Mesacanthoides flagellatum, Pastor & Russo, 2021

Pastor, Catalina & Russo, Virginia Lo, 2021, Two new species of Enoplolaiminae (Enoplida: Thoracostomopsidae) from Río Negro and Chubut, Argentina, Zootaxa 5020 (2), pp. 337-351 : 346-347

Mesacanthoides flagellatum sp. n.

( Figure 1A , 4 , 5 ; Table 1, 3)

Measurements. See Table 1.

Description. Males: Body cylindrical, tapering posteriorly and ending in a flagellate tail. Cuticle finely striated. Head rectangular in shape, not set off, with three lips (one dorsal and two ventrosublateral) with striations and separated by deep clefts, 18 µm high. The striations are delicate and can only be seen along the lips borders. Each lip bearing a small glandular area on the tips of the lips, and two inner labial sensilla, 0.3 corresponding body diameters in length. Six outer labial sensilla, 1.4 corresponding body diameters in length, inserted at the level of posterior margin of cephalic capsule. A second crown of four cephalic setae 0.6 corresponding body diameters in length. Two crowns of cervical setae in dorsosublateral and ventrosublateral position. First crown of 4 setae, 0.7 corresponding body diameters in length, second crown of 4 setae, 0.4 corresponding body diameters in length. Amphids not seen. Body with somatic setae scattered along anterior pharyngeal area.

Cephalic capsule well developed, shorter than wide. Anterior margin at the base of the lips. Posterior margin with incisions accommodating the first crown of outer labial setae and cephalic setae. Cuticularized arch surrounding the setae insertion. Three arched, mandibular plates with anterior curved bar, to the extremes claws-like, with no conspicuous pointed tips, projecting into stoma. Mandibular plates with “v” shaped incision posterior to the anterior bar. Mandible surface is densely mottled, but sufficiently transparent to see the onchia. Three onchia, one dorsal and two ventrosublateral, equal in size, at the base of each mandible. Mandibular ring with three cuticularized semicircular structures between the mandibles, at level of half of the onchia length. Amphids, excretory pore and metanemes not observed. Anterior part of the pharynx around buccal cavity globose, then cylindrical.

Reproductive system diorchic, anterior testis outstretched, posterior one reflexed. Spicules with diagonal reinforcement (4.2 cloacal body diameters in length), with pointed distal end. Gubernaculum complex, “s” shaped with a dorsal small apophysis, a dorsoventral knob and a distal cannula part ending in two small teeth. One precloacal supplement present. No setae or papillae were observed in pre and post cloacal area. Tail flagellate, 10.9 cloacal body diameter long.

Females: Similar to male in general body shape, but much larger. No differences in shape and number of labial and cephalic setae. Six labial sensilla 4.3 corresponding body diameters in length, six long outer labial sensilla 1.2 corresponding body diameters in length and four short cephalic setae 2.5 corresponding body diameters in length, inserted at posterior margin of cephalic capsule. One crown of four cervical setae 3.7 corresponding body diameters in length. Three arched, mandibular plates with pointed tips projecting into stoma. Mandibular plates same in shape as in males and with mottled surface. Three onchia, one dorsal and two ventrosublateral, equal in size. Reproductive system didelphic, amphidelphic, ovaries reflexed. Vagina with thick walls and well developed sphincter muscles. Tail flagellate, 16.1 anal body diameters in length.

Material examined. Holotype Male, CNP-NEM N° 1282, collected on 17/6/2006 at “Balneario San Antonio Oeste” (lat. 40 º 43’ 55”S, long. 64 º 56’ 52”W), during low tide. Collector: Catalina Pastor. Paratypes one male and one female, same data as holotype mounted on slide numbers CNP—NEM N° 1283‒1284 .

Type locality and habitat. “Balneario San Antonio Oeste”, winter, temperature = 9.6 °C, salinity = 33.5 psu. They were found 26 cm depth in sediment, at low tide. Characteristics of sediment: mean particle size Q2 (50%) = 0.4; VCS = 96.8%; SO index = 0.54; Organic matter / 100 g sediment = 4.1 mg.

