Introduction
Marine tardigrades are found across the globe (Kaczmarek et al., Reference Kaczmarek, Bartels, Roszkowska and Nelson2015) and 263 species are known to date (Guidetti et al., Reference Guidetti, McInnes and Kristensen2025). Among marine tardigrades, the subfamily Euclavarctinae Renaud-Mornant, 1983 (Halechiniscidae Thulin, 1928) is the only family-group taxon with its described members (10 species) exclusively found from the deep sea, specifically from the water depths ranging from 200 to 5,281 m (Kaczmarek et al., Reference Kaczmarek, Bartels, Roszkowska and Nelson2015; Bai et al., Reference Bai, Wang, Zhou, Lin, Meng and Fontoura2020; Saluenko et al., Reference Saulenko, Maiorova, Martínez Arbizu and Mordukhovich2022). The genus and species of this subfamily (no subfamily rank at the time), Euclavarctus thieli Renaud-Mornant, 1975, was first described from off Somalia, Indian Ocean (water depths: 2,600–2,650 m) (Renaud-Mornant, Reference Renaud-Mornant1975). Subsequently, this species was reported from off Peru, Pacific Ocean (3,200 m) accompanied with further morphological observation (Renaud-Mornant, Reference Renaud-Mornant1981). Another record of this species was reported from off France, Atlantic Ocean (2,205–3,039 m) when its own subfamily Euclavarctinae was erected together with descriptions of the following four new species including three new genera: Clavarctus falculus Renaud-Mornant, 1983 from Mozambique Channel, Indian Ocean (275–500 m), Euclavarctus convergens Renaud-Mornant, 1983 from Mozambique Channel (500–3700 m), Exoclavarctus dineti Renaud-Mornant, 1983 from off France (1,369 m), and Proclavarctus fragilis Renaud-Mornant 1983 from Mozambique Channel (type locality; 3,700 m) and off France (3,039 m) (Renaud-Mornant, Reference Renaud-Mornant1983). Renaud-Mornant (Reference Renaud-Mornant1983) recognized differences among these taxa primarily in the cephalic sensory organs and diagnosed this subfamily to have the following character states: a body without alae, a conical head, primary and secondary clavae of different morphology, primary clavae and lateral cirri situated apart from each other, legs with indistinct tarsus, simple digits terminating in crescent-shaped claws, claws of the internal digits with or without a dorsal spur. A year after the erection of this subfamily, a problematic new genus and species with characteristic aliform expansions, Parmursa fimbriata Renaud-Mornant, 1984, was described from Mozambique Channel (depth range: 400–770 m) (Renaud-Mornant, Reference Renaud-Mornant1984). Although this genus and species had character states that contradict Euclavarctinae’s diagnosis (presence of unique alae, a trapezoid head, and a common pedestal for the primary clava and lateral cirrus), for the overall morphology of the cephalic sensory organs, legs, and also seminal receptacles which is not included in the diagnosis, Renaud-Mornant (Reference Renaud-Mornant1984) assigned it to Euclavarctinae without amending the diagnosis. Bussau (Reference Bussau1992) described a new genus and species that conforms to the subfamily’s diagnosis of Renaud-Mornant (Reference Renaud-Mornant1983), Moebjergarctus manganis Bussau, 1992, from a manganese nodule area off Peru (4,140–4,170 m) (Bussau Reference Bussau1992). These are the six genera known to date and excluding Parmursa with aforementioned differences, they are distinguished from each other almost solely by their cephalic cirri (length, partition, and shape) (Renaud-Mornant Reference Renaud-Mornant1983; Bussau Reference Bussau1992) [also see the section comparing these genera in the keys of Fontoura et al., [Reference Fontoura, Bartels, Jørgensen, Kristensen and Hansen2017] and Hansen and Kristensen [Reference Hansen, Kristensen and Schmidt-Rhaesa2020]). Since then, three new species have been described, Pa. torquata Hansen, 2007 from Faroe Bank, Atlantic Ocean (200 m; the shallowest record of the subfamily) (Hansen Reference Hansen2007), M. clarionclippertonensis Bai et al., 2020 from the Clarion-Clipperton Fracture Zone, Pacific Ocean (5,135–5,281 m) (Bai et al., Reference Bai, Wang, Zhou, Lin, Meng and Fontoura2020), and Moebjergarctus okhotensis Saluenko et al., 2022 from Sea of Okhotsk, Pacific Ocean (3,351–3,366 m) (Saulenko et al., Reference Saulenko, Maiorova, Martínez Arbizu and Mordukhovich2022). Also, several new records expanded the distributions of described species, Eu. convergens from Gulf of Mexico, Atlantic Ocean (2,743 m) (Romano III et al., Reference Romano III, Gallo, D’Addabbo, Accogli, Baguley and Montagna2011), Eu. thieli from Gulf of Mexico (2,635 m) (Romano III et al., Reference Romano III, Gallo, D’Addabbo, Accogli, Baguley and Montagna2011), M. clarionclippertonensis from three localities around the border of the Atlantic Ocean and Southern Ocean (2981–3650 m) (Trokhymchuk et al., Reference Trokhymchuk, Schmidt-Rhaesa, Utevsky, Kristensen and Kieneke2024).
