Degmion Gąsiorek, Morek & Michalczyk, 2024

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 : 57-59

Degmion Gąsiorek, Morek & Michalczyk gen. nov.

ZooBank registration: urn:lsid:zoobank.org:act:B97D39A1-69B5-4E06-81F6-F671444BFFC6

Diagnosis: Cuticle distinctly sculptured in the caudal portion of the trunk; more anterior portions with a weaker sculpturing or smooth. Apophyses for the insertion of the stylet muscles (AISMs) in the shape of semilunar hooks. Buccal tube followed by a flexible annulated pharyngeal tube; the pharyngeal annulation is semi-complex (annuli single laterally and forking dorsoventrally). A DABT is present. There is a spherical pharynx with two macroplacoids and a septulum. Macroplacoids are arranged rhomboidally (◊). Claws of the Hypsibius type, i.e. asymmetrical both with respect to the sequence of primary and secondary branches (2-1-2-1) and with respect to their size, with external and posterior claws always evidently larger than the internal and anterior claws. Pseudolunulae below claws I–IV may be present; only basal cuticular bars below claws IV may be present, with the remaining types of bars absent .

Composition: Degmion burti comb. nov., Degmion nodulosum comb. nov., Degmion nuominense comb. nov., Degmion oculatum comb. nov. (type species), Degmion opisthoglyptum comb. nov., and Degmion rugocaudatum comb. nov. et nom. inq.

Etymology: The name honours Dr. Peter Degma, a renowned Slovakian tardigradologist and a great friend.

Differential diagnosis: Within the Pilatobiinae , Degmion gen. nov., Pilatobius , and Fontourion gen. nov. possess a pharyngeal tube (absent in Notahypsibius ) but:

• Degmion gen. nov. has the dorsal sculpturing restricted to the caudal part of the trunk or the caudal sculpturing is much better developed than that found anteriorly and lacks dorsolateral gibbosities, whereas Pilatobius either exhibits gibbosities and/or its dorsal cuticle is uniformly sculptured throughout the trunk;

• Degmion gen. nov. always exhibits an obvious caudal sculpturing, whereas the cuticle is smooth in Fontourion gen. nov.

Fontourion Gąsiorek, Morek & Michalczyk gen. nov.

ZooBank registration: urn:lsid:zoobank.org:act:ACDC00E5-6B25-4823-BFE8-79CEA2A9457E

Diagnosis: Cuticle smooth. The AISMs are in the shape of semilunar hooks. Buccal tube is followed by a flexible annulated pharyngeal tube; the pharyngeal annulation is semi-complex (annuli single laterally and forking dorsoventrally). A DABT is present. There is a spherical pharynx, with two macroplacoids and a septulum. Macroplacoids are arranged rhomboidally (◊). Claws are of the Hypsibius type, i.e. asymmetrical both with respect to the sequence of primary and secondary branches (2-1-2-1) and with respect to their size, with external and posterior claws always evidently larger than the internal and anterior claws. Pseudolunulae below claws I–IV may be present; only internal and posterior cuticular bars may be present .

Composition: Fontourion boreale comb. nov., Fontourion brevipes comb. nov., Fontourion glaciale comb. nov., Fontourion islandicum comb. nov., Fontourion recamieri comb. nov. (type species), and Fontourion secchii comb. nov. et nom. inq.

Etymology: The name honours Dr. Paulo Fontoura, an esteemed Portuguese tardigradologist.

Differential diagnosis: Within Pilatobiinae , Fontourion gen. nov., Degmion gen. nov., and Pilatobius possess a pharyngeal tube (absent in Notahypsibius ) but:

• Fontourion gen. nov. has a smooth dorsal cuticle and lacks dorsolateral gibbosities, whereas Pilatobius either exhibits gibbosities and/or its dorsal cuticle is uniformly sculptured throughout the trunk;

• for the distinction with Degmion gen. nov., see above.

