Gammanema lunatum, Leduc & Zhao, 2025

Gammanema lunatum sp. nov.

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Figs 6–8 View Fig View Fig View Fig ; Table 2 View Table 2

Diagnosis

Gammanema lunatum sp. nov. is characterised by body length 754–1817 µm, cuticle with minute spines mostly visible in tail region, amphideal fovea large and loop-shaped in males and small and unispiral in females, inner and outer labial setae of similar length (2–4 µm), conical and on broad cuticle bases, cephalic setae 0.25–0.35 cbd long, spicules relatively short (length = 0.8–0.9 body diameter at level of cloacal opening), gubernaculum ca ⅔ of spicule length, six to ten sup-shaped precloacal supplements, vulva at almost ⅔ of body length from anterior extremity, tail length 1.3–1.6 cloacal/anal body diameters.

Differential diagnosis

The new species differs from most other species of the genus, except Gammanema agglutinans Leduc, 2013 , in males having loop-shaped amphidial fovea instead of unispiral ( G. conicauda Gerlach, 1953 ) or multispiral amphidial fovea (all other species of Gammanema ). The new species also differs in the shape of the amphidial fovea in females, which is multispiral in all other species of the genus for which observations are available, except G. conicauda which is characterised by the unispiral amphidial fovea. In addition, the new species differs from most congeners, except G. agglutinans , in having minute cuticular spines.

Gammanema lunatum sp. nov. differs from G. agglutinans in the shape of the amphidial fovea in males (¾ vs ½ turn in G. agglutinans ) and females (1.0 vs 1.5 turns in G. agglutinans ), larger amphidial fovea size in males (17 vs 10 µm wide in G. agglutinans ) and females (8 vs 5 µm in G. agglutinans ), and the shape of precloacal supplements (cup-shaped vs tubular in G. agglutinans ). Gammanema lunatum differs from G. conicauda in the shorter body (754–1817 vs 1985–3724 µm in G. conicauda ), markedly lower a ratio (13–16 vs 30–43 in G. conicauda ), shorter outer labial setae (2–4 vs 16 µm in G. conicauda ), shorter cephalic setae (8–20 vs 30–33 µm in G. conicauda ), and fewer precloacal supplements (6–10 cup-shaped supplements vs 22 papilliform precloacal supplement in G. conicauda ).

Etymology

The species name is derived from the Latin ‘ lunatus ’ (= ‘shaped like a crescent moon’) and refers to characteristic shape of the amphids in males.

Type material examined

Holotype NEW ZEALAND • ♂; Otago Fan complex, Papanui Canyon ; 45.8668° S, 171.0415° E; 680 m water depth; 9 Feb. 2024; sandy mud sediments; voyage TAN2402, stn 2; NIWA 181633 View Materials . GoogleMaps

Paratypes NEW ZEALAND • 1 ♂, 1 ♀; same data as for holotype; NIWA 181634 View Materials GoogleMaps .

Type habitat and locality

Upper continental slope, New Zealand.

