Mesacanthion sanantoniensis, Russo & Pastor De Ward, 2021

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, pp. 17-31 : 26-30

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scientific name

Mesacanthion sanantoniensis

sp. nov.

Mesacanthion sanantoniensis sp. nov.

Figs 4 View Fig , 5B, E View Fig


Named after the locality where this species was found.

Material examined

Holotype ARGENTINA • ♂; Río Negro, San Matías Gulf, San Antonio Oeste ; 40°43ʹ S, 64°58′ W; 10 Oct. 2006; C. Pastor leg.; lower littoral; fine sand sediments; CNP-NEM 849 . GoogleMaps

Paratypes ARGENTINA • 1 ♂; same collection data as for holotype; CNP-NEM 848 GoogleMaps 1 ♂; Río Negro, San Matías Gulf, San Antonio Oeste ; 40°43ʹ S, 64°56′ W; 17 Jun. 2006; C. Pastor leg.; lower littoral; fine sand sediments; MACN-In 43853 GoogleMaps .



See Table 1 View 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.



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.