During our survey of samples collected on board T/RV Toyoshiomaru (Hiroshima University), we encountered a peculiar Parmursa-like undescribed species from a deep-sea locality (596–606 m) in the Okinawa Islands, Japan. Herein, we describe this taxon of Euclavarctinae as a new genus and species.
Material and methods
The sediment containing the undescribed species was collected at St. 11 (26°19.180′ N, 127°25.560′ E; depth range: 596–606 m), north of Kuroshima island, Kerama Islands, Okinawa Islands, Ryukyu Islands, Japan on 24th May 2003 during a research cruise of T/RV Toyoshiomaru (for details of collection, see Ohtsuka and Boxshall [Reference Ohtsuka and Boxshall2004]). On board, sediments were washed in a bucket filled with sea water several times and the supernatants were filtered with a small plankton net (0.1 mm in mesh size). Hyperbenthic and meiobenthic organisms retained on the plankton net were kept in 10% neutralised formalin (Ohtsuka and Boxshall, Reference Ohtsuka and Boxshall2004). In 2024, the meiobenthic organisms were further concentrated by the floatation method (e.g., Giere, Reference Giere2009) using Ludox HS-40 colloidal silica and a 63-μm-opening mesh net. Specimens of the undescribed species were sorted from this extract using a stereomicroscope.
Differential interference contrast microscopy (DIC) and scanning electron microscopy (SEM) followed Fujimoto and Jimi (Reference Fujimoto and Jimi2020), except that for DIC observation, H-S slides (Shirayama et al., Reference Shirayama, Kaku and Higgins1993) were used and most observations and drawings of the undescribed species were made using an Olympus BH-2 microscope equipped with a drawing tube. Further, only one paratype was observed in distilled water and the drawing of the buccal apparatus was made based on a video recording of this observation. Videos and micrographs of this specimen are available at MorphoBank (O’Leary and Kaufman, Reference O’Leary and Kaufman2012) under the project number: 5987 (http://morphobank.org/permalink/?P5987). DIC micrographs and measurements (to the closest micrometre) were taken using an Olympus BX53 microscope equipped with an Olympus DP73 digital camera and cellSens software. The type material was deposited at the National Museum of Nature and Science (NMST), Tsukuba, Japan. Figures were prepared using Affinity Designer 2 (Serif (Europe) Ltd.). Terminology followed Hansen (Reference Hansen2007) except that the paired lateral alae were further divided into three pairs, lateral alae I–III, corresponding to our observation and interpretation (see Results).
Results
Systematics
Class Heterotardigrada Marcus, 1927
Order Arthrotardigrada Marcus, 1927
Family Halechiniscidae Thulin, 1928
Subfamily Euclavarctinae Renaud-Mornant, 1983
Ranarctus gen. nov.
urn:lsid:zoobank.org:act:2FA7C4E1-855F-4363-890B-E5BA14340AA9
Diagnosis. Euclavarctinae with wrinkled, cuticular aliform expansions without any distinct ribs, trapezoid head, primary clava and lateral cirrus on common pedestal, cephalic cirri each with constant-width scapus, digits with wrinkles, internal digits longer than external digits, and internal claws each with dorsal spur.
