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 bifidum sp. nov. urn:lsid:zoobank.org:act: 62E01288-0803-428A-BB5E-D51B592D1591 Figs 2 , 5D Etymology In reference to the proximal part of the gubernaculum that ends in two small pointed teeth, from the Latin word ‘bifidum’. 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 . Table 1 (continued on next page). Measurements (μm) of Mesacanthion bifidum sp. nov. , M. longigubernaculum sp. nov. and M. sanantoniensis sp. nov. , including mean values (range).

M. bifidum sp. nov.		M. longigubernaculum sp. nov.		M. sanantoniensis sp. nov.
Character	Holotype ♂	♀♀	Holotype ♂	♂♂	Holotype ♂	♂♂
Number of specimens	1	3	1	2	1	2
L	2990	3363.3 (3150–3515)	1450	1445.7 (1380–1507)	2540	2466.7 (2120–2740)
a	19.9	25 (22.8–26.2)	55.8	52.9 (47.8–55.8)	20.8	24.7 (20.8–29.4)
b	4.9	4.9 (4.6–5.2)	4.1	4.5 (4.1–5.4)	4.2	4.1 (3.7–4.3)
c	13.8	14.7 (13.8–15.7)	13.9	14.6 (13.9–15.1)	12.3	13.1 (12.3–13.9)
Inner labial setae length	7	8 (7–9)	8.5	8 (7.5–8.5)	7.5	7.3 (7–7.5)
Outer labial setae length	23	20.5 (20–21)	18	19 (16–23)	36	33 (28–36)
Cephalic setae length	18	13 (12–14)	13	12 (9–14)	21	20.2 (17.5–22)
Buccal cavity width	13	22.3 (18–27)	8	10.3 (8–13)	16	15 (14–16)
Buccal cavity length	18	19.3 (17–21)	12	10.7 (10–12)	16	16.7 (16–17)
Cephalic diameter at cephalic setae level	29	39.3 (38–41)	18	18.8 (18–20)	34	31.3 (29–34)
Amphid aperture width	2	1.9 (1.8–2)	3	3.5 (3–4)	2	1.8 (1.5–2)
Body diameter at amphid level	36	50.7 (48– 52)	21	21 (21–21)	38	36.3 (32–39)
Amphid width/cbd (%)	5.6	3.8 (3.5–4.2)	14.3	16.7 (14.3–19)	5.3	5.1 (3.8–6.3)
Cephalic capsule length	29	34 (32–36)	17	16 (14–17)	29	27.8 (26.5–29)
Cervical setae	10	15 (14–16)	9.5/20	13.3 (6.5–20)	9/13	10.2 (7.5– 14)
Nerve ring from anterior end	205	241.7 (210–265)	125	110 (92–125)	210	200 (170–220)
Body diameter at nerve ring level	70	89.7 (87–94)	25	26.7 (24–31)	76	67 (48–77)
Pharynx length	615	686.7 (680–700)	350	325.3 (256–370)	600	603.3 (570–640)
Body diameter at pharynx base level	96	121.7 (115–132)	24	25.7 (23–30)	102	91.3 (60–112)
Maximum body diameter	150	134.7 (132–138)	26	27.5 (25–31.5)	122	103 (72– 122)
Anal/cloacal distance from anterior end	2774	3133.7 (2922–3291)	1345.5	1346.5 (1287–1407)	2334	2277 (1967–2530)
Anal/cloacal body diameter	63	59.7 (57–63)	22.5	22.5 (20–25)	55	46.7 (36–55)
Spicule length as arc	93	–	22	22.7 (22–23)	71.5	71.2 (69–73)
Spicule length as cloacal diameter	1.5	–	1	1 (0.9–1.1)	1.3	1.6 (1.3–1.9)
Gubernaculum length	24	–	14	13.7 (10–17)	33	30.3 (27–33)
Gubernaculum apophysis length	21	–	–	–	–	–

Table 1 (continued).

M. bifidum sp. nov.		M. longigubernaculum sp. nov.		M. sanantoniensis sp. nov.
Character	Holotype ♂	♀♀	Holotype ♂	♂♂	Holotype ♂	♂♂
Gubernaculum as spicule length (%)	25.8	–	63.6	60.3 (43.5–73.9)	60	59.1 (42.5–75)
Precloacal organ length	34	–	5	4.7 (4–5)	21	19.3 (17–21)
Precloacal organ from anterior end	2520	–	1290	1317 (1233– 1428)	2310	2163.3 (1790–2390)
Vulva from anterior end/Body length (%)	–	51.5 (51.1–52)	–	–	–	–
Tail length	216	229.7 (224–237)	104.5	99.2 (93–104.5)	206	189.7 (153–210)
c’	3.4	3.9 (3.6–4.2)	4.6	4.4 (4–4.7)	3.7	4.1 (3.7–4.3)

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 . 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. Fig. 1. Map showing the study sites; map of sampling areas. A . ʻSan Antonio Oeste river estuaryʼ, Río Negro Province. B . ʻPuerto Madryn cityʼ and ʻBahía Kaiserʼ, Chubut Province. C . ʻRada Tillyʼ, Chubut Province. Fig. 2. Mesacanthion bifidum sp. nov. A . Cephalic sense organs on anterior end of female paratype (CNP- NEM 983). B . Cephalic sense organs on male holotype (CNP-NEM 936). C . Female paratype (CNP- NEM 983) showing vulva and gonadal apparatus. D . Precloacal supplement organ of male holotype (CNP-NEM 936). E . Posterior end of male holotype (CNP-NEM 936). F . Copulatory apparatus, spicules and gubernaculum of male holotype (CNP-NEM 936). G . Gonads of male holotype (CNP-NEM 936). H . Posterior end of female paratype (CNP-NEM 983). Scale bars: 1 = 20 μm; 2 = 100 μm; 3 = 200 μm. 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.