Diphascon scoticum, Murray, 1905

Gąsiorek, Piotr, Blagden, Brian, Morek, Witold & Michalczyk, Łukasz, 2024, What is a ‘ strong’ synapomorphy? Redescriptions of Murray’s type species and descriptions of new taxa challenge the systematics of Hypsibiidae (Eutardigrada: Parachela), Zoological Journal of the Linnean Society 202 (1), pp. 1-63 : 8-12

D. scoticum ; Forth Valley, Scotland; Murray (1906b).

D. scoticum ; Achill and Clare Island, Belclare, Ireland; Murray (1911).

Hypsibius scoticus ; Wrockwardine, Shropshire, England; Le Gros (1955) .

H. (D.) scoticus ; Perthshire, Ross and Cromarty, England, Isle of Man; Morgan and King (1976).

Adropion scoticum scoticum ; Battle Hill, Huntly, Aberdeenshire; Fochabers , Winding Walks Wood, Morayshire; Invermoriston, Loch Ness , Highland; Lake of Menteith , Stirlingshire; Slug Road, Stonehaven , Aberdeenshire, Scotland; Barna Woods , County Galway, Ireland; Blagden et al. (2020) .

A. scoticum ; Norway, Scotland; Gąsiorek and Michalczyk (2020).

Material examined: 77 individuals in total (for details, see Supporting Information, Table S1).

Neotype material: Neotype and 49 specimens (slides GB.043.01– 09 and GB.054.01–12) from Scotland, Drumoak, Drum Castle (57°5 ʹ 42″N, 2°20 ʹ 16″W; 114 m a.s.l.), leaf litter (mixed Quercus robur L. + Rhododendron L.) collected from soil, April 2016, Blagden coll., are deposited in the Institute of Zoology and Biomedical Research, Jagiellonian University, Kraków. Eight specimens (slides GB.054.13–17; NHMD Catalogue number 1651660–4) are deposited in the Natural History Museum, University of Copenhagen, Denmark .

Comparative material: Scotland, Scone Palace near Perth (56°25 ʹ 21″N, 3°26 ʹ 16″W; 28 m a.s.l.), rural vicinity, lichen from tree branch, 7 April 2016, Jakubczak coll. (three specimens); Battle Hill, Huntly (57°27 ʹ N, 2°47 ʹ W; ~ 130 m a.s.l.), rural vicinity, leaf litter from soil, April 2016, Blagden coll. (one specimen) ; Poland, Stubno , Starzawa Nature Reserve (49°53 ʹ 18″N,

23°01 ʹ 44″E; 201 m a.s.l.), peatbog, moss from peat, April 2014, Gąsiorek coll. (eight specimens).

Redescription: Body medium in size ( Table 3), elongated and white, cuticle smooth, without pores or granulation ( Fig. 10A); however, in larger specimens (> 300 μm), the caudal end is wrinkled and can be brownish. Cribriform areas not visible under PCM. Legs short and plump. Eyes absent in live animals. Buccopharyngeal apparatus of the Adropion type ( Fig. 10A, B). The OCA not visible under PCM ( Fig. 10B). Furcae of the Hypsibius type, but with relatively elongate and divergent arms ( Fig. 10C). Pharyngeal tube annulated over its entire length, i.e. the last annuli extend to the ring-shaped thickening ( Fig. 10B, D); pharyngeal apophyses small ( Fig. 10D). Pharynx oval ( Fig. 10B; non-deformed pharynx length-to-width ratio: 1.6–2.0). Macroplacoid length sequence 2 <1 <3, all elongated and thin, with smooth margins. The third macroplacoid much longer than the first and second ( Fig. 10B, D) and with an evident subterminal constriction ( Fig. 10D). Microplacoid large, oval in PCM ( Fig. 10B), but clearly rod-like under SEM ( Fig. 10D).

Claws of the Hypsibius type, large, with slightly widened bases and slightly protruding accessory points on the primary branches of claws I–III ( Fig. 11A, C). Claws IV with primary branches with more obvious accessory points and widened, calyx-like bases ( Fig. 11B, D). Pseudolunulae visible under the bases of the anterior claws IV ( Fig. 11B). Cuticular bars of three types present at the claw bases (all detectable solely under PCM): (i) small, roundish median bars between external and internal claws I–III ( Fig. 11A, filled arrowhead, often barely visible); (ii) large, internal oblique bars with irregular margins at the internal claw bases ( Fig. 11A, incised arrowhead), near to the internal pulvini; and (iii) long, thick posterior bars joining the bases of the anterior and posterior claws ( Fig. 11B, filled arrowhead) .