Type species. Ranarctus kondoi gen. et sp. nov.
Etymology. The genus name (masculine gender) is a combination of two latin words, rana and arctus. The former word means “frog” referring to the holotype’s posture that resembles the frog winning a sumo bout against a rabbit in the first scroll (Ko) of Choju jinbutsu giga, a Japanese artwork. The latter word means “bear”, commonly used in genus names of marine tardigrades.
Ranarctus kondoi gen. et sp. nov.
urn:lsid:zoobank.org:act:230A41E0-1356-4F09-96BD-5C3084526F81
Drawings of Ranarctus kondoi gen. et sp. nov.: (a) ventral view of habitus, holotype, NSMT-Tg 352 (alae and transition zones with wrinkles in grey shade and these two areas separated by broken lines); (b) dorsal view of head, holotype, NSMT-Tg 352; (c) buccal apparatus (incomplete), paratype, NSMT-Tg 354. Scale bars: A, B, 50 μm; C, 20 μm. Abbreviations: an, anus; bt, buccal tube; ca, caudal ala; cE, cirrus E; ec, external cirrus; fa, frontal ala; go, gonopore; ic, internal cirrus; laI–III, lateral alae I–III; lc, lateral cirrus; mc, median cirrus; mo, mouth; pb, pharyngeal bulb; pl, placoid; pc, primary clava; sc, secondary clava; soI–IV, leg I–IV sensory organ; sr, seminal receptacle; st, stylet.
Ranarctus kondoi gen. et sp. nov., habitus of holotype, NSMT-Tg 352 (DIC): (a) dorsal view; (b) ventral view. Scale bars: 50 μm.
Ranarctus kondoi gen. et sp. nov., holotype, NSMT-Tg 352 (DIC): (a) frontal ala and median cirrus; (b) lateral ala I; (c) lateral ala II; (d) lateral ala III, caudal ala, and two leg sensory organs; (e–h) serial view of cephalic sensory organs; (i) cirrus E; (j) leg I sensory organ; (k) leg II sensory organ; (l) digits of leg III; (m) digits of leg IV; (n) female genital structure. Scale bars: a–h, 20 μm; i–n, 10 μm. Abbreviations: an, anus; black arrowhead, proximal portion of cirrus E’s annulation; ca, caudal ala; cE, cirrus E; ec, external cirrus; ed, external digit; fa, frontal ala; go, gonopore; hollow arrowhead, boundary of caudal ala; ic, internal cirrus; id, internal digit; laI–III, lateral alae I–III; lc, lateral cirrus; mc, median cirrus; mo, mouth; pb, pharyngeal bulb; pc, primary clava; sc, secondary clava; soI–IV, leg I–IV sensory organ; sr, seminal receptacle; white arrowhead, duct of seminal receptacle.
Ranarctus kondoi gen. et sp. nov., paratypes (DIC): (a) cross-section-like view of caudal ala, NSMT-Tg 358; (b) lateral view of head, NSMT-Tg 357; (c) dorsal view of head with evident scapi recognised for median cirrus and internal cirrus, NSMT-Tg 354; (d) lateral cirrus with evident scapus, NSMT-Tg 354; (e) partly dissolved buccal apparatus, NSMT-Tg 354; (f) female genital structure, NSMT-Tg 357; (g), (h), head of two smaller specimens, NSMT-Tg 359 and NSMT-Tg 360 respectively. Scale bars: 10 μm. Abbreviations: an, anus; bt, buccal tube; ca, caudal ala; ec, external cirrus; fa, frontal ala; go, gonopore; hollow arrowhead, boundary of caudal ala; ic, internal cirrus; lc, lateral cirrus; mc, median cirrus; pb, pharyngeal bulb; pl, placoid; pc, primary clava; sc, secondary clava; sr, seminal receptacle; st, stylet.
