Rhynchonema pulchrum Fadeeva & Karpova 2024
Creators: name: Fadeeva, Natalia P.
name: Karpova, Alena A.
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Nematoda
Chromadorea
Monhysterida
Xyalidae
Rhynchonema
Rhynchonema pulchrum

description: Rhynchonema pulchrum sp. nov.
(Figures 4–5, Table 1)
Type material. Holotype adult male (MN 21052017) and six male paratypes (MN 130722, MN 210517), four females (MN 130722, MN 210517), formalin-fixed, mounted on slides, in glycerin, deposited in the nematode collections of the Division of Biological Sciences, Zoological Museum of Far Eastern Federal University, Vladivostok, Russia. Type locality. Holotype (MN 21052017) was all collected from intertidal fine sandy sediments, 0.4 depth in Vostok Bay, Sea of Japan, 21 May 2017. Five males paratype and one female paratype were all collected from intertidal fine sandy sediments, 0.4 to 0.5 m depth in Vostok Bay, Sea of Japan, 21 May 2017.
Additional locality. One male and three females paratypes (MN 130722) intertidal fine sand 0.30 depth, sandbar Nazimova, Primorsky Krai, the Sea of Japan, 13 July 2022.
Etymology. The species epithet is derived from the Latin “ pulchra ”, meaning “beautiful”.
Descriptions
Males. Body slender and tapering anterior to the amphideal fovea. Anterior region from the level of the amphideal fovea is elongated (Figure 5A). The cuticle is coarsely striated (annuli 2 μm large). Annuli are anteriorly orientated in the anterior region, and posteriorly in the posterior region of the body; inversion point of the annuli at 48–51 % of the body length (Figures 4A, 4B, 5E). Labial sensilla not visible in light microscopy. Six cephalic setae, 2–3 μm long. Somatic setae 5–10 μm long distributed, not arranged in pairs, over the entire body surface.
The oval-shaped amphideal fovea comprise 65–67 % of the corresponding body diameter, occupying a width of 3–4 annuli and situated at the end of the pharyngostome (Figure 5C). From the anterior end of the body to the anterior edge of the amphideal fovea there are 37–38 annuli.
Cheilostome about 4–5 μm long, unarmed, pharyngostome slightly enlarged at its base; small, oval cardia protrudes slightly into the cylindrical intestine. Nerve-ring difficult to distinguish, and excretory pore not observed (Table 1).
Reproductive system diorchic with two opposed testes, the anterior testis to the left, the posterior one to the right of the intestine. Spicules symmetrical, ventrally curved, cephalated, 1.2 times cloacal body diameter long. Gubernaculum well developed, dorsally oriented, about one-third of spicules length; its anterior most part surrounding the distal part of the spicules and the paddle-like apophysis measuring 6–8 μm long. Annuli slightly inflated at the level of cloaca (Figure 4C).
Tail conical, ventrally curved, end not annulated; 3.4–4.2 times cloacal body diameter long (Figure 5H).
Females. Entire body size, shape, structure and measurements of various body parts, except for female specific characters, similar to males (Figures 4D, 5B). Amphideal fovea сircular, smaller than those in males (Figure 5D). Female genital system monodelphic, prodelphic and outstretched; transverse vulva, covered by a thin cuticle flap (Figure 5F). Tail similar in shape in both sexes, short, conoid, terminus sclerotized, ventrally curved, 4 abd (Figure 5G). Tail setae not seen.
Diagnosis. Rhynchonema pulchrum sp. nov. is characterized by having 478–629 µm long body, sharply tapering in the anterior end, covered with somatic setae. Cuticle strongly annulated. Inversion point at 48–51 % of the body length; 37–38 annuli in males, 34 in females from the anterior end to the edge of the amphideal fovea end. Amphideal fovea oval, larger in males than in females (сircular), situated at the level of the end of the pharyngostome. Transverse vulva, covered by a thin cuticle flap. Spicules are equal in size, curved; with proximal capitulum; gubernaculum with paddle-like apophysis, annuli of cloacal region inflated.
Differential diagnosis. Rhynchonema pulchrum sp. nov. can be easily distinguished from all known congeneric species in having a combination of the following features: sexual dimorphism on the number of annulations from the anterior end to the anterior edge of the amphideal fovea (37–38 in males and 34 in females); amphideal fovea form and size (oval in males vs сircular in females); large cephalated spicules, gubernaculum with paddle-like apophysis, inflated precloacal annuli.
The new species differs from R. amakusanum Aryuthaka, 1989 and R. ornatum Lorenzen, 1975 by the possession of sexual dimorphism in the shape of amphideal fovea, lacking two rows of lateral longitudinal ridges in the cuticle and the number of annulations from the anterior end to the anterior edge of the amphideal fovea (37–38 vs 26–28 in R. amakusanum and vs 33 in R. ornatum). The new species also differs from R. ornatum by the size of the spicules (26–28 μm vs 22 μm).
The species R. brevituba Gerlach, 1953 is also similar to R. pulchrum sp. nov. in many measurements but differs in having shorter spicules (22 vs 26–28 μm), shorter pharyngostome (27–33 μm long vs 58–64 μm); presence of 18 vs 37–38 annuli anterior to amphideal fovea.
R. megamphidum Boucher, 1974 is similar to the new species in body length and tail shape, but it differs from the new species in amphideal fovea (more than 100 % of the corresponding diameter wide vs 65–67 % in R. pulchrum sp. nov.), spicules shape (short arcuate vs curved with proximal capitulum) and size (16–22 μm vs 26–28μm in R. pulchrum sp. nov.), in having gubernaculum with dorsal apophysis and inversion point just behind the half of the body. The new species also resembles R. chiloense Lorenzen, 1975, nonetheless both species can be easily separated by their body length (478–629 µm in R. pulchrum sp. nov. vs 400–430 µm in R. chiloense), length of pharyngostome (58–64 µm in R. pulchrum sp. nov. vs 38–45 µm in R. chiloense) and number of annulli anterior to amphideal fovea (37–38 in R. pulchrum sp. nov. vs 46–54 in R. chiloense).
The new species differs from R. cemae Bezerra, Smol & Vincx, 2014 by symmetrical, ventrally curved spicules, with a globular capitulum, 26–34 μm long vs asymmetrical spicules (right spicule 23–33 μm long, left 23–31 μm long). It also differs from R. cemae by larger amphideal fovea (4.4–7 μm vs 6–7 μm) located more posterior from the anterior end (43–46 μm vs 36–44 μm).
description: Published as part of Fadeeva, Natalia P. & Karpova, Alena A., 2024, New species of Metadesmolaimus, Rhynchonema and Gairleanema (Nematoda) from the sandy beaches of the Sea of Japan, pp. 115-132 in Zootaxa 5537 (1) on pages 122-125, DOI: 10.11646/zootaxa.5537.1.6, http://zenodo.org/record/14239338