Remarks: Large pores used by Buda et al. (2018) to characterize F. islandicum comb. nov. are muscle attachment points that have no taxonomic value. With the exception of F. brevipes comb. nov., the species in the recamieri complex are extremely similar ( Zawierucha et al. 2020), and we are of the opinion that the morphometric differences used to delineate them could result from discrepancies in the morphometry conducted by the various researchers. The species are clearly delineated genetically ( Fig. 1 ), thus until proper redescriptions are performed, species identification in this lineage should rely on DNA barcoding.

Pilatobius Bertolani et al., 2014

Amended diagnosis: Cuticle uniformly sculptured throughout the dorsal part of trunk; dorsolateral gibbosities may be present. The AISMs in the shape of semilunar hooks. Buccal tube followed by a flexible annulated pharyngeal tube; the pharyngeal annulation is semi-complex (the annuli are single laterally and forking dorsoventrally). A DABT is present. A spherical pharynx with two macroplacoids and a septulum. Macroplacoids arranged rhomboidally (◊). Claws are of the Hypsibius type, i.e. asymmetrical both with respect to the sequence of primary and secondary branches (2-1-2-1) and with respect to their size, with external and posterior claws always evidently larger than the internal and anterior claws. Pseudolunulae below claws I–IV may be present; cuticular bars of various types (internal, anterior, posterior, and orthogonal) present .

Composition: P. aculeatus ( bullatus group), P. bullatus ( bullatus group, type species), P. granifer ( rugosus group), P. iltisi ( rugosus group), P. patanei ( bullatus group), P. procerus ( rugosus group), P. ramazzottii ( rugosus group), P. rugosus ( rugosus group), P. sexbullatus ( bullatus group), and P. ziliense ( rugosus group).

Remarks: We consider the genus Pilatobius as artificial (nonmonophyletic) in its present composition. Additional DNA data should contribute to disentangling its phylogenetic affinities, and the topology of the pilatobiin tree.

Problems with hypsibiid classification

This study provides redescriptions of some crucial hypsibiid species and descriptions of new species that were included in the paper devoted to hypsibiid phylogeny ( Gąsiorek and Michalczyk 2020). We provide a good starting point for sound taxonomic research on the genera Adropion, Guidettion , and Platicrista , at the same time stressing that without a reliable redescription of Astatumen trinacriae from Sicily, a trustworthy species identification within that genus remains impossible. A higher-level classification of Hypsibiidae illustrates the conflict between the morphological vs.phylogenetic approaches to tardigrade systematics, which had been ripening over the last few years and has recently erupted independently in several contributions. Although the currently recognized four hypsibiid subfamilies ( Pilato and Binda 2010, Bertolani et al. 2014) have been recovered independently as clades ( Gąsiorek and Michalczyk 2020, Tumanov 2020, Tumanov and Tsvetkova 2023, Zawierucha et al. 2023), their current morphological diagnoses are meaningless owing to the erroneous inclusion of some taxonomic features within each clade (originally, Pilatobiinae were defined as having an elongated pharyngeal tube, Itaquasconinae – without the DABT, and Diphasconinae – as having macroplacoids in the pharynx; all these characteristics have recently been demonstrated as diagnostically questionable).

Recently, Tumanov and Tsvetkova (2023) proposed that Itaquasconinae and Pilatobiinae should be elevated to the ranks of separate families within Hypsibioidea . However, in this paper, we have maintained the previous classification of broadly understood Hypsibiidae , for several reasons. The main arguments for dividing Hypsibiidae were their morphological heterogeneity and the lack of monophyly in the molecular analyses of Tumanov and Tsvetkova (2023). However, the first reason is highly subjective (we do not consider traditionally defined Hypsibiidae as more diverse than, e.g. Macrobiotidae ), and the elevation of two of the subfamilies to the rank of families does not change the fact that none of the four hypsibiid groups has a coherent diagnosis. Moreover, neither Itaquasconinae nor Pilatobiinae differ consistently from the clade Diphasconinae + Hypsibiinae [ Tumanov and Tsvetkova (2023) proposed a ‘transfer’ of Diphasconinae to Hypsibiidae , which was redundant, because diphasconins had already been included in this family at that time] and between each other, which would justify the taxonomic elevation of their status. The lack of hypsibiid monophyly can be explained by the highly fragmented (e.g. only the 18S rRNA marker available for some taxa), incomplete, and taxonomically biased (many unusual hypsibiid morphotypes are still not sequenced) genetic dataset at our disposal. In short, based on the available data, we do not recognize the validity of the proposal put forward by Tumanov and Tsvetkova (2023). Nevertheless, we do not exclude the possibility of such moves in the future. Currently, the evidence is simply not there.