Ranarctus kondoi gen. et sp. nov., paratypes, NSMT-Tg 361 and NSMT-Tg 362 (SEM): (a) Fronto-lateral view, NSMT-Tg 361; (b) ventral view, NSMT-Tg 362; (c) lateral view, NSMT-Tg 362; (d) ventro-lateral view of head, NSMT-Tg 362; (e) frontal view of head, NSMT-Tg 362; (f) torn cirrus E, NSMT-Tg 362; (g)–(j) legs I–IV sensory organs, NSMT-Tg 362; (k) claw of internal digit (leg IV), NSMT-Tg 362; (l) caudal view, NSMT-Tg 362. Scale bars: a–c, 20 μm; d, e, 10 μm; f–k, 5 μm; l, 10 μm. Abbreviations: an, anus; black arrowhead, proximal portion of cirrus E’s annulation; ca, caudal ala; cE, cirrus E; ds, dorsal spur; ec, external cirrus; fa, frontal ala; go, gonopore; hollow arrowhead, boundary of caudal ala; ic, internal cirrus; laI–III, lateral alae I–III; lc, lateral cirrus; mc, median cirrus; mo, mouth; pc, primary clava; sc, secondary clava; soI–IV, leg I–IV sensory organ.
Measurements (μm) of Ranarctus kondoi gen. et sp. nov.
A dash indicates an unmeasured trait.
Diagnosis. Ranarctus with eight aliform expansions (frontal ala, paired lateral alae I–III, and caudal ala), wrinkled primary clavae without proximal swellings, ovoid secondary clavae, secondary clavae slightly shorter than primary clavae, leg IV sensory organs each consisting of papilla and short distal spine, and seminal receptacles each opening lateral to gonopore and duct running laterally, then anteriorly, and finally bent posteriorly terminating in large, elongated-U-shaped vesicle.
Material examined. Holotype: NSMT-Tg 352, adult female on a H-S slide. Paratypes: NSMT-Tg 353–358, adult females on H-S slides; NSMT-Tg 359 and NSMT-Tg 360, male or four-claw juvenile on H-S slides; NSMT-Tg 361 and NSMT-Tg 362, two adult females on the same aluminium stub for SEM. All specimens were collected together with the holotype from the type locality.
Type locality. North of Kuroshima island, Kerama Islands, Okinawa Islands, Japan (26°19.180′ N, 127°25.560′ E; depth range: 596–606 m).
Etymology. The specific name is given in honour of Yusuke Kondo for his contribution to marine biology and devoted support to visiting researchers on board of T/RV Toyoshiomaru.
Description of holotype. Adult female with ovoid body shape from dorso-ventral view (Figures 1a, 2) and fringed with cuticular aliform expansions. Body 207 μm long, from anterior margin of cephalic region between paired internal cirri to posterior margin of caudal ala, and widest at level between legs II and III including lateral alae, 96 μm. Aliform expansions consists of frontal ala (fa in Figures 1b, 3a), at level immediately posterior to primary clavae and lateral cirri, paired lateral alae I (laI in Figures 1a, 3b), spanning from level of frontal ala to level between legs I and II, paired lateral alae II (laII in Figures 1a, 3c), spanning from level between legs I and II to level between legs II and III, paired lateral alae III (laIII in Figures 1a, 3d), spanning from level between legs II and III to level of legs IV, and caudal ala (ca in Figures 1a, 3d), present posterior to level of legs IV, marking posterior margin of body. Expansions characterized by its thin cuticle, numerous wrinkles that run from proximal to distal margin, absence of external supporting structures, specifically ribs in Hansen (Reference Hansen2007), and absence of intracuticular pillars [for pillars see Kristensen (Reference Kristensen1984) and Kristensen and Neuhaus (Reference Kristensen and Neuhaus1999)]. Wrinkles could be present beyond expansions to adjacent dorso-ventrally flattened body regions, transition zones from body to expansions (Figures 1a, 3d). Base of transition zones difficult to delimit.
