Metoncholaimus paracavatus, Russo & Pastor De Ward, 2019

Russo, Virginia Lo & Pastor De Ward, Catalina T., 2019, Three new species of the free-living marine nematode Metoncholaimus Filipjev 1918 (Enoplida, Oncholaimidae) from Argentinean coasts, Zootaxa 4688 (1), pp. 86-100 : 94-98

Metoncholaimus paracavatus sp. n.

( Figure 4 ; Table 3 )

Measurements. See Table 3 .

Description. Males: Body cylindrical. Cuticle smooth. Head slightly constricted at level of amphids. Endocupola 5 μm long. Buccal cavity rectangular. Three teeth, one left tooth 17 μm length, one dorsal and one right 9–10 μm in length. Lip region with six small labial papillae (R1), 6 + 4 outer labial sensilla (R2) + cephalic sensilla (R3), 8–10 μm in length and four 4–8 μm long respectively. Amphideal fovea oval (26–36% of corresponding diameter). Two crowns of six cervical setae, 4–6 μm length, distant about 27 and 44 μm from anterior end. Pharynx cylindri- cal. Cardia present. Reproductive system with paired testes of same size one in right position relative to intestine. Spicules paired, symmetrical, short, barely more than one anal diameter long. Gubernaculum small, surround the proximal part of the spicules. Tail is curved forming a bursa inside the body surrounded by six pairs of setae (7 μm). Inside this bursa a plate with several small spines covers the anterior and sub-lateral portion of the cloaca opening ( Figure 4C ), posteriorly there is huge post-cloacal papilla in all males (N=5) observed. Tail conical and curved with 1/3 cylindrical. Two papillae near tail tip, plus two pairs of subventral setae and two pairs of submedian-dorsal setae different in length.

Female: Similar to males in general body shape, anterior sensilla and amphids. Reproductive system monodelphic, ovary anterior, reflexed, in right position to intestine. Oblique vagina with at least three pairs of glands around it. Demanian system present ( Figure 5 ). Uvette was not observed. One osmosium located at ca 270 µm and a double canal (70 µm in length) that opens into two pores in preanal lateral position. Tail conical with two papillae near tail tip, one subventral pair of setae and two pairs of sublateral median setae.

Type material. Holotype and some paratypes are deposited in the CNP-NEM collection in the Centro Nacional Patagónico (Chubut, Argentina). One male paratype is deposited in the National Collection of Invertebrates of the Museo Nacional de Ciencias Naturales ‘ Bernardino Rivadavia’ ( Argentina).

Type locality and habitat. Holotype: adult male. Registration number CNP-NEM 1539; type locality: Cormo- ranes Beach (San Matías Gulf); coordinates: 40°53ʹ S; 65°07ʹ W; upper littoral, medium sand sediments. Collected by C.T. Pastor, 4 March 2006. Paratypes: Four males and one female. Registration numbers MACN-In 38924 (male) and CNP-NEM 21995/22119/22122 (males) and CNP-NEM 1540 (female). Same data as holotype.

Etymology. Named for its resemblance to Oncholaimus cavatus .

Discussion and conclusion. The new species is closely related to Oncholaimus cavatus Gerlach 1956 , a species described for Brazil. Both species have relatively same size (a little longer in M. paracavatus sp. n.) and de Man’s ratios are similar in males and equal in females. Other measurements are similar too ( Table 3 ). Both species have a bursa formed as an invagination inside the body. Gerlach (1956) described it as a pincushion going around the cloacal opening: “Ausserdem ist unmittelbar vor dem after eine kissenförmige cuticularverdickung vorhanden”. In the Argentinean’ species this plate with minute spines is around the cloaca on the anterior and lateral sub-ventral sides, posteriorly only a conspicuous papilla has been observed.