In concordance with Tumanov and Tsvetkova (2023), the presence of the DABT (= drop-like thickening sensu Guidetti et al. 2013 /drop-like apodeme sensu Tumanov and Tsvetkova 2023) can no longer serve as the main distinguishing criterion between Adropion and Diphascon (and, ipso facto, between Itaquasconinae and Diphasconinae ). We revealed that this structure can take various forms, and it should be analysed under SEM to determine its shape accurately. In this regard, Dastych et al. (1990) have already noted that the straightforward distinction between the lack ( Adropion ) and presence ( Diphascon ) of an apodeme (Pilato 1987) is contravened by the slight thickening of the buccal tube wall in Diphascon higginsi Binda, 1971 , Diphascon puniceum Jennings, 1976 in Jennings (1976b), and D. sanae Dastych et al., 1990 . The apodeme is reduced and flattened in D. higginsi ( Fig. 2A, B ), which has contributed to confounding views regarding the affinity of the species ( Ramazzotti and Maucci 1983, Pilato and Binda 1988). Furthermore, the apodeme in the D. pingue group is not a solid drop-shaped thickening, as it would appear in LCM. Instead, when observed in SEM, it is a pair of looped filaments with mostly empty lateral portions (possibly joined by one or more irregular cuticular septa in the middle; Fig. 3C, D ).

As noted above, Tumanov (2020) described a remarkable case of paraphyly: the genus Notahypsibius , characterized by a hypothetically plesiomorphic rigid buccal tube ( Gąsiorek and Michalczyk 2020), has been placed within Pilatobius , which exhibits an advanced, elongated posterior portion to its buccopharyngeal apparatus referred to as the pharyngeal tube ( Bertolani et al. 2014). This topology is maintained in the new phylogeny presented herein ( Fig. 2 ), which also includes several recently described taxa.

The issue of the paraphyletic taxa in classification, which affects many tardigrade lineages, is a topic long discussed in both botany and zoology ( Schwenk 1994, Brummitt 1996, Hörandl and Stuessy 2010). Putting aside the fact that a natural consequence of anacladogenesis is the presence of paraphyletic taxa, we are of the opinion that natural classification should aim at distinguishing onlymonophyletictaxa.Besidesthe Notahypsibius – Pilatobius case, which we resolve, in part, by establishing Degmion and Fontourion , a similar puzzling state is found in Hypsibiinae , in which Borealibius ( Pilato et al. 2006) and Cryobiotus ( Dastych 2019) , i.e. genera with clear autapomorphic conditions, are probably (although genetic data are scarce) embedded within the ‘plesiomorphic’ Hypsibius (see also Tumanov and Tsvetkova 2023). Hypsibius scabropygus Cuénot, 1929 is typically recovered as sister to all other hypsibiins and should be erected together with a few other Hypsibius species as a separate genus ( Gąsiorek et al. 2018). An analogous problem was recently shown in Macrobiotidae , in which the strongly modified genus Xerobiotus ( Bertolani & Biserov, 1996) is nested within Macrobiotus and thus should be considered a species group within Macrobiotus ( Stec et al. 2021) . We do not exclude the possibility that some of these taxa are only superficially artificial, because DNA sequences are lacking for the vast majority of species, and that the phyletic relationships might eventually turn out to be different from what we currently know, but this issue seems topical and requires further study.