Trapezoid head (or cephalic region) (Figures 1a, b, 3a, e–h) with unpaired median cirrus, paired internal cirri, paired external cirri, paired lateral cirri, paired primary clavae, paired secondary clavae, and mouth cone. Median cirrus (Figures 1b, 3a) positioned 12 μm posterior from anterior margin of head, where internal cirri (Figures 1b, 3e) arise. Each of external cirrus (Figures 1a, 3h) and secondary clava (Figures 1a, 3g) paired and positioned on common swelling. Swellings and two sensory organs positioned approximately 8 μm posterior from anterior margin and latero-ventral to internal cirri. Secondary clavae latero-dorsal to external cirri. Each of lateral cirrus (Figures 1a, b, 3e) and primary clava (Figures 1a, b, 3f) paired and positioned on common pedestal. These pedestals and two sensory organs positioned 21 μm posterior from anterior margin and mark widest part of head (69 μm between distal tips of pedestals). Lateral cirri dorsal to primary clavae. Each cephalic cirrus consists of constant-width or slightly narrowing scapus and tapering flagellum. Primary clavae elongated, constant-width, and wrinkled. Secondary clavae erect, elongated, and ovoid. Each clava with van der Land organ at its base. Mouth cone antero-ventrally directed and telescopic with mouth opening at distal tip (Figures 1a, 3h). Other than pharyngeal bulb (13 μm wide), most of buccal apparatus dissolved before observation in holotype.
Paired cirri E (Figures 1a, 3i) positioned dorsally at level between legs III and IV. Each cirrus on short cirrophore and consists of annulated portion followed by tapering spine. Proximal-most portion of annulation appears slightly thicker than rest of annulation.
Some parts of legs torn in holotype. Each telescopic leg consists of coxa, femur, tibia, tarsus, and four claw-equipped digits. Boundary of coxa and femur indistinct and on legs I–III of these partitions, sensory organs present. Leg I sensory organ (Figures 1a, 3j) as unsegmented, short, tapering spine. Leg II sensory organ (Figures 1a, 3k) and leg III sensory organ (Figures 1a, 3d) as unsegmented, tapering spines. Leg IV sensory organ (Figures 1a, 3d), on each of leg IV’s coxa, as elongate papilla with short, terminal spine and basal van der Land organ. Tarsus and digits lack peduncles and other supporting structures. Digits have wrinkles. Internal digits longer than external digits (Figures 1a, 3l, m). Digits of legs IV longer than those of legs I–III. Each claw simple, crescent-shaped and lacks calcar (for dorsal spur see Additional information from paratypes).
Genital structure (Figures 1a, 3n) consists of gonopore and paired seminal receptacles. Gonopore opens ventrally at level between legs III and IV and rosette-like (8 μm wide, 9 μm high). Each seminal receptacle opens lateral to gonopore at level immediately posterior to it. Seminal receptacle duct runs laterally, then anteriorly, and makes turn posteriorly, and terminates in stout U-shaped vesicle. Anus (Figures 1a, 3n) positioned 9 μm posterior from gonopore and consists of three wrinkled lobes, two lateral and one caudal (9 μm wide, 12 μm high).
Cephalic and caudal vesicles not recognized. However, these vesicles might be obscured by retracted tissue (artefact) of specimen.
Measurements provided in Table 1.
Additional information from paratypes. The paratypes for DIC conform to the description of the holotype but also provide additional information on the aliform expansions, buccal apparatus, cephalic sensory organs, seminal receptacles, and post-embryonic development. A paratype prepared in lateral position (NSMT-Tg 358) showed the transition from the body to the caudal ala in a cross-section-like view (Figure 4a). The thickness of the ala appeared thinner than both the dorsal and ventral cuticle. Another paratype in lateral position (NSMT-Tg 357) provided a lateral view of the cephalic sensory organs (Figure 4b). It shows that the external cirrus and the secondary clava are on a common swelling and the lateral cirrus and the primary clava are situated close together. Further, for the cephalic cirri, some of the partitions (scapus and flagellum) appeared indistinct. However, we deem the indistinctness is a disturbed state, introduced by fixation, preservation, or preparation, and when the specimen is in a good condition, the scapus is evident (Figure 4c, d). The buccal apparatus was observed in one paratype, NSMT-Tg 354 (Figures 1c, 4e). The buccal apparatus had a buccal tube leading to the pharyngeal bulb with three simple placoids devoid of proximal apophyses and terminal branches. Paired stylets were present but the furca were not recognised (probably dissolved). Stylet supports were absent. All seminal receptacles in the paratypes had U-shaped vesicles with both branches of the U with similar width (Figure 4f), conforming to the holotype’s stout (or thick) ones. Paratypes included two four-claw juveniles, NSMT-Tg 359 (Figure 4g) and NSMT-Tg 360 (Figure 4h). Both specimens were smaller than the adult females (NSMT-Tg 352–358). The shape of the secondary clava in NSMT-Tg 359 (Figure 4g) appeared different from NSMT-Tg 360 and adult females. Additional specimens are required to interpret this difference. Measurements are provided in Table 1.
