigment spots. Three high lips with points facing outward. Each lip carries two inner labial setae (7 µm long). Six outer labial setae (32 µm long) double length of four cephalic setae (16 µm long), located at middle of cephalic capsule arranged in single crown. Near and posterior to cephalic capsule, short subcephalic setae (about 8 – 9 µm), two in subventral position, two lateral and two in subdorsal position. About 25 µm after cephalic capsule, toward nerve ring, pairs of cervical setae in subventral and subdorsal positions (same size as previous). Same size setae present between nerve ring and end of oesophagus, but not in pairs and scattered. Rest of body presents few shorter somatic setae (about 4 µm) until cloaca, except for longer one (8 µm) located just before precloacal organ. Amphideal aperture pore-shaped, small (7 % of cbd), located at middle of cephalic capsule. Metanemes not seen. Funnel-shaped buccal cavity with wide opening that diminishes its width towards level of mandibles to their base. Its armature consists of three mandibles composed of two vertical rods (15 µm) united by arcuate bar (9 µm) at top and ending as claws. Each mandible associated with tooth forming a unit. Dorsal tooth slightly smaller (9 µm) than two ventrosublateral teeth (11 µm). Tip of teeth not very pointed. Pharynx cylindrical with irregular contours. Cardia barely visible, triangular and going into intestine. Nerve ring lying at about 33 % of pharynx length from anterior end. Secretory-excretory system not visible. Reproductive system diorchic, with opposed and outstretched testes in right position relative to intestine. One ventral precloacal bar-shaped supplement located about 2.4 abd anterior to cloaca. Subventral precloacal setae present. Spicule paired, arcuate, with slight seam forming manubrium (1.5 abd). Gubernaculum with dorsal apophysis (29 % of spicule), perpendicular to body axis. Tail 4.1 abd long, conical. It presents few setae in subventral and subdorsal positions (8 µm). Caudal gland bodies in anal region. Terminal setae not present. Female Unknown.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
03A942223A3DFFE6FDDFFA92FB264A1D.taxon	diagnosis	Differential diagnosis Mesacanthion sanantoniensis sp. nov. is characterized by its long and stout body, striated cuticle, long cephalic setae, onchia of different sizes, amphidial fovea pouch-shaped, spicule arcuate, gubernaculum with dorsal apophysis, and tail conical without terminal setae. As M. longigubernaculum sp. nov., M. sanantoniensis sp. nov. is similar to M. longissimesetosum and M. pali, according to the key of Jeong et al. (2019). They have males with a supplementary organ lying further away from the cloaca, presence of gubernaculum, and presence of subventral precloacal setae. Mesacanthion sanantoniensis sp. nov. has shorter inner and outer labial setae than M. pali (7.3 and 33 vs 24 and 92 µm, respectively), shorter cephalic setae (20.2 vs 84 µm), lower a index (24.7 vs 50), and higher b (4.1 vs 3.3) and c′ (4.1 vs 2.8) indices, shorter mandibles (15 vs 25 µm) and a smaller precloacal organ (19.3 vs 31 µm). Mesacanthion longissimesetosum and M. sanantoniensis sp. nov. have differences in mandible shape. Mesacanthion sanantoniensis sp. nov. has mandibles ending in jaws, posteriorly straight and slightly smaller. Mesacanthion longissimesetosum has mandibles posteriorly arcuate and larger. Mesacanthion sanantoniensis sp. nov. has a distinct amphid and three crowns of cervical setae. Mesacanthion longissimesetosum has no amphid, and the cervical setae are not arranged in crowns. Mesacanthion longissimesetosum has two tiny precloacal setae just before the spicule vs none in M. sanantoniensis sp. nov. At the posterior part of the tail, the new species has two pairs of setae plus one single seta, and M. longissimesetosum has none. Considering the similarities between M. longigubernaculum sp. nov., M. sanantoniensis sp. nov., M. longissimesetosum and M. pali, we provide a small key for an easy differentiation.	en	Russo, Virginia Lo, Pastor De Ward, Catalina T. (2021): Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts. European Journal of Taxonomy 787: 17-31, DOI: https://doi.org/10.5852/ejt.2021.787.1611, URL: http://dx.doi.org/10.5852/ejt.2021.787.1611
