Arctodiphascon tenue ( Thulin, 1928 ), 2023

Arctodiphascon tenue ( Thulin, 1928) , comb.nov.

( Figs 1–3)

Diphascon tenue Thulin, 1928: 255–256 View in CoL , fig. 26.

New material examined. 172 specimens and one exuviae with eggs mounted on slides in Hoyer’s medium (six of these specimens used for DNA extraction); 15 specimens mounted on stubs for SEM investigation. Norway, West Spitsbergen , Nordvest-Spitsbergen National Park, 79°15′37.0″N 11°31′41.3″E, moss on soil, 17 Aug. 2019, N. Shunatova leg., slides SPbU 259(001–107), SEM stub SPbU Tar _38 GoogleMaps .

Type material examined. Syntype. Denmark, Faroe Is., Trangesvaag, 250 m a.s.l., moss, slide ZMUC- TAR-774 ( NHMD) [Thulin’s collection, labelled as “type”] .

Morphological description (based on new material). Body elongate, relatively thin, slightly widened on its caudal end, with evidently narrowed head region ( Fig. 1; Table 1). Body transparent or whitish, without eyespots. Cuticle smooth in LM, with poorly developed rugosity in SEM, more evident dorsally ( Fig. 1B–D). No cephalic sensory structures visible ( Fig. 1E), except for two elongate porous areas laterally to mouth opening (visible in SEM only; Fig. 1F, G, white arrowheads), possibly being sensory structures or muscle attachment zones. Mouth opening anteroventral, surrounded by six peribuccal lobes (visible in SEM only; Fig. 1F, G).

Bucco-pharyngeal apparatus of Diphascon - type sensu Pilato & Binda (2010), with very thin and long bucco-pharyngeal tube ( Fig. 2A–C). Oral cavity armature not visible in LM. Dorsal and ventral apophyses for insertion of stylet muscles ( AISM) in shape of “semilunar hooks”, asymmetrical with respect to frontal plane, with dorsal apophyses being distinctly higher, shorter and thicker than ventral ones ( Fig. 2E–G), caudal processes of both apophyses pointed posteriorly and laterally, better developed dorsally ( Fig. 2F, G). Stylet furcae typically developed, with swollen apices ( Fig. 2D). Stylet supports well-developed ( Fig. 2A–C). Pharyngeal tube with fine indistinct annulation ( Fig. 2H). Most caudal zone of pharyngeal tube inside pharynx with slightly thickened walls and less developed annulation; annulation in this zone often not visible ( Fig. 2K, black arrow). A well-developed drop-like dorsal apodeme present between buccal tube and pharyngeal tube ( Fig. 2A–C, white arrowhead); surface of apodeme with annulation looking as continuation of pharyngeal tube annulation; annulation also present on pharyngeal tube wall below apodeme ( Fig. 2I, J). Extremely small pharyngeal apophyses rarely discernible with LM ( Fig. 2L, black arrow) present; small thickenings of pharyngeal tube walls usually visible only caudally ( Fig. 2C, black arrow). Pharyngeal bulb subspherical, slightly elongate, with three rod-like macroplacoids, first and second being almost equal, third the longest ( Fig. 2A–C, L). Third macroplacoid often with poorly developed pre-terminal incision ( Fig. 2L, white arrow). No microplacoid, septulum or pseudoseptulum.

All legs with small claws of Hypsibius - type sensu Pilato & Binda (2010), slightly increasing in size from legs I to legs IV, with developed accessory points ( Fig. 3A–E). No lunules (pseudolunules) at claw bases, but claw bases widened (visible in SEM only) ( Fig. 3D, E). No cuticular bars below claws, but muscle attachment zone often forming a deep fold on ventral surface of legs I–III, which sometimes looking as a poorly developed bar-like structure in LM ( Fig. 3A, B, D, white arrowhead).

Eggs laid in exuviae. Egg chorion with numerous short pillars, forming a granulated pattern on egg surface ( Fig. 3F–H).

Comparison of the new material with the original description and the syntype of Diphascon tenue . Thulin (1928) described D. tenue from four syntypes. The original description given by Thulin (1928) is rather brief and does not meet modern standards for tardigrade species descriptions. However, it is accurate enough to provide essential details for the species recognition.

Our material corresponds perfectly to Thulin’s description (1928) in all details, including some morphometric indices introduced by Thulin. These were ms (length of the structure in relation to body length) and claw length index (ratio between posterior and anterior claws lengths for legs IV). We did not use Thulin’s cph index (length of the structure in relation to pharynx length), because the soft tissues can be strongly affected by the slide preparation process and mounting medium. Measurements provided by Thulin were the following (measurements of our material are in parentheses): body length 231 µm (131–234 µm); pharynx length 21.6 µm (23.3 µm in a single measured specimen); length of posterior claw of legs IV 6.7 µm (5.3–8.2 µm); ms for posterior claws of legs IV 2.8% (2.9–3.7%); claw length index for legs IV 1.33 (1.09–1.42).

The presence of a drop-like apodeme of the bucco-pharyngeal tube was not mentioned in the original description, but it is visible in the Thulin’s drawing (1928: 255, fig. 26). Moreover, this structure is well-recognisable in the syntype examined ( Fig. 4). Unfortunately, specimens from the Thulin’s collection are in poor state now because of the mounting medium degradation.

DNA sequences . The sequences of good quality for 18S rRNA and 28S rRNA gene fragments were obtained from six individuals [voucher slides SPBU 259 (57–61)], those for COI gene fragment from five specimens [voucher slides SPBU 259 (57–61)], and for ITS-2 marker from three specimens [voucher slides SPBU 259 (57–59)]. No genetic polymorphism was revealed for all genes fragments: all sequences were identical for each of four genes. All obtained sequences were deposited in GenBank (https://www.ncbi. nlm.nih.gov/genbank/) under the following accession numbers: OQ351311–OQ351316 (18S rRNA); OQ357540–OQ357545 (28S rRNA); OQ352269–OQ352273 ( COI); OQ357877 – OQ357879 (ITS-2) .