The paratypes, NSMT-Tg 361 and NSMT-Tg 362, were observed by SEM (Figure 5). These specimens provided decisive evidence that aliform expansions lack ribs and only have wrinkles (Figure 5a–c). In addition, the discontinuity of the alae is evident from the presence of ala-free regions between the adjacent alae (however, at least between the lateral alae III and caudal ala, the transition zone is present in this region). It is also noteworthy that the alae and the transition zones have different shades of grey in SEM (especially pronounced in the caudal ala). The SEM’s three-dimensional view of the head in NSMT-Tg 362 (Figure 5d, e) shows the swellings with the external cirri and the secondary clavae as more distinct structures than by DIC. However, in the other specimen, NSMT-Tg 361, the swellings are not as pronounced with no deep depressions between the swellings and the internal cirri (Figure 5a). The cephalic cirri’s scapi were all wrinkled giving a soft impression. The soft nature of the scapi may have caused the difficulty in identifying some of the distal ends of the scapi by DIC. The cirri E were broken, but the proximal thicker portion of the annulation was recognised as in DIC (Figure 5f). The leg sensory organs were recognised conforming DIC (Figure 5g–j). The digits were not in good condition for observation and most of the claws were in their sheaths. A claw of the internal digit of the fourth pair of legs had a dorsal spur (Figure 5k). The gonopore and the anus were recognised (Figure 5l).
Discussion
Ranarctus kondoi gen. et sp. nov. conforms to the diagnosis of Parmursa (2 species) by the presence of the aliform expansions and trapezoid head (Hansen, Reference Hansen2007). Further, in these three species, (i) the primary and secondary clavae exhibit different morphology, (ii) the primary clava and lateral cirrus are inserted on the same pedestal, (iii) the seminal receptacles’ ducts make turns in the anterior–posterior directions and vesicles U-shaped (unknown for Pa. torquata), and (iv) the claws are simple, crescent-shaped with or without dorsal spurs on the internal ones (Renaud-Mornant, Reference Renaud-Mornant1984; Hansen, Reference Hansen2007). For these similarities, we place the new genus and species in Euclavarctinae following the same reasoning for placing Parmursa in this subfamily (Renaud-Mornant, Reference Renaud-Mornant1984) (see Introduction). However, R. kondoi gen. et sp. nov. does not have ribs in its aliform expansions and its cephalic cirri’s scapi are not funnel-shaped as diagnosed for Parmursa (Hansen, Reference Hansen2007). Differences are also found in the number of lateral alae, digit pattern, and morphology of the seminal receptacles. R. kondoi gen. et sp. nov. has three pairs of lateral alae, whereas in Pa. torquata, the lateral expansion on each side of the body is continuous and therefore only one pair is present (Hansen, Reference Hansen2007). Renaud-Mornant (Reference Renaud-Mornant1984) did not describe the number of lateral alae for Pa. fimbriata. R. kondoi gen. et sp. nov. has internal digits longer than external digits, whereas equal-length digits are present in P. fimbriata according to figures 3, 4D in Renaud-Mornant (Reference Renaud-Mornant1984) and also in Pa. torquata, stated in its description (Hansen, Reference Hansen2007). There are two differences between the seminal receptacles of R. kondoi gen. et sp. nov. and Pa. fimbriata. The first point is the shape of the ducts: each duct of the former species open posterior to the gonopore, run laterally, then run anteriorly, and makes a turn posteriorly, whereas each duct of the latter species open anterior to the gonopore, makes a turns immediately and runs posteriorly, makes a turn and runs anteriorly, and makes a turn posteriorly (Renaud-Mornant, Reference Renaud-Mornant1984). The second point is the thickness of the U-shaped vesicles: the two branches of the U have a similar thickness in the former species, whereas the branch connecting to the duct is thin compared to the other branch in the latter species (Renaud-Mornant, Reference Renaud-Mornant1984). The seminal receptacles exhibit similar morphology in this subfamily (Renaud-Mornant, Reference Renaud-Mornant1981, Reference Renaud-Mornant1983, Reference Renaud-Mornant1984; Bai et al., Reference Bai, Wang, Zhou, Lin, Meng and Fontoura2020; Saulenko et al., Reference Saulenko, Maiorova, Martínez Arbizu and Mordukhovich2022) except those of Eu. thieli from its first report (Renaud-Mornant, Reference Renaud-Mornant1975) and it is likely that this character is not useful in distinguishing genera in this subfamily. Below, we further discuss the significance of the differences observed in the aliform expansions, scapi, and digits to justify the erection of the new genus.
