Aegialoalaimus tereticauda, Leduc & Zhao, 2021

Leduc, Daniel & Zhao, Zeng Qi, 2021, Molecular characterization of free-living nematodes from Kermadec Trench (Nematoda: Aegialoalaimidae, Xyalidae) with description of Aegialoalaimus tereticauda n. sp., Zootaxa 4949 (2), pp. 341-352 : 344-347

Aegialoalaimus tereticauda n. sp.

( Figs 1 & 2 )

Measurements. See Table 1.

Type specimens. Holotype male (139255), collected 3 December 2017 (voyage TAN1711, station 25, site 3). One paratype male and two paratype females (139256), same data as holotype.

Type habitat. Kermadec Trench (30.3815° S, 176.6417° W), water depth: 9540 m, sediment depth: 4–5 cm.

Etymology. The species name is derived from the latin teretis (= rubbed off, rounded, cylindrical) and cauda (= tail).

Description. Males. Body slender, cylindrical over most of its length, tapering slightly towards anterior extremity. Cuticle with transverse striations, without lateral differentiation; somatic setae not observed. Body pores and epidermal glands not observed. Labial region rounded, continuous with rest of body, lips fused. Inner and outer labial sensilla indistinct. Four papilliform cephalic sensilla, located close to anterior body extremity. Subcephalic setae absent. Amphideal fovea large, circular, with lightly cuticularized outline, its anterior end located well posterior to cephalic sensilla bases. Ocelli absent. Oral opening terminal. Buccal cavity narrow, tubular, undistinguishable from cuticularized lumen of the pharynx. Pharynx divided into two sections: anterior tubular part and posterior bulb. Tubular part of pharynx is 97–105 µm long, 1.3 µm wide at base and gradually narrowing to 0.8–0.9 µm anteriorly, surrounded by thin sheath of tissue. Posterior bulb is muscular, with dorsal pharyngeal gland sometimes visible in middle of dorsal sector of posterior bulb. Subventral pharyngeal glands indistinct. Nerve ring located near middle or slightly anterior to middle of pharynx. Secretory-excretory system difficult to distinguish; excretory pore located slightly anterior to posterior bulb, renette cell not observed. Cardia 4–5 µm long, not embedded in intestine.

Reproductive system with two testes. Anterior testis outstretched, located to the left or right of intestine; posterior testis located on same side of intestine as anterior testis, may be directed posteriorly and outstretched, or possibly directed anteriorly and reflexed (with proximal portion obscured). Mature sperm globular, 3× 4 µm. Spicules paired, equal, 1.5–1.6 cloacal body diameters long, arcuate, tapering distally and slightly swollen proximally. Gubernaculum small, plate-like, without lateral pieces, without apophyses. Precloacal supplements absent. Ejaculatory glands not observed. Tail cylindrical with rounded tip; three caudal glands present in posterior half of tail with common opening.

Female. Similar to males but excretory pore located 1 cbd anterior to nerve ring and slightly longer tail. Reproductive system with two opposed and reflexed ovaries both located to the right of intestine. Vulva located slightly posterior to middle of body length. Vagina perpendicular to body surface; vaginal glands not observed.

Diagnosis. Aegialoalaimus tereticauda n. sp. is characterised by body length 755–864 µm, cephalic sensilla papilliform (<1 µm long), amphideal fovea 64–78% cbd wide, excretory pore located slightly anterior to posterior bulb in males and slightly anterior to nerve ring in females, arcuate spicules 18–22 µm (1.5–1.6 cloacal body diameters) long, gubernaculum present, precloacal supplements absent, and cylindrical tail 58–64 µm long (cʹ = 4.4–5.3) with rounded tip.

The new species is most similar to A. leptosoma Gagarin, 2012 in the short body length (<900 µm) and structure of the spicular apparatus (arcuate spicules with gurbenaculum), but can be distinguished from the latter by the absence of precloacal supplements (versus 3–5 precloacal papillae in A. leptosoma ), tail shape (cylindrical versus conical in A. leptosoma ) and position of caudal gland nuclei (in posterior half of tail versus anterior half of tail in A. leptosoma ). Other minor differences include the longer tail (cʹ = 4.4–5.3 versus 3.1–4.0 in A. leptosoma ), shorter cephalic sensilla (<1 versus 1.5–2.0 µm in A. leptosoma ), and shorter spicules (18–22 versus 23–24 µm in A. leptosoma ). The new species is also similar to A. bratteni Holovachov, 2015 in having papilliform cephalic sensilla and in lacking precloacal supplements, but can be differentiated from the latter by the shorter body (<900 versus 1491– 1754 µm), longer tail (cʹ = 4.4–5.3 versus 4.2 in A. bratteni ), spicule shape (arcuate versus straight in A. bratteni ), presence of gubernaculum (versus absent in A. bratteni ), and tail shape (cylindrical versus conical in A. bratteni ).

Molecular phylogenetic relationships. Near full-length SSU (1506–1571 bp) and D2–D3 of LSU sequences (685–720 bp) were obtained for Aegialoalaimus sp., Manganonema sp., and Daptonema amphorum . For Metasphaerolaimus constrictus , partial SSU (850 bp) and near full length D2–D3 of LSU sequences (762 bp) were obtained. Among the orders included in our SSU phylogenetic analysis, only the Desmodorida, Desmoscolecida and outgroup Chromadorida were recovered as monophyletic ( Figure 3 ). The two Aegialoalaimus sequences formed a distinct, well supported clade (100% posterior probability and 98% bootstrap support) without clear relationships with the Plectida , Araeolaimida , Monhysterida, Isolaimiida , or any of the orders included in our analysis. The Xyalidae sequences, including Manganonema sp. and Daptonema amphorum , formed a moderately to well-supported monophyletic clade (100% posterior probability and 69% bootstrap support). Within the Xyalidae , Daptonema amphorum was grouped with sequences of Daptonema Cobb, 1920 spp., Zygonemella striata Cobb, 1920 and Metadesmolaimus Schuurmans Stekhoven, 1935 sp. with 98% posterior probability but less than 50% bootstrap support. Manganonema sp. was most closely related to Theristus agilis ( de Man, 1880) Filipjev, 1918 (93% posterior probability and 60% bootstrap support). Metasphaerolaimus constrictus and the other Sphaerolaimidae sequences formed a well-supported monophyletic clade (100% posterior probability and bootstrap support). The Monhysterida was not monophyletic due to the placement of taxa belonging to the Siphonolaimoidea Filipjev, 1918 ( Monhysterida II and III ; Figure 3 ) outside of the main monhysterid clade ( Monhysterida I; Figure 3 ).

Figures

FIGURE 1. Aegialoalaimus tereticauda n. sp. A: Female anterior body region; B: male anterior body region; C: female cephalic region; D: female posterior body region; E & F: male posterior body region. Scale bar: A & B = 40 µm, C = 15 µm, D & F = 32 µm, E = 22 µm.

FIGURE 2. Aegialoalaimus tereticauda n. sp. A: Entire male; B: female reproductive system. Scale bar A = 125 µm, B = 100 µm.

FIGURE 3. Bayesian tree inferred from SSU sequences, aligned using the MUSCLE alignment algorithm under the general time-reversible (GTR) + I (proportion of invariable sites) + gamma distribution (G) model. Posterior probabilities (left) and bootstrap values (right) greater than or equal to 50% are given on appropriate clades. New sequences are shown on grey background and underlined font. The scale stands for substitutions per site. *: no bootstrap support; -: less than 50% bootstrap support.