Description

Males

Body cylindrical, relatively stout, tapering slightly towards both extremities; no colouration except for light to dark brown intestine wall. Cuticle ca 2–3 µm thick, with faint transverse annulations ca 1 µm apart and transverse rows of punctations without lateral differentiation; minute spines visible mainly in tail region, may be present in mid-body region and apparently absent in pharyngeal region. Eight longitudinal rows of somatic setae, 4–7 µm long in pharyngeal and tail regions, 2–4 µm long in mid-body region; pore complexes not observed. Cephalic region blunt, slightly rounded. Mouth opening surrounded by 12 membranous projections of varying shape located in internal circle surrounded by the lips. Partly fused lips each bearing a conical inner labial seta, 2–4 µm long, on a broad cuticle base. Six outer labial setae of same size and shape located further posteriorly at base of lip region; four longer (ca 0.25–0.35 cbd long) and thinner cephalic setae located immediately posterior to outer labial setae, without broad base. Amphideal fovea relatively large, loop-shaped, situated slightly posterior to cephalic setae; amphidial aperture slightly shorter than amphidial fovea. Buccal cavity (pharyngostome) divided into anterior (gymnostome) and posterior portions (stegostome). Anterior portion of buccal cavity cup-shaped, ca 14–23 µm deep, with three sets of cuticularised rhabdions, 18–20 µm long, terminating posteriorly in pairs of small teeth or denticles; posterior portion of buccal cavity narrower, cylindrical, surrounded by three Y-shaped pairs of cuticularised rhabdions with swollen bases, of similar length to anterior rhabdions. Pharynx cylindrical, muscular, without anterior or posterior bulb; pharyngeal lumen not conspicuously cuticularized or partitioned. Nerve ring at ca 40% of pharynx length from anterior. Secretory-excretory system not observed. Cardia small, surrounded by intestine. Intestine and vas deferens join posteriorly into cloaca, opening to the exterior via cloacal opening.

Reproductive system diorchic with two short and opposed testes located ventrally relative to intestine; anterior testis outstretched, posterior testis reflexed. Sperm cells globular, longest diameter up to 23 µm, ca 1.5–2.0 greater than shortest diameter. Spicules paired, slightly curved, tapering distally, length 0.8– 0.9 body diameters at level of cloacal opening; gubernaculum consisting of two flat detached lateral pieces (crurae), ca ⅔ of spicule length; median portion of gubernaculum (corpus and cuneus) apparently absent. Six to ten cup-shaped precloacal supplements; posteriormost supplement 18–32 µm anterior to cloacal opening, supplements located 5–17 µm apart. Tail short, conical; three caudal glands and spinneret present.

Female

Similar to males, except for conspicuously smaller, unispiral amphidial fovea and aperture. Reproductive system didelphic-amphidelphic, with reflexed ovaries located ventrally relative to intestine. Vulval opening on conspicuously swollen body region, situated almost two thirds of body length from anterior extremity. Proximal portion of vagina surrounded by constrictor muscle; several vaginal glands present surrounding vulval region. Rectum and anus present.

Molecular phylogenetic relationships

Partial SSU (526 bp) and near full-length D2–D3 of LSU sequences (742 bp) were obtained for Gammanema lunatum sp. nov. and near full length SSU (1327 bp) and D2–D3 of LSU sequences (710 bp) were obtained for Halichoanolaimus funestus . The Selachinematidae formed monophyletic clade in both the consensus SSU and D2–D3 of LSU trees ( Figs 9–10 View Fig View Fig ; 100% posterior probability and bootstrap support). Two similar clades were recovered in both consensus trees: clade 1 comprised the genera Halichoanolaimus de Man, 1886 , Bendiella Leduc, 2013 , Cobbionema Filipjev, 1922 and Demonema Cob, 1894 with strong support (100% posterior probability and 96–100% bootstrap support), and clade 2 comprised the genera Gammanema , Latronema Wieser, 1954 and Choanolaimus with varying levels of support (95–100% posterior probability and up to 75% bootstrap support). The remaining genera ( Choniolaimus Ditlevsen, 1918 , Cheironchus Cobb, 1917 , Pseudocheironchus Leduc, 2013 , Synonchiella Cobb, 1933 ) formed a third clade in the D2–D3 of LSU tree with moderate or no support (76% posterior probability and <50% bootstrap support), but not in the SSU tree. Gammanema lunatum formed a monophyletic clade together with the other Gammanema sequences in both SSU and D2–D3 trees (100% posterior probability and 99–100% bootstrap support). We found no support for the monophyly of the genus Halichoanolaimus in either SSU or D2–D3 of LSU trees; in the D2–D3 of LSU tree, the Halichoanolaimus funestus Leduc, 2020 sequence was grouped together with Bendiella longicauda Leduc & Zhao, 2016 .