Parmursa has ribs in the aliform expansions that appear as striations with light microscopy (Renaud-Mornant, Reference Renaud-Mornant1984; Hansen, Reference Hansen2007). The SEM of Pa. torquata, Hansen (Reference Hansen2007) revealed that these striations are external cuticular supports of the aliform expansions (figure 4A, C, D in Hansen (Reference Hansen2007)). Striation-like “lines” are also recognized in R. kondoi gen. et sp. nov. with DIC, but our SEM decisively revealed that these “lines” are fine wrinkles. Considering that the structures of the aliform expansions are used to distinguish genera (presence/absence of caesti and ramuli) in Florarctinae (Halechiniscidae) (Kristensen, Reference Kristensen1984; de Zio Grimaldi et al., Reference de Zio Grimaldi, Lamarca, D`Addabbo Gallo and Pietanza1999; Hansen et al.,, Reference Hansen and Kristensen2022), we deem distinguishing genera based on ribs and wrinkles are plausible.
In Euclavarctinae, the morphology of the scapus is considered an important character and Euclavarctus is the only genus without a note of this character state in the diagnosis (Renaud-Mornant, Reference Renaud-Mornant1983; Bussau, Reference Bussau1992; Hansen, Reference Hansen2007). The scapi of R. kondoi gen. et sp. nov. are tubular with no distal expansion in diameter, whereas those of Parmursa species are funnel-shaped (Renaud-Mornant, Reference Renaud-Mornant1984; Hansen, Reference Hansen2007). Further, although the possibility of an artefact during the SEM preparation process cannot be eliminated, SEM observations show that the former species’ scapi have a soft texture compared to those of the latter species [figure 4B in Hansen (Reference Hansen2007)]. These differences in the scapus morphology support the erection of the new genus.
Pollock (Reference Pollock1995) suggested that the digit pattern (originally ‘foot pattern’) is a stable character at subfamily-level for Halechiniscidae. However, the diagnosis of Euclavarctinae does not include this character (Renaud-Mornant, Reference Renaud-Mornant1983) and even for genus-level, Moebjergarctus is the only genus with this character included in the diagnosis (Bussau, Reference Bussau1992). Focusing on genera with more than one species, Parmursa (2 species) has equal-length digits as aforementioned and Moebjergarctus (3 species) has longer internal digits (Bussau, Reference Bussau1992; Bai et al., Reference Bai, Wang, Zhou, Lin, Meng and Fontoura2020; Saulenko et al., Reference Saulenko, Maiorova, Martínez Arbizu and Mordukhovich2022). In the other non-monotypic genus Euclavarctus (two species), it is not clear whether the digit pattern is stable or not: Eu. theili was originally reported to have internal digits (9–10 μm) longer than external digits (7–8 μm) (Renaud-Mornant, Reference Renaud-Mornant1975) but in the drawing of a specimen from another locality, the digits appear equal-length (figure 1 in Renaud-Mornant [Reference Renaud-Mornant1981]) and Eu. convergens seems to have equal-length digits as there was no specific mention of difference in digit lengths in its description (Renaud-Mornant, Reference Renaud-Mornant1983). Further, the two reports of Eu. theili show different seminal receptacle morphology (Renaud-Mornant, Reference Renaud-Mornant1975, Reference Renaud-Mornant1981), suggesting the possibility that the second report is a different species. Although there is an uncertainty in the stability of the digit pattern in Euclavarctus, from our knowledge of Moebjergarctus and Parmursa, the digit pattern is likely to serve as a genus-distinguishing character in Euclavarctinae. Therefore, the difference in digit patterns between R. kondoi gen. et sp. nov. and Parmursa is likely a genus-level difference.