Etymology. “ flagellatum ” from latin word “flagellum”, in relation to the tail shape.

Differential diagnosis. M. flagellatum sp. n. is characterized by high striated lips, with a striation only visible along the borders of the lips. The mandible texture is weakly cuticularized and mottled. Mandibular plates with pronounced “v” shaped incision posterior to the anterior bar. Cephalic capsule shorter than wide, with setae in posterior position. Males with long spicules, with “s” shaped gubernaculum, precloacal supplement and flagellate tail.

M. flagellatum sp. n. is related to M. fibulatus Wieser & Hopper, 1967 , M. psittacus Wieser & Hopper, 1967 and M. magna Coles, 1977 . For more information see Table 3.

M. fibulatus has similar body size, cephalic capsule proportions (height/width), spicules length in anal diameters. But it differs in having unstriated lips, different length of inner labial sensilla (14-16 µm vs 7 µm), cephalic setae length (73–80 µm vs 21 µm), mandible length (30.0–32.0 µm vs 18 µm), tail length (10.6–11.7 c.b.d. vs 20 c.b.d.) and de Man’s ratio “a” (71.9–75.6 vs 57.3), “b” (6.1–6.4 vs 8.5) and “c” (6.8–7.2 vs 12.3).

M. psittacus has similar cephalic capsule height/width (0.5 vs 0.4–0.5), mandible height/width (2.0), inner labial sensilla length (15.0 µm vs 14.0–16.0 µm), and cephalic setae length (30–32 µm vs 32.7 µm). But it differs in having unstriated lips, different de Man’s ratios “a, b, c”, spicule length in c.b.d (4.2–4.4 vs 1.0), gubernaculum length in c.b.d (0.4–0.5 vs 1.9) and tail length in c.b.d. (10.6–11.7 vs 5.0).

M. magna has similar length of inner labial sensilla (10–15 µm vs 10.0–15.0 µm), outer labial sensilla (73–80 µm vs 50.0–100.0 µm), cephalic setae length (30–32 µm vs 40 µm), mandible length (30.0–32.0 µm vs 45.0–50.0 µm), onchia length (23.0–24.0 µm vs 25.0–27.0 µm) and De Man’s ratios “a” (71.9–75.6 vs 61.0–70.4) and “b” (6.1–6.4 vs 2.1–6.2). But differs in the small size (5750.0 –5900.0 µm vs 12200.0–15500.0 µm), in having unstriated lips, in spicule length in c.b.d. (4.2–4.4 vs 7.6–9.0), tail length in c.b.d (10.6–11.7 vs 2.0) and De Man’s ratio “c” (6.8–7.2 vs 69.7–83.8).

Figures

FIGURE 1. Map showing the study sites. Map of sampling areas:A. “San Antonio Oeste river estuary” beach, Río Negro Province; B. “El Límite” beach, Chubut Province.

FIGURE 4. M. flagellatum sp. n. Line drawings. A. Cephalic sense organs on anterior end of male holotype; B. Cephalic sense organ on female paratype; C. Buccal cavity and mandibles of male holotype; D. Buccal cavity and mandibles of female paratype; E. Posterior end of male holotype; F. Vulva and gonadal apparatus of female paratype; G. Posterior end of female paratype; H. Detail of precloacal organ in male holotype; I. Copulatory apparatus, spicules and gubernaculum of male holotype. Scale bar: 1= 20 μm; 2= 100 μm.

FIGURE 5. M. flagellatum sp. n. Micrographs of slide mounted specimens. A. Cephalic sense organs on female paratype; B. Cephalic sense organs of male holotype; C. Buccal cavity and mandibles of female paratype; D. Mandibles and onchia on male paratype; E. Mandibles and onchia in female paratype; F. Vulva opening in female paratype; G. Detail of gubernaculum of male paratype; H. Lateral view of precloacal organ, gubernaculum and spicules of male holotype. Scale bar: 1= 20 μm.