Remarks: Adropion scoticum ommatophorum ( Thulin, 1911) , described from Degerfors (Örebro, Sweden), has stouter claws, with short, broad bases and more curved secondary branches, and with the same arrangement of bars as described above for A. scoticum ( Fig. 11E–H). Moreover, the holotype of this subspecies has evident eyes, a condition unknown in populations of the type subspecies. Therefore, it is elevated to a species rank as A. ommatophorum comb. nov., pending a thorough redescription. In contrast, the Chinese subspecies Adropion scoticum qinlingensis ( Li & Liu, 2005) does not exhibit traits that would allow its current taxonomic status to be ascertained (specimens are undistinguishable from the neotype series, because the shape of claw bases, i.e. narrow and calyx-like, presented by the authors as a differentiating trait, is the same for European and Asian individuals). As a result, we do not recognize the validity of the second subspecies. Consequently, all subspecies of A. scoticum are either elevated or supressed.

Etymology: The name signifies a Scottish animal. An adjective in nominative singular.

Differential diagnosis: The most similar Adropion species share three bar-shaped macroplacoids and a microplacoid in the pharynx, thus only two taxa must be compared with A. scoticum : Adropion marcusi ( Rudescu, 1964) sp. dub. and A. ommatophorum comb. nov. (see also the key to Adropion spp. ). Specifically, A. scoticum differs from:

• Adropion marcusi sp. dub., known from Central–East Europe, by the presence of bars (three types of bars in A. scoticum vs. no bars in A. marcusi sp. dub.) and different macroplacoid formula (2 <1 <3 in A. scoticum vs. 1 ≤ 2 <3 in A. marcusi sp. dub.);

• Adropion ommatophorum comb. nov., described from Central Sweden, by a different claw morphology (see above) and the presence of eyes (absent in A. scoticum vs. present in A. ommatophorum comb. nov.).

Composition of Adropion

The genus Adropion was inferred as polyphyletic by Bertolani et al. (2014). Gąsiorek and Michalczyk (2020) confirmed the non-monophyly of Adropion and established Guidettion (the latter having only three bar-like macroplacoids in the pharynx). However, the genus is still heterogeneous in terms of the number of macroplacoids: the majority of species exhibit three placoids, whereas there is a small group of species with only two macroplacoids, i.e. A. belgicae , A. diphasconiellum comb. nov. et stat. rev., and A. gani . The adoption of molecular phylogenetic methods in tardigrade research has shown that the number and shape of placoids in the pharynx is indicative of common ancestry (e.g. Gąsiorek et al. 2019, Guidetti et al. 2022). Thus, it is plausible that A. belgicae and A. diphasconiellum comb. nov. et stat. rev. are closely related and represent a separate evolutionary lineage. If reciprocal monophyly is confirmed molecularly, they should be erected as a new genus. However, A. gani is designated as nomen inquirendum, because having two rod-like macroplacoids arranged in parentheses (i.e. as in the members of the genus Pilatobius ) it does not fit the current diagnosis of Adropion . Moreover, recently three species of Adropion , each with three placoids, were transferred to Kararehius ( Zawierucha et al. 2023) . Their systematic affinity remains to be verified genetically.

Taxonomic key to the genus Adropion and species of Diphascon lacking a DABT

Remark: Adropion gani requires further investigation (see above) in order to ascribe it to the correct genus (i.e. a new genus or Pilatobius ). Therefore, it is excluded from the key, together with A. marcusi sp. dub. Note that although D. greveni and D. mauccii Dastych & McInnes, 1996 belong within Diphasconinae ( Tumanov and Tsvetkova 2023) , they cannot be distinguished morphologically from Adropion based on the currently recognized taxonomic criteria and are thus included in the key.

1. Macroplacoids granular ............................................................................................................................................................................... 2 – Macroplacoids bar-shaped .......................................................................................................................................................................... 4 2. Internal bars I–III present ......................................................................................................................................... Adropion gordonense – Internal bars I–III absent ............................................................................................................................................................................. 3 3. Macroplacoid length sequence 1 <2 <3, dorsal granulation absent ............................................................... Adropion linzhiensis – Macroplacoid length sequence 2 <1 <3, dorsal granulation present .......................................................... Adropion afroglacialis 4. Three macroplacoids present ...................................................................................................................................................................... 5 – Two macroplacoids present ........................................................................................................................................................................ 9