(Smol et al. 2014; emended). Large nematodes (up to 172 mm). Six inner labial sensilla mostly papilliform, six outer labial and four cephalic sensilla papilliform or setiform. Amphids pocket-shaped. Large number of metanemes with caudal filament usually present: dorsolateral and ventrolateral or only dorso-lateral orthometanemes and loxometanemes of type I. Many species with ocelli. Buccal cavity narrow, sometimes with tooth-like thickening. Pharynx inserts into the body cuticle in the region of buccal cavity, the cephalic capsule is variable in the form. Three pharyngeal glands open in the buccal cavity. Pharynx always smooth in outline. Secretory-excretory system, if present, usually restricted to the pharyngeal region, may consists of two cells. Female reproductive system didelphic-amphidelphic with antidromously reflexed ovaries. Males with two testes opposed. Gonad positions relative to intestine variable in species, with anterior and posterior gonad position reversed. Subventral or ventral precloacal papillae (never tubules) often present. Caudal glands mostly present, extending into the precaudal region. Marine and parasites of marine invertebrates.

(emended after Tchesunov and Spiridonov 1985; Miljutin 2003, 2014a). Very large nematodes; at the larval stage parasitize marine invertebrates. Adult worms are free-living. Three lips. Pharynx cylindrical, muscular, with tri-radial internal lumen. Rectum and anus present. Outer labial and cephalic sensilla papilliform, situated in one circle. Amphideal fovea small, pore-like. Cervical setae absent. Excretory-secretory system consists of two cells. Female reproductive system didelphic, amphidelphic, ovaries hologonic. Male reproductive system didelphic, testes outstretched. Tail convex-conoid, broadly rounded.

Aborjinia eulagiscae (Tchesunov & Spiridonov, 1985): Özdikmen 2010, by original designation [= Australonema eulagiscae Tchesunov & Spiridonov, 1985].

Aborjinia corallicola Westerman, de Moura Neves, Ahmed & Holovachov, 2021.

= Paraborjinia corallicola (Westerman, de Moura Neves, Ahmed & Holovachov, 2021), comb. nov.

Body 22.9–27.7 mm long in males. Six outer labial and cephalic sensilla papilliform, situated 19–21 µm from anterior end. Amphideal aperture located 37–40 µm from anterior end. Pharynx tubular without any valves or bulbs, tightly surrounded by the glandular tissue. Nerve ring situated ~ 40% of pharynx length from anterior end. Intestine well developed with wide lumen. Spicules slightly bent, 364–372 µm long. No pre- or postcloacal sensilla or supplements. Spinneret present.

Three males (holotype and two paratypes). The holotype (SMF 14457) and paratype (SMF 14458) are deposited in the Senckenberg Museum, Frankfurt am Main, Germany. Paratype (MIMB 42307) is deposited in the Zoological Museum of A.V. Zhirmunsky National Scientific Center of Marine Biology, Vladivostok, Russia.

One formalin-preserved specimen (male) and two DESS-preserved specimens (males). The Kuril-Kamchatka Trench, water depth 5477 m (45.0°N, 151.1°E), 6114 m depth (45.9°N, 152.8°E) Deposited in the Zoological Museum of A.V. Zhirmunsky National Scientific Center of Marine Biology, Vladivostok, Russia (MIMB 42308).

The Kuril-Kamchatka Trench, water depth 8235 m (44.7°N, 151.5°E) (Fig. 1, Table 1).

The Kuril-Kamchatka Trench, water depth 5477 m (45.0°N, 151.1°E), 6114 m depth (45.9°N, 152.8°E) (Fig. 1, Table 1).

Species name derived from the Latin profundus that means deepwater and refers to the deepwater habitat of described species.

GenBank accession numbers OP600452.1, OP600453.1 (small subunit ribosomal RNA gene, partial sequence; internal transcribed spacer 1, 5.8S ribosomal RNA gene, and internal transcribed spacer 2, complete sequence; and large subunit ribosomal RNA gene, partial sequence); OP407645.1, OP407646.1 (large subunit ribosomal RNA gene, partial sequence).

Large nematodes, 22.9–27.7 mm long, with an average diameter 0.2–0.4 mm. Body cylindrical, tapering towards both extremities (Figs 2, 3). The cuticle finely striated under SEM, and thin (~ 5 µm; Fig. 6). Hypodermis and muscle layers are thin, cords are prominent (Fig. 5). Body pores distinct, irregularly arranged. Measurements tabulated in Table 2.

