Adropion camtchaticum, Tumanov & Kalimullin, 2024

Tumanov, D. V. & Kalimullin, A. A., 2024, Adropion camtchaticum, a new species of Tardigrada (Eutardigrada: Itaquasconidae) from the Russian Far East, Zoosystematica Rossica (China) 33 (1), pp. 19-27 : 21-26

Adropion camtchaticum sp.nov.

( Figs 1, 2)

Holotype. Sex not determined; Russia, Kamchatka Terr., Kamchatka Peninsula, Elizovo distr., foothill of Mutnovskaya Sopka Volcano , 52°31′54.7″N

158°11′46.5″E, ca. 750 m, moss on soil, 16 Oct. 2022, P. V. Grebenkin leg., SPbU 327(5) .

Paratypes. Sex not determined; 4 adults, same data as for holotype, SPbU 327(1–4) .

The holotype and three paratypes are kept at the Department of Invertebrate Zoology, St Petersburg University , Russia and one paratype, at the Zoological museum of Far Eastern Federal University .

Morphological description. Body large (up to 596 µm), elongate, body width equal along most part of its length, with head region slightly narrowed ( Fig. 1A; for morphometrics, see Table 1). Body whitish in small specimens, with brown pigment in larger specimens; no eyespots visible in mounted specimens. Body surface without granulation but with strong cuticular wrinkles dorsally, between well-developed cribrose areas ( Fig. 1B: white arrowheads), sometimes forming reticular pattern on lateral body surfaces ( Fig. 1C). No cephalic sensory structures visible. Mouth opening anteroventrally, on well-developed mouth cone ( Fig. 1D, G).

Bucco-pharyngeal apparatus of Diphascon - type sensu Pilato & Binda (2010), with thin and long bucco-pharyngeal tube ( Fig. 1D). Oral cavity armature ( OCA sensu Michalczyk & Kaczmarek, 2003) consisting of dorsal and ventral rows of round dots (teeth) and two pairs of thin laterodorsal and lateroventral crests in caudal part of oral cavity ( Fig. 2A, B: black arrows). Dorsal and ventral apophyses for insertion of stylet muscles ( AISM) in shape of “semilunar hooks” (following Pilato & Binda, 2010), symmetrical with respect to frontal plane; caudal processes of both apophyses pointing backwards and sideways ( Fig. 2A, B). Stylet furcae typically developed, with swollen apices ( Fig. 1F). Stylet supports well-developed ( Fig. 1D–F). Pharyngeal tube with well-developed annulation of Adropion / Guidettion -type (see Gąsiorek & Michalczyk, 2020, for the description of annulation type) ( Fig. 1H). Most caudal part of pharyngeal tube inside pharynx with evidently thickened walls and less developed annulation, often not visible ( Fig. 1I). Pharyngeal apophyses and drop-like dorsoposterior apodeme of buccal tube ( DABT sensu Gąsiorek et al., 2023) absent ( Fig. 1E); one paratype with a vestigial appendage on dorsal surface of bucco-pharyngeal tube in its part where DABT present in Diphascon species ( Fig. 1F: black arrowhead). Pharyngeal bucco-pharyngeal apparatus ( DIC); E, anterior part of bucco-pharyngeal apparatus (PhC); F, anterior part of bucco-pharyngeal apparatus (PhC; black arrowhead indicates vestigial appendage on the caudal end of buccal tube); G, mouth cone ( DIC); H, annulation of pharyngeal tube ( DIC); I, caudal end of pharyngeal tube (PhC). Scale bars: A – 100 µm; B–D – 20 µm; E–G – 10 µm; H, I – 5 µm.

Note. Measurements are given in µm, pt values in % (the p t index is the percentage ratio between the length of a structure and the length of the buccal tube). SD – standard deviation.

bulb elongate, with three long thin macroplacoids with jagged (irregularly serrate) edges, second the shortest, third the longest ( Figs 1D, 2C–E). Third macroplacoid with thickened caudal end ( Fig. 2C–E). Microplacoids clearly visible, slightly elongate ( Figs 1D, 2C–E). Oblique cuticular rods usually present in anterior part of pharynx ( Fig. 2C: inset, black arrow).

All legs with large robust claws of Hypsibius - type sensu Pilato & Binda (2010), slightly increasing in size from legs I to legs IV, with clearly visible internal septae ( Fig. 2F) and well-developed accessory points. Free apices of accessory points usually located on both sides of main claw branch ( Fig. 2G), sometimes poorly discernible in lateral projection ( Fig. 2F). All claws with lunules (pseudolunules), which better developed on internal claws ( Fig. 2F, G: white arrowheads). Bases of internal claws convex; bases of external claws widened, usually concave. Bases of all claws with minute teeth; teeth better visible on internal claws, less developed on claws of legs IV ( Fig. 2H, I: white arrows). Legs I–III with strong internal and median cuticular bars ( Fig. 2F, H, I: black arrowheads); legs IV with posterior bars only ( Fig. 2G: black arrowhead).

Eggs unknown.

Comparison. In the genus Adropion , only A. scoticum and A. ommatophorum possess three long thin macroplacoids, evident microplacoid without septula, and cuticular bars between the claw bases of legs I–III. The new species differs from A. scoticum (its morphometric data are given according to Gąsiorek et al., 2023) in the larger body size (body length up to 404 µm in A. scoticum vs. up to 596 µm in A. camtchaticum sp. nov.), in having a thinner buccal tube in relation to the buccal tube length [buccal tube external width pt value is 8.1–12.4 (mean 10.8 ± 1.1) in A. scoticum and 6.5–8.3 (mean 7.6 ± 0.7) in A. camtchaticum sp. nov.], in the cuticle with evident cribrose areas, a row of teeth in OCA, and well-developed lunules and teeth on the bases of claws ( vs. only poorly developed lunules, which sometimes can be visible on claws of A. scoticum ).

The new species differs from A. ommatophorum in the larger body size (body length up to 318 µm in A. ommatophorum vs. up to 596 µm in A. camtchaticum sp. nov.), in having lunules and teeth on the bases of claws (see Gąsiorek et al., 2023).

The new species differs from A. belgicae and A. diphasconiellum in having three macroplacoids and wrinkled cuticle with evident cribrose areas; from all other species of the genus Adropion ( A. afroglacialis , A. gordonense , A. linzhiensis , and A. onorei ) in having no septula.

Adropion marcusi is considered a nomen dubium ( Dastych, 2015; Gąsiorek et al., 2016, Gąsiorek et al., 2023) because of unclear and incomplete description, but it can be differentiated from A. camtchaticum sp.nov. in the first macroplacoid shorter than or equal to the second one, while in the new species the second macroplacoid is the shortest, and in having no cuticular bars on legs.

Etymology. The new species is named after the Kamchatka Peninsula, where the type specimens were collected.