Daptonema gelida, Timchenko & Portnova, 2024

Timchenko, Anna I. & Portnova, Daria A., 2024, A new sympagic nematode species Daptonema gelida sp. n. (Monhysterida, Xyalidae) from the White Sea landfast ice, Zootaxa 5433 (2), pp. 183-206 : 186-192

Daptonema gelida sp. n.

( Figures 2 A−E, 3A−B, 4A−C, 5A−C and Table 2 )

Type material. Ten males and nine females. All the type specimens were mounted in permanent glycerin slides. Holotype: male (slide number: М1/38). Paratypes: nine males (slide numbers: M1/39, M1/40, M1/41, M1/42, M1/43) and nine females (slide numbers: M1/38, M1/39, M1/40, M1/41, M1/42, M1/43). Holotype and paratypes were deposited in the collection of the Shirshov Institute of Oceanology of the Russian Academy of Sciences, Moscow, Russia.

Type locality and habitat. Russia, the Kandalaksha Bay of the White Sea, the Ermolinskaya Inlet (66.555 N 33.041 E); first-year thin landfast ice (35 cm thickness); the lowermost 10 cm of ice core; often associated with the presence of macroalgae Fucus vesiculosus in ice samples; water depth 2.9 m .

Etymology. The species epithet is derived from the Latin word gelida , meaning “frozen-in”, and refers to the habitat type.

Descriptions.

Males. Body slender and elongated, about 1.3–1.6 mm long. Body diameter gradually tapering towards both ends. Cuticle finely striated annulations. Short somatic setae uniformly distributed throughout the body, 3–6 µm in length, except for a few that increase toward the cardia. Head rounded, clearly set off, six inner labial papillae, six long outer labial setae and four cephalic setae in single circle, 0.4 corresponding body diameter (cbd) long. Amphideal fovea circular, with clear cuticular edging, 5–7 μm in diameter, and occupy 24.9–34.2% of cbd, posterior to the buccal cavity and located at 1.0–1.4 cbd from the anterior end. Buccal cavity conical-shaped with slightly sclerotized walls. Pharynx narrow, cylindrical, muscular, and gradually expands toward the cardia. Cardia short and heart-shaped, 6–7 µm long. Nerve ring at mid-length of pharynx. Secretory-excretory system not observed.

Reproductive system diorchic with two outstretched testes; the anterior testis to the left of the intestine, longer and extending to the anterior part of the intestine; the posterior testis shorter, right of the intestine. Spicules paired, equal, well cuticularized, thin, strongly curved in middle portion, L-shaped, 1.4–2.0 anal body diameters (abd) long; spicules length along arc of 35−45 µm and on chord of 24−31 µm; manubrium with round-shaped. Gubernaculum distinct, well cuticularized, with short dorsal-caudal apophysis by triangular-shaped outline, 5–8 µm long. Precloacal supplements absent. Tail conico-cylindrical, elongated, narrowed, 6.9–9.0 abd in length. Tail tip slightly swollen with two terminal setae at the end. Three caudal gland cell bodies occupy the space dorsally from the anus to the end of the tail.

Females. Similar to males in most characteristics but with higher c’ (8.7–10.5 vs 6.9–9.0). Short somatic setae are equally distributed over the body. Reproductive system, monodelphic with ovary outstretched, to the left of intestine, extending anteriorly to near cardia region. The width of the biggest egg in the oviduct was 8.1 µm and the length 25.8 µm. Post-vulval uterine sac present and located to the left of intestine, well developed, conspicuously cuticularized with rounded distal end, 1.8–2.6 times the length of cbd; numerous sperm cells observed.

Species diagnosis. Daptonema gelida sp. n. is characterized by elongated and very thin body; short somatic setae equally distributed, 3–4 µm long, but longer (up to 6 µm) on cardia in male, circular amphideal fovea; spicules thin, curved, L-shaped, with round-shaped manubrium; gubernaculum with short dorsal-caudal apophysis by triangular-shaped outline; post-vulval uterine sac present; conico-cylindrical narrowed tail with slightly swollen tip with two terminal setae.

Differential diagnosis. The new species Daptonema gelida sp. n. resembles several Daptonema species such as D. robustum ( Tchesunov, 1980) Tchesunov, 1990b ; D. hirsutum ( Vitiello, 1967) Lorenzen, 1977 ; D. svalbardense ( Gerlach, 1965) Lorenzen, 1977 ; especially in the structure of the copulatory apparatus and the latter two in the relatively distant position of the amphideal fovea from the anterior end of body, D. normandicum ( de Man, 1890) Lorenzen, 1977 and D. limnobia Wu & Liang, 2000 in the L-shaped and length of spicules, presence of a post-vulval uterine sac, and short terminal setae of the latter, and somewhat resembles D. ostentator Wieser & Hopper, 1967 and D. levis Rieger & Ott, 1971 in body size and de Man’s indices.