Head narrow, bluntly rounded with three lips. Inner labial sensilla papilliform, hardly visible under light microscope. Papilliform outer labial sensilla and cephalic sensilla in one circle, 1–2 µm long, situated 19–21 µm from anterior end (Fig. 6A, B). Amphideal opening pore-like, located 37–40 µm from anterior end. Pharynx tubular without any valves or bulbs, tightly surrounded by the glandular tissue (Figs 3A, 4). Nerve ring situated ~ 40% of pharynx length from anterior end. Intestine well developed with wide lumen. Ventral gland consists of two cells. Cell bodies ~ 80 μm long and 75 μm wide (~ 30% of corresponding body diam.), arranged in tandem and situated 725–792 µm from anterior end (Fig. 4D, E). Excretory pore not observed.

Male reproductive system didelphic, testes outstretched. Spicules paired, symmetrical, slightly bent, 364–372 µm long (Figs 3, 4). Gubernaculum not found. No pre- or postcloacal sensilla or supplements. Rectum and anal opening functional. Tail conical with terminal spinneret, caudal glands not observed.

The new species differs from A. eulagiscae by the location of the ventral gland cell bodies (posterior to the nerve ring vs posterior to the cardia), by the smaller body size (23–28 mm vs 103–132 mm), and the shorter tail (193–263 µm vs 500–850 µm).

Anterior and posterior parts of the DESS fixed female. Eastern slope of the Kuril Islands, water depth 3374 m (Fig. 1, Table 1).

OP600454.1 (small subunit ribosomal RNA gene, partial sequence; internal transcribed spacer 1, 5.8S ribosomal RNA gene, and internal transcribed spacer 2, complete sequence; and large subunit ribosomal RNA gene, partial sequence); OP407647.1 (large subunit ribosomal RNA gene, partial sequence).

Body opaque, cylindrical, slightly narrowing to both ends, 17.2 cm long (Figs 2, 7). Cuticle smooth under the light microscope. Inner labial sensilla papilliform, hardly visible under light microscope. Papilliform outer labial sensilla and cephalic sensilla in one circle (Fig. 8C). Amphideal opening pore-like. Buccal cavity small, narrow. Pharynx tubular without any valves or bulbs, cardia small. Intestinal lumen distinct only in its anterior most part. Secretory-excretory system consists of two big cells situated 5053 µm from anterior end (Fig. 8F). Excretory pore not observed. Reproductive system didelphic, amphidelphic with outstretched ovaries. Uteri large, tubular. Vulva located at midbody, a transverse slit. Tail conico-cylindrical. Anal opening present. Spinneret very vestigial (Fig. 8D), caudal glands not observed.

In total, six specimens were sequenced for the 18S rDNA, 28S rDNA (D2-D3 region), and ITS (including ITS1, 5.8S rDNA, and ITS2) genes. Of these, two specimens belong to Leptosomatides Filipjev, 1918, two specimens to Aborjinia profunda sp. nov., and one female and one juvenile belong to Aborjinia sp. The sequence length of 18S rDNA was 1694–1716 bp (14 variable sites, 11 parsimony-informative characters), 28S rDNA – 659–671 bp (66 variable sites, 59 parsimony-informative characters). The length of ITS for the genus Aborjinia was 1054–1094 bp (61 variable sites, 9 parsimony-informative characters) whereas for Leptosomatides sp. length was 1267 bp (3 variable sites).

To calculate genetic distances, as well as to reconstruct phylogenetic relationships, we used all available sequences of leptosomatids from GenBank (Suppl. materials 2, 3). The BI phylogeny using 18S rRNA reveal Deontostoma Filipjev, 1916 as the earliest branching lineage within Leptosomatidae. However, this genus was shown to be polyphyletic which occupied two of the branches of the polytomous clade. A well supported clade (Bayesian PP, BPP = 1; ML bootstrap value percent, ML = 89) united the four samples of Pseudocella Filipjev, 1927. Another moderately supported clade uniting Thoracostoma microlobatum and two species of genus Proplatycoma Platonova, 1976 was uncovered (BPP = 0.78). Two species of PlatonovaMordukhovich et al., 2019 and Synonchus Cobb, 1894 were placed as monophyletic clade with moderate support (BPP = 0.96; ML = 98), whereas the placement of Cylicolaimus de Man, 1889 was unsupported. Thoracostoma trachygaster Hope, 1967 was sister to obtained sequences and Paraborjinia corallicola (BPP = 0.99, ML = 86) and not monophyletic to Thoracostoma microlobatum and Thoracostoma sp., rendering that genus polyphyletic. A clade containing two samples of Leptosomatides (vouchers L1, L2) and a clade uniting samples of Aborjinia (excluding Paraborjinia corallicola) and Leptosomatides (HM564626) were high supported (BPP = 1; ML = 94 and BPP = 0.99; ML = 96, respectively). Paraborjinia corallicola was sister to previous clade with high Bayesian support (BPP = 0.99, ML = 72).

