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 : 49-57

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.

Composition of Pilatobius

The genus Pilatobius comprises species exclusively with two macroplacoids, arranged in parentheses, and a septulum in the pharynx. With the exception of the P. recamieri complex, Pilatobius members typically exhibit varying degrees of dorsal cuticular sculpturing ( Figs 33C, 35C, 38B, 40B, 42D, F, G; see below). The punctiform microplacoid mentioned in the original description of P. opisthoglyptus is most probably a mistake, because the interval between the second macroplacoid and the septulum seems too small to leave space for additional structures ( Fig. 42A). The claws of this species are stumpy, with short primary branches; those of the internal claws with a clear hump, and accessory points held tightly adjacent to the claw ( Fig. 42B).

Two general morphotypes of the pharyngeal structures can be distinguished within Pilatobius : (i) short and robust macroplacoids, partly overlapping with the septulum ( Fig. 43A, B); and (ii) long and bar-like macroplacoids, followed by a clear gap between the septulum ( Fig. 43C–F). The first macroplacoid is typically subdivided at 40–50% of its length by a deep constriction. The second has the constriction positioned (sub)terminally ( Fig. 43A–D, F). Only rarely are the constrictions poorly developed ( Fig. 43E). The morphometry of pharyngeal structures is, together with the existence of the cuticular sculpture, of crucial importance to the taxonomy of Pilatobius , thus redescriptions of ‘old’ taxa and descriptions of new species should provide extensive measurements of these structures and their relative positions.

Shortly after Pilatobius was established ( Bertolani et al. 2014), it was revealed that the genus is paraphyletic, because Notahypsibius , which exhibits only a rigid buccal tube, is embedded among the pilatobiins ( Tumanov 2020). The new phylogeny ( Fig. 2) corroborates the previous findings, placing Notahypsibius as sister to the P. recamieri group, whereas the clade P. oculatus group + P. cf. bullatus is sister to that clade. In an attempt to make the systematics of Pilatobiinae closer to the natural state, we propose the division of the genus into four coherent morphological groups, in a similar manner to the way in which we approached the reorganization of Isohypsibius and Doryphoribius ( Gąsiorek et al. 2019) . Hence, the recamieri group comprises species with a smooth cuticle that lack dorsal gibbosities, whereas the bullatus group (= Pilatobius s.s. owing to the inclusion of P. bullatus , its type species) comprises species with a sculptured cuticle and dorsal gibbosities. Neither the oculatus group nor the rugosus group exhibit dorsolateral gibbosities; however, the cuticular sculpturing is developed differently in each of these species complexes: in the oculatus group, the cuticle is noticeably sculptured only caudally, although some small sculpturing may appear more anteriorly; whereas in the rugosus group, the cuticle is intensely sculptured over the whole dorsum. Two of these groups (the bullatus , oculatus, recamieri , and rugosus groups) are erected as new genera (see below) to reflect the phylogeny of Pilatobiinae ( Fig. 2). Unfortunately, owing to our stringent selection criteria for the composition of the genetic dataset, we were forced to eliminate P. nodulosus ( rugosus group), P.patanei ( bullatus group), and P. ramazzottii ( rugosus group) from our analysis, because only 18S rRNA barcodes were available in GenBank.

In agreement with the doubts raised by Ramazzotti and Maucci (1983), P. elongatus ( Mihelčič, 1959) sp. dub. is designated as synonym novum of P. bullatus owing to the lack of separating characters.Given that the alleged rows of ‘minor’ gibbosities in Pilatobius bisbullatus ( Iharos, 1964) seem to be only folds of cuticle between otherwise typical gibbosities and are an artefact resulting from shrinkage of the specimens in the mounting medium, we designate the species as invalid. Other species ( P. gerdae ( Mihelčič, 1951) sp. dub., P. nonbullatus ( Mihelčič, 1951) sp. dub. (both in bullatus group), P.latipes ( Mihelčič, 1955) sp. dub. ( rugosus group)) designated as dubious by Dastych (2015) are also considered as such and rejected in this paper.