Richtersia japonica, Revkova & Sergeeva, 2024

Revkova, Tatiana N. & Sergeeva, Nelli G., 2024, Free-living marine nematodes of the genus Richtersia Steiner, 1916 (Desmodorida: Richtersiidae) from the Sea of Japan, Zootaxa 5543 (1), pp. 123-134 : 125-128

Richtersia japonica sp. nov.

( Figures 2–3 , Table 1 )

Type material. Eight males and four females. Holotype male mounted on slide Meib.18.N.h ., four paratype males mounted on slides: Meib.17.N.p. (2 males), Meib.21.N.p. , Meib.60.N.p. and two paratype females: Meib.24.N.p. , Meib.59.N.p. in pure glycerin; three paratype males mounted on slides: Meib.16.N.p. (2 males), Meib.20.N.p. and two paratype females: Meib.20.N.p ., Meib.56.N.p. in glycerine-gelatin.

Type locality and habitat. Specimens were collected from the Sea of Japan in June 2021 on station 7, water depth 22.1 m, pelitic dense silt with mica plates, fluffy detritus on the surface, coordinates: N 42°50.5, E 131°26.4.

Etymology. The species epithet is derived from the Latin adjective meaning “Japanese”.

Descriptions.

Males. Body short, cylindrical, rounded anteriorly, tapering posteriorly. Cuticle annulated, annuli with numerous short and stout spines from posterior to cephalic region to near tail tip. These spines arranged heterogeneously in regular longitudinal rows from about the level of the middle of the pharynx. Each annule carrying spines arranged in groups of longitudinal rows: about 50–53 rows at the level of the middle of the pharynx, about 31–33 rows in the mid-body region, and about 16–19 rows in the cloacal region. Posterior one-third of the tail lacking spines and with a smooth tail tip. Somatic setae 3–9 μm long, irregularly situated in eight rows between longitudinal rows of spines. Longitudinal rows of robust spines on the ventral side that begin at the level of the middle of the pharynx and end about 17–32 µm from the cloaca. Six internal labial setae 6–8 μm long, situated on a hyaline labial membrane; six external labial setae 5–6 μm long and four cephalic setae 4–5 μm long located at almost the same level. Subcephalic setae 4–5 μm long, one at each side of amphidial fovea. Amphidial fovea 40.7–60% of chd (corresponding head diameter), round and ventrally spiralled with 3.5 turns, surrounded by cuticular spines. Labial region with 12 lobes, strengthened by 12 cuticular rods, forming a pyramidal invagination of cheilostome wall into the stoma; invagination alternating with 12 triangular prisms. Stoma wide, funnel-shaped, unarmed and partly surrounded by pharyngeal tissue. Pharynx long, cylindrical and lacking a posterior bulb. Cardia not observed. The intestinal cells containing dark brown granules evenly dispersed all along the intestine. The nerve ring and secretory-excretory system indistinguishable.

Reproductive system monorchic, with outstretched testis. Two spicules unequal in shape and size: right spicule very fine, 4.3 (3.4–5.0) times cloacal body diameter; left spicule wider and 1.5 (1.2–2.2) times cloacal body diameter. Right spicule 2.8 (2.2–3.0) times longer than left spicule; left spicule with a capitulum. Gubernaculum surrounding distal ends of spicules, with an anterior apophysis on each side. Tail short, 1.2 (0.7–1.3) cbd (cloacal body diameter) long, with two caudal glands.

Females. General morphology similar to that of males. Spines arranged in regular longitudinal 49–51 rows beginning close to the buccal cavity, 37–39 rows in the mid-body region, and 22–24 rows in the cloacal region. Somatic setae 3–6 μm long. Amphidial fovea smaller, 26–39.1% of chd, with one turn. Reproductive system didelphic, amphidelphic. Ovaries reflexed. Spermatozoa present in uterus. One female with a mature egg ( Figure 3F ). The tail is covered with numerous rows of short hairy spines ( Figure 3H ), in which substrate particles can accumulate.

Diagnosis. Richtersia japonica sp. nov. is characterised by 450–655 μm long body; presence of multispiral amphidial fovea with 3.5 turns in males and one turn in females; longitudinal rows of spines located approximately from the level of the middle of the pharynx in males and behind the buccal cavity in females; two unequal spicules and gubernaculum with anterior apophyses, short conical tail; tail with hairy spines in females.

Differential diagnosis. Based on the amphidial fovea structure and unequal spicules R. japonica sp. nov. is similar to R. beibuwanensis Fu, Cai, Boucher, Cao & Wu, 2013 , R. staresensis Soetaert & Vincx, 1987 , R. coomansi Soetaert & Vincx, 1987 and R. inaequalis Riemann, 1966 . However, the new species differs from R. coomansi and R. inaequalis by having an anterior apophysis on gubernaculum (vs. absent); length of the left (51–68 μm vs. 124– 185 μm in R. coomansi and 39–47 μm in R. inaequalis ) and right 139–173 μm vs. 66–85 μm in R. coomansi and 113–126 μm in R. inaequalis ) spicules; longitudinal rows of spines begin at the level of the middle of the pharynx in males and behind buccal cavity in females (vs. at the level of amphidial fovea in R. coomansi and R. inaequalis ). The new species differs from R. coomansi by the number of amphidial fovea turns in males (3.5 turns vs. 5.25 turns) and the shape of internal labial setae (usual vs. prominent base).

The new species mostly resembles R. beibuwanensis and R. staresensis in the arrangement of longitudinal rows of spines, but differs from it in de Man a value in males (8–11.9 vs. 7.1–7.4 in R. beibuwanensis and 15.4 in R. staresensis ) and lengths of the spicules: right (139–173 μm vs. 44–50.9 μm in R. beibuwanensis and 87–93 μm in R. staresensis ) and left (51–68 μm vs. 98.2–108 μm in R. beibuwanensis and 41–46 μm in R. staresensis ). R. japonica sp. nov. also differs from R. beibuwanensis by having a relatively longer body (450–655 μm vs. 373.7– 399.6 μm in males, 450–561 μm vs. 405.6 μm in females) and subcephalic setae (present vs. absent). Furthermore, the new species differs from R. staresensis by having a smaller amphidial fovea in males (40.7–60% chd with 3.5 turns vs. 90% chd with 4 turns).

Figures

FIGURE 2. Richtersia japonica sp. nov. A: holotype male, entire body; B. holotype male, spicules; C. holotype male, buccal cavity; D. paratype female, amphid level; E. holotype male, amphid level; F. paratype female, buccal cavity; G. paratype female, entire body. Scale bar: A, G = 100 μm; B–F = 20 μm.

FIGURE 3. Richtersia japonica sp. nov. A. paratype female, entire body; B. holotype male, entire body; C. paratype female, amphid level; D. holotype male, amphid level; E. paratype female, buccal cavity; F. paratype female, mature egg; G. paratype male, tail region; H. paratype female, tail region and setae (arrows); I. paratype female, cuticula on vulval region; J. paratype female, vulva (arrow). Scale bar: A, B = 100 μm; C–J = 20 μm.