The phylogenetic relationships using 28S rRNA reveals opposite topology compared with 18S rRNA. Paraborjinia corallicola, the rest Aborjinia species and Leptosomatides (vouchers L1, L2) were the earliest branching lineages but supports of these clades were moderate or low. Genus Thoracostoma was also polyphyletic. Deontostoma was placed in one clade with Thoracostoma microlobatum (BPP = 0.99, ML = 83). Pseudocella and one out of three Thoracostoma were sister to Platonova (including Synonchus) (BPP = 0.95, ML = 88).

The average intergeneric p-distances within Leptosomatidae were 1.96% (0.57%–4.66%) and 13.30% (9.15%–17.12%) for 18S rDNA and 28S rDNA respectively if the two non-monophyletic species Thoracostoma trachygaster and Thoracostoma microlobatum Allgén, 1947 as well as Paraborjinia corallicola and the remaining Aborjinia belongs to different genera. Genus Aborjinia (including sequences HM564626 and HM564855, excluding Paraborjinia corallicola) differed from other genera of the family by 1.49% and 11.57% in average for 18S rDNA and 28S rDNA, respectively. Same values for genus Leptosomatides were 1.71% and 13.84% and for Paraborjinia corallicola were 3.78% and 16.15%, respectively.

The interspecific p-distance for 18S rDNA between Aborjinia profunda sp. nov. and Aborjinia sp. (voucher M10) was 0.24%, for 28S rDNA this value was 1.82%, and for ITS2 5.78% (Table 3). Using the programs RNA structure and 4SALE, homologous regions of AborjiniaITS2 as well as Aborjinia sp. MZ504143 sequences were generally folded as comparable secondary structural motifs. Analyses revealed single secondary structure for all sequences contained four universal helices (Fig. 9). Comparison of sequences across taxa identified several hemi-compensatory base changes (hemi- CBCs, Table 4, Fig. 9) which in turn belonged to different types of changes (H1-H3). Various comparison pairs of Aborjinia species gave 3–7 hemi-CBCs while no double-sided changes (CBC) were found (Table 4).

To date only three species (including the present material) were originally described in the genus Aborjinia: Aborjinia corallicola, Aborjinia eulagiscae, and Aborjinia profunda sp. nov. but Aborjinia corallicola is here transferred to Paraborjinia sp. nov. Both species (A. eulagiscae and A. profunda sp. nov.) are characterized by the outer labial and cephalic sensilla situated in one circle and the presence of two cells of secretory-excretory system. In the description of P. corallicola provided by Westerman et al. (2021) it is mentioned that outer labial sensillae and cephalic sensilla are situated in one circle. However, on the photograph provided in that paper these sensilla are situated in two separate circles. In addition, in P. corallicola the secretory-excretory system was not found, contrasting with the two giant and clearly visible cells in other Aborjinia. It should also be noted that, unlike other representatives of the genus, P. corallicola is parasitic as an adult. Based on above we assume that P. corallicola belongs to another genus. Our conclusion is also supported by the molecular phylogenetic tree (Fig. 10), values of intergeneric p-distances within Leptosomatidae, and interspecies p-distances within Aborjinia (Table 3). We propose the new genus Paraborjinia gen. nov. for A. corallicola.

Paraborjinia corallicola (Westerman, de Moura Neves, Ahmed & Holovachov, 2021). Type locality: Atlantic Ocean, Labrador Shelf (60.6083°N, 61.7428°W), 426 m depth. Type host: Acanella arbuscula.

Parasitic life style. Distinct body pores along the body. Outer labial and cephalic sensilla papilliform, situated in two circles. Amphideal aperture pore-like. Muscular and uniformly cylindrical pharynx. Intestine not modified into trophosome. Hologonic ovaries in females. Presence of caudal glands.

Within the family Leptosomatidae the new genus differs from all genera except Aborjinia by having and endoparasitic lifestyle and hologonic ovaries. Paraborjinia gen. nov. differs from Aborjinia by the position of cephalic sensitive organs (outer labial and cephalic papilla in two separate circles in Paraborjinia vs outer labial and cephalic papilla in one circle in Aborjinia). Paraborjinia gen. nov. differs from Aborjinia, Ananus, and Thalassonema by the parasitic adult (vs free-living in Aborjinia, Ananus, and Thalassonema). Paraborjinia gen. nov. differs from Ananus by the presence of rectum and anus. In addition, in all described species of Aborjinia and Ananus the secretory-excretory system is well developed and consists of two prominent cells while in Paraborjinia the secretory-excretory system was not found.