Based on the differences found in the two genus-distinguishing characters (aliform expansions and scapi of cephalic cirri) and one likely genus-distinguishing character (digit pattern), the status of R. kondoi gen. et sp. nov. is justified. For a subfamily with only 10 species and three of them comprising monotypic genera, it is not easy to determine what morphological characters are appropriate to delimit genera. To further understand this subfamily, we need to describe more taxa. In addition to describing new taxa, which is sometimes troublesome in collecting appropriate specimens, there are published records of specimens identified to Euclavarctinae genera (Hansen, Reference Hansen2007; Romano III et al., Reference Romano III, Gallo, D’Addabbo, Accogli, Baguley and Montagna2011; Wang et al., Reference Wang, Bai, Wang, Lu, Li, Lin, Huang and Fontoura2023) and detailed description of these specimens could also expand our understanding of this deep-sea subfamily’s morphological diversity. In addition to this morphological approach, molecular data for this group are awaited to evaluate the validity of the present taxonomic characters and to understand Euclavarctinae’s morphological evolution. The phylogenetic positions of the anomalous genera, Parmursa and Ranarctus gen. nov., is an important topic which we could not analyse in this study for the specimens’ condition. Fujimoto et al., (Reference Fujimoto, Suzuki, Ito, Tamura and Tsujimoto2020) demonstrated that it is feasible to obtain molecular data from deep-sea tardigrade specimens and there are eDNA metabarcoding studies that detect tardigrade sequences from deep sea (e.g., Kitahashi et al., Reference Kitahashi, Sugime, Inomata, Nishijima, Kato and Yamamoto2020). The molecular tools are ready for us to investigate not only its taxonomy and phylogeny but also its abundance and distribution in the deep sea.
Data
The type material was deposited at the National Museum of Nature and Science, Tsukuba, Japan (NSMT-Tg 352–362) and videos and micrographs of one of the paratypes, NSMT-Tg 354, are available at MorphoBank (http://morphobank.org/permalink/?P5987).
Acknowledgements
We thank captain Akio Go and the crew and the research team on board the research cruise no. 2003-03 of T/RV Toyoshiomaru, Hiroshima University, for the collection of the sediment sample. We also thank Kuniaki Sasaki, Organization for Research Promotion, Iwate University for his assistance in SEM observation; Yusuke Kondo, Hiroshima Institute of Technology, for his support in managing the samples collected by T/RV Toyoshiomaru; late Daisuke Shimada, formerly of Nihon University, Shogo Sekiguchi, Hibiki Sakai, and Choji Tsukagoshi, Hokkaido University, for their collaboration in the project to reinvestigate the samples collected by T/RV Toyoshiomaru between 1988 and 2023.
Author contributions
Shinta Fujimoto: Conceptualization, Formal analysis, Investigation, Writing – original draft. Hiroshi Yamasaki: Funding acquisition, Investigation, Project administration, Writing – review and editing. Susumu Ohtsuka: Investigation, Resources, Writing – review and editing.
Funding
The authors acknowledge The Nippon Foundation-Nekton Ocean Census Programme (https://oceancensus.org/) for supporting the description of this genus and species. This is Ocean Census Species Number 211.
Competing interests
None.
