Three new species of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) from Argentine coasts Author Russo, Virginia Lo A67891F9-E1BD-407D-AA67-E2ED5C722A21 Laboratorio de Meiofauna Marina (LAMEIMA) - Instituto de Diversidad y Evolución Austral (IDEAus-CONICET), Blvd. Alte. Brown 2915, U 9120 ACF Puerto Madryn, Chubut, Argentina. lorusso@cenpat-conicet.gob.ar Author Pastor De Ward, Catalina T. 421DA5CB-6FDD-4219-BC2C-79AE72A2E8F1 Laboratorio de Meiofauna Marina (LAMEIMA) - Instituto de Diversidad y Evolución Austral (IDEAus-CONICET), Blvd. Alte. Brown 2915, U 9120 ACF Puerto Madryn, Chubut, Argentina. pastor@cenpat-conicet.gob.ar text European Journal of Taxonomy 2021 2021-12-28 787 17 31 http://dx.doi.org/10.5852/ejt.2021.787.1611 journal article 2666 10.5852/ejt.2021.787.1611 84081e4a-a912-4d46-aa7d-55684073019a 2118-9773 5817931 F386403A-8DDA-4BC3-9197-A57E7AA05732 Mesacanthion longigubernaculum sp. nov. urn:lsid:zoobank.org:act: CBD8B1B5-E4C4-44A4-8B45-3180296C8AC3 Figs 3 , 5A, C Etymology In reference to the length of the gubernaculum. 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 . 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. Fig. 3. Mesacanthion longigubernaculum sp. nov. , ♂, holotype (CNP-NEM 27442). A . Cephalic sense organs on anterior end. B . Posterior end, showing copulatory apparatus and precloacal supplement organ. C . Gonads. D . Copulatory apparatus, spicules and gubernaculum. Scale bars: 1 = 20 μm; 2 = 100 μm; 3 = 10 μm. Female Unknown. 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.