Pilatobius sexbullatus ( Ito, 1995 )

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 : 49-52

Pilatobius sexbullatus ( Ito, 1995)

Diphascon (Diphascon) sexbullatum ; Narusawa-mura, Minamitsuru-gun, Mount Fuji, Japan; Ito (1995).

D. (D.) sexbullatum ; Arhangaj Ajmak, Chubsugul Ajmak, Mongolia; Kaczmarek and Michalczyk (2006).

Proabably also the following record:

D. iltisi ; Pektu-San Mountain, Ryanggang-do Province, North Korea; Dastych (1974).

Material examined: 13 individuals in total (for details, see Supporting Information, Table S1 ).

Comparative material: Russia, Baikal Lake , Olchon (53°16 ʹ 08″N, 107°33 ʹ 05″E; 525 m a.s.l.), shrubland, moss from soil, 14 September 2016, Barczyk coll. (one specimen) .

Amended description: Body small ( Table 14 ) and corpulent ( Fig. 40A ). A single median line of six large, poorly delineated hemispherical gibbosities: three at the level of legs I–III, two between legs I–II and II–III, and one in the segment behind the third leg pair ( Fig. 40A ). Cuticle greatly sculptured on the entire dorsal surface by sub-round polygons of variable size: the largest present on the gibbosities and in the cephalic region, and the smallest on the lateral sides of the trunk ( Fig. 40A, B ). Cribriform areas not visible under PCM. Legs short, plump, and barely delimited from the trunk ( Fig. 40A ). Eyes present in living animals, but usually quickly dissolving in Hoyer’s medium. Buccopharyngeal apparatus of the Pilatobius type, with a short pharyngeal tube ( Fig. 40C ). The OCA not visible under PCM. Furcae of the Hypsibius type. The DABT large; pharynx circular, with large pharyngeal apophyses ( Fig. 40C ). Macroplacoid length sequence 2 <1, the first with delicate constriction at twofifths of its length. Septulum large, but clearly shorter than the second macroplacoid.

Claws of the Hypsibius type, with very short basal portions; those of the external claws also slightly broadened ( Fig. 41A ). The most remarkable are the bases of the posterior claws, which are extremely short, with margins evidently pointed upwards ( Fig. 41B ). Primary branches of the internal and anterior claws much longer than the secondary branches ( Fig. 41 ). Both branches of the external and posterior claws elongated. Accessory points barely divergent ( Fig. 41 ). Pseudolunulae absent. Two types of cuticular bars present: (i) internal, long thickenings at the bases of claws IIII ( Fig. 41A ); and (ii) posterior, short rods ( Fig. 40A ); the latter may be absent ( Fig. 41B ).

Remarks: Specimens depicted by Dastych (1974) fit the description of P. sexbullatus . If their conspecificity is confirmed with DNA barcodes, all published records of this species (from Korea, Mongolia, and Russia, apart from the locus typicus in Japan) would strongly indicate an Eastern Palaearctic species. However, it must be noted that Dastych examined the types of P. iltisi , confirming the similarity of the Korean and American individuals [importantly, an unidentified Pilatobius species ( Diphascon at that time) was recorded from the Polish Tatras by Dastych (1980), and P. iltisi was indicated as the taxon morphologically closest to that single specimen; the detailed drawing of that specimen shows a strong similarity to P. sexbullatus ]. The dorsal cuticular thickenings of P. iltisi are likely to be an extremely flattened row of the dorsolateral gibbosities typically found in P. sexbullatus (slides made by Schuster and Grigarick usually contain strongly compressed tardigrades, which can cause artefacts; P. Gąsiorek, pers. obs.). Therefore, a putative synonymy of P. iltisi and P. sexbullatus (with a potential younger synonym being P. sexbullatus owing to an inadequate description of P. iltisi ) should be considered. The zoogeographical implication of such action would be a single Holarctic species. All members of the bullatus group have paired dorsal gibbosities, thus they are easily distinguishable from P. sexbullatus with its autapomorphic condition of a single median line of gibbosities.

Figures

Figure 40. General morphology of Pilatobius sexbullatus (Ito, 1995) (PCM): A, specimen in toto (dorsal view); B, cuticular sculpturing of the centromedial region with three gibbosities; C, buccopharyngeal apparatus in toto. Scale bars are in micrometres.

Figure 41. Claws of Pilatobius sexbullatus (PCM): A, claws II (black arrowhead indicates internal bars); B, claws IV (empty arrowhead indicates pseudolunule). Scale bars: 10 μm.