The differences between the species from Argentina in respect to the Brazilian one are the unequal cephalic setae length, the presence of two crowns of cervical setae near anterior end, a pair of caudal papillae near the tip of the tail in both sexes and an extra postcloacal papilla in males (inside the bursa and posterior to cloacal opening). The most striking difference is the presence of gubernaculum in M. paracavatus sp. n. against the absence of it in O. cavatus . This, plus the Demanian system type are the reasons to describe this population as a new species of Metoncholaimus .

Metoncholaimus paracavatus sp. n. is similar to M. intermedius , M. murphyi , M. parasimplex and M. simplex in the general body and tail shape. However, M. paracavatus sp. n. has a shorter body length and spicules (in anal diameters) than all of them ( Table 4 ), presence of bursa and presence of gubernaculum (instead of absence in M. intermedius , M. parasimplex and M. simplex ). In addition, it can be differentiated from all the species with gubernaculum in the combination of the length of the spicules and tail ( Table 4 ), the shape of the tail and the presence of bursa.

Figures

FIGURE 4. Metoncholaimus paracavatus sp. n.: (A) Head region (external) of holotype male (CNP-NEM 1539); (B) Head region (internal) of holotype male (CNP-NEM 1539); (C) Posterior end of holotype male (CNP-NEM 1539); (D) Cloacal region of holotype male (CNP-NEM 1539) showing the gubernaculum and papilla; (E) Head region (external) of paratype female (CNP-NEM 1540); (F) Head region (internal) of paratype female (CNP-NEM 1540); (G) Posterior end of paratype female (CNP-NEM 1540). Scale bars: 1= 20 µm.

FIGURE 5. Reproductive and demanian system of paratype females: (A) Metoncholaimus sanmatiensis sp. n. (CNP-NEM 1655); (B) Metoncholaimus perdisus sp. n. (CNP-NEM 16959); (C) Metoncholaimus paracavatus sp. n. (CNP-NEM 1540). Scale bars: 1= 500 µm; 2= 200 µm; 3= 100 µm.

Tables

TABLE 3. Measurements (μm) of Metoncholaimus paracavatus sp. n. including mean values (range) and the original values of Oncholaimus cavatus.

Species	Metoncholaimus paracavatus sp. n.			Oncholaimus cavatus
Locality	Río Negro, Argentina			São Pablo, Brazil
Sex	Holotype ♂	4 ♂	1 ♀	Holotype ♂	1 ♀
L	1790	1687 (1590–1790)	1820	1460	1432
a	48.4	52.6 (48.4–56.4)	55.2	47	55
b	6	5.8 (5.2–6.9)	5.8	4.8	4.8
c	44.8	39.8 (35.3–44.8)	47.9	47	45
Cephalic sensilla (R3)	4	6.1 (4–8)	5	–	–
Outer labial sensilla (R2)	9	9.2 (8–10)	8	8	–
Buccal cavity diameter	12	12.1 (11.5–13)	12	9	–
Buccal cavity length	19	19.5 (18–21)	21	18	–
Lip region diameter	20	20.9 (20–22)	19	18	18
Amphid width	8	7.3 (7–8)	6	–	–
Width of amphideal fovea as percentage of corresponding body diameter (%)	36.4	32.4 (25.9–36.4)	37	47.4	–
Cervical setae length	5.5	5.3 (4–6)	4	8	–
Excretory pore from anterior end	30	34.2 (27–43)	35	36	–
Pharynx length	300	291 (250–305)	315	305	300
Maximum body diameter	37	32.2 (29–37)	33	28	26
Anal/cloacal body diameter	23	22.2 (21–23)	25	30	18
Spicule length	30	29.4 (27–31)	–	28	–
Spicule length as cloacal diameter	1.3	1.3 (1.3–1.4)	–	1.4	–
V (%)	–	–	71.4	–	73
Tail length	40	42.6 (40–46)	38	30	32
c’	1.7	1.9 (1.7–2.1)	1.5	1.5	1.8

TABLE 3. Measurements (μm) of Metoncholaimus paracavatus sp. n. including mean values (range) and the original values of Oncholaimus cavatus .