Pilatobius

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 : 53-57

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.

Figures

Figure 2. Phylogenetic relationships within Hypsibiidae in the Bayesian and maximum likelihood analyses (identical topologies; the scale refers to the Bayesian tree and represents substitutions per position; posterior probability values are provided above the nodes, whereas bootstrap values can be found below the nodes; *, maximal support; #, no support).Calohypsibius ornatus constitutes an outgroup. Newly added species and populations are in bold.

Figure 33. General morphology of Pilatobius bullatus (Murray, 1905) in Murray (1905a) (PCM, topotypes): A, dorsal view; B, ventral view; C, close up of dorsal gibbosities and sculpturing.Scale bars are in micrometres.

Figure 35. General morphology of Pilatobius oculatus (Murray, 1906) in Murray (1906b) (PCM): A, neotype in ventral view; B, cuticular wrinkling at the level of third pair of legs (III), with muscle attachments visible as large white ovals, note marginal sculpturing; C, caudal sculpturing, note smaller muscle attachments.Scale bars are in micrometres.

Figure 38. General morphology of Pilatobius cf. rugosus (Bartoš, 1935) (all but C, PCM): A, specimen in toto (ventral view); B, cuticular sculpturing of the caudal region; C, cuticular sculpturing of the caudal region (SEM); D, buccopharyngeal apparatus in toto. Scale bars are in micrometres.

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 42. Various Pilatobiinae, morphological details (PCM): A, P.opisthoglyptus (Maucci, 1987), paratype, buccopharyngeal apparatus in toto (empty white arrowhead indicates septulum); B, P.opisthoglyptus, paratype, claws I; C, P.nodulosus (Ramazzotti, 1957), claws I; D, P.nodulosus, claws IV; E, P.secchii (Bertolani & Rebecchi, 1996) nom.inq., claws II; F, P.ramazzottii (Robotti, 1970), caudal sculpturing; G, P.granifer (Greven, 1972), paratype, cephalic sculpturing (scale unknown). Filled incised white arrowheads indicate bars at the limb bases. Scale bars:10 μm.

Figure 43. Various Pilatobiinae, morphology of pharyngeal structures (SEM): A, P.oculatus; B, P.bullatus; C, P.ramazzottii; D, P.recamieri (Richters, 1911); E, P.patanei (Binda & Pilato, 1971), lateral view; F, P.patanei, dorsal view. Scale bars: 5 μm.