In fact, the only hypsibiid subfamily that seems well defined morphologically from the remaining clades are the Hypsibiinae , whose morphology of the buccopharyngeal apparatus includes plesiomorphic eutardigrade characters [a likely ancestral type, comprising only a rigid buccal tube and granular or bar-like macroplacoids without signs of thinning and forming single, thin and long macroplacoids typical of many itaquasconins and some diphasconins; however, see the alternative hypothesis postulated by Bertolani et al. (2014) and Tumanov (2020), according to which the presence of only the buccal tube within the buccopharyngeal apparatus is considered apomorphic]; and claws that are relatively homogeneous within Hypsibiidae (see Guil et al. 2013). This means, and is the main point of our criticism, that narrow definitions of newly erected supraspecific taxa, which focus on autapomorphies (always termed subjectively as ‘strong’ or ‘weak’) and neglect the topology of the evolutionary tree, will inevitably lead to creation of paraphyletic taxa characterized by advanced characters and nested among groups of a plesiomorphic nature ( Fig. 44 ). Therefore, we advocate for the phylogenetic approach and the rule of reciprocal monophyly ( Hennig 1966) in tardigrade systematics.

The paraphyletic status of some tardigrade taxa is not the only problem affecting hypsibiid classification. The addition of A. tenue and Arctodiphascon wuyingensis ( Sun et al., 2020) blurred the distinction between Diphasconinae and Itaquasconinae further ( Tumanov and Tsvetkova 2023). This means that some Diphascon spp. with a DABT might, in fact, belong to the Itaquasconinae , ordinarily apodemeless (the apodeme is small and may be reduced in A. wuyingensis ). The intricacy of this situation stems from the lack of a morphological apomorphy that would allow A. wuyingensis to be placed in a new genus (Arctodiphascon was erected with clear evidence of non-monophyly; Tumanov and Tsvetkova 2023). However, this discrepancy might be resolved when the species is studied in more detail, because Tumanov (2020) and Tumanov and Tsvetkova (2023) pointed out that traits previously neglected in hypsibiid taxonomy, such as ornamentation of eggs deposited in exuviae, might prove useful in monophyletic classification.

To sum up, the phylogenetic and morphological diversity of hypsibiids must be researched to a greater extent in order to define accurately the four main clades that are currently recognized as subfamilies. Based on our analyses, we postulate that the morphology of the DABT might bear an important evolutionary value. The morphological characterization of pedal bars, summarized in this paper, aims at unifying the anatomical terminology to aid taxonomic descriptions and comparisons. Integrative revisions of type species were, once again, demonstrated as pivotal to the decluttering of tardigrade classification. It appears that, in many instances, the evolutionary significance of particular phenotypic characters in tardigrade systematics is misunderstood. As a result, subjective assessments of morphology as the only basis for erection of new higher taxa, i.e. finding (‘strong’) apomorphies, lead to the erection of taxa defined by characters (phenetic systematics), and not by factual evolutionary affinities (i.e. reciprocal monophyly; e.g. see de Queiroz and Gauthier 1990) .

Figures

Figure 1. Schematic depiction of six types of cuticular pedal bars and other leg structures found in the Hypsibiidae. Note that the edges of convex pulvini may overlap with fully sclerotized cuticular bars but are generally faint and weakly defined when observed under light microscopy. Historically, these edges of pulvini were sometimes described as ‘bars’ [e.g. see Dastych (1988) for Adropion belgicae or Gąsiorek et al. (2016) for Mesocrista].

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 3. Various Diphasconinae, morphology of the DABT (SEM): A, Diphascon higginsi Binda, 1971 (dorsal view); B, Diphascon higginsi (lateral view); C, Diphascon pingue (Marcus, 1936) (lateral view); D, Diphascon pinguiforme Pilato & Binda, 1997/8 (dorsolateral view). Scale bars: 1 μm.

Figure 44.Evolution scheme for a tardigrade clade, where the blue triangle and red rectrangle signify newly erected genera (‘apical taxa’ in the paper by Tumanov and Tsvetkova 2023) with ‘strong apomorphies’, i.e. the set of well-defined, advanced morphological characters, distinguished from the previous broadly defined genus, whose members (marked with black circles) are still classified as the original genus, which became artificial but cannot be divided further because the lineages group species with mainly (or solely) plesiomorphic characters.