The new species differs from D. robustum by shorter spicules (35–45 µm vs 56–63 µm) and cephalic setae (6–8 µm v s 20–22.5 µm), a smaller amphideal fovea (5–7 µm vs 11.2 µm) located farther from the anterior end (24–28 µm vs 18.7–22.5 µm). The new species differs from D. hirsutum by both shorter spicules (35–45 µm vs 64 µm ( Vitiello 1967) and 68–75 µm ( Boucher & Helléouët 1977)) and smaller gubernaculum (10–13 µm vs 30 µm), and shorter somatic and terminal setae (4–6 µm vs 50 µm ( Vitiello 1967) or 45–67 µm ( Boucher & Helléouët 1977)). The differences between D. gelida sp. n. and D. svalbardense include longer body length (1293–1632 µm vs 984 µm ( Gerlach 1965) or 870–920 µm ( Lorenzen 1977)), longer spicules (35–45 µm vs 33 µm ( Gerlach 1965) or 23–25 µm ( Lorenzen 1977)), and strongly-curved spicules (versus barely curved spicules with the spicules tips bilobed distally and pointing in opposite directions); the absence of gubernaculum apophysis in D. svalbardense ( Lorenzen 1977) . The new species differs from D. normandicum by having more anteriorly located amphids, slightly widened proximal end of spicules (versus hooked and bifurcated), and posteriorly located vulva (70−85% vs 58%). The new species can be separated from D. limnobia by the longer body length (1293–1632 µm vs 944–1025 µm), the more anteriorly located amphids (24–28 vs 4–7 µm), smaller gubernaculum (10–13 µm vs 20–23 µm), absence of ejaculatory glands and posteriorly located vulva (70−85% vs 58−62%). D. gelida sp. n. can be differentiated from the latter five species by elongated and slender body (a=59.4–71.2 vs 25.8–27 ( D. hirsutum ), 27 ( D. svalbardense ) 37 ( D. normandicum ) 17–23 ( D. limnobia ), 22−28.6 ( D. robustum )).

The following characteristics differentiate the new species from D. ostentator : the more anteriorly located amphids, lack of pronounced sexual dimorphism in amphideal fovea width (male and female 5–7 µm vs male 15 µm /female 8 µm), shape of spicules (curved vs straight), shape of gubernaculum (triangular-shaped outline vs plateshaped with subterminal constriction and terminal hook) and smaller size (10–13 µm vs 17–18 µm), shorter terminal setae (4–6 µm vs 20 µm). Differences in D. levis from the D. gelida sp. n. consist of higher b (7–7.3 vs 4.9–5.6), c (9.5–9.8 vs 7.4–8.1), and straight spicules without gubernaculum.

Sequence data. The 18S rDNA fragment (886 bp) sequenced for one specimen: 25DWS (station number: WSBS20 /21 4−9 1) and deposited to the NCBI /GenBank database ( Benson et al. 2012) under accession number OQ925937 .

Molecular genetics. We compared the obtained 18S rDNA sequence with data in GenBank, which confirmed placing Daptonema gelida sp. n. within the genus Daptonema . D. gelida sp. n showed significant relatedness to D. hirsutum (AM236231; AY854223) and D. setosum ( Bütschli, 1874) Lorenzen, 1977 (AM234045) (94.26% identity, 100% query coverage), Metadesmolaimus sp. (AJ966491) and Daptonema sp. (FJ040463) (94.14% identity, 100% query coverage), as well as to D. carnulentum Leduc & Zhao, 2023 (OK317212) (96.37% identity, 90% query coverage).

The alignment of 18S, including GenBank data, comprised 23 sequences with a length of 891 bp. Both ML and BI phylogenetic reconstructions demonstrated a similar topology, with specimens in the Daptonema genus subdivided into two clades ( Figure 6 ). The tree showed a strongly supported clade I (99.6% SH-aLRT, 100% posterior probability), with specimens corresponding to the Daptonema genus as well as species previously reported as Zygonemella striata Cobb, 1920 ( Neres et al. 2010, Cunha et al. 2013) and Metadesmolaimus sp. ( Meldal et al. 2007). In the BI tree, D. gelida sp. n. formed a separate, highly supported branch (posterior probability = 100%) and became a sister group to D. carnulentum as well as other Daptonema species, including D. oxycerca ( de Man, 1888) Lorenzen, 1977 , D. procerum ( Gerlach, 1951) Lorenzen, 1977 , D. setosum , D. hirsutum , Metadesmolaimus sp. , and Daptonema spp. However, the two methods did not resolve the relationships between the new sympagic species in the same way. In the ML tree, D. gelida sp. n. grouped together with D. carnulentum , with low SH-aLRT support (<32%), while the positions of D. amorphum Leduc, 2015 , D. normandicum and Daptonema sp. (MG669735) in clade II were weakly supported (75.4% SH-aLRT, 67% posterior probability) and unstable.

Figures

FIGURE 2. Daptonema gelida sp. n. Holotype male (slide number: M1/38): A. Anterior region; B. Head region; E. Posterior region and F. Spicules and gubernaculum. Paratype female (slide number: M1/38): C. Vulval region and D. Posterior region. Scale bars: A, 50 µm; B, C, D, E and F, 20 µm.

FIGURE 6. Phylogenetic tree (18S rDNA) derived from Bayesian inference (BI) and Maximum Likelihood (ML) analyses. Branch support provided as ML/BI (SH-aLRT support (%)/posterior probability (%)). The sequence of the new Daptonema species is marked in bold.