Adropion belgicae (Richters, 1911)

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 : 7-8

Adropion belgicae (Richters, 1911)

Diphascon belgicae ; terra typica: Advent Fjord, Hopen, Svalbard; Richters (1911).

D. belgicae ; Albert I Land,Atomfjella, Svalbard; Dastych(1985).

Material examined: 21 individuals in total (for details, see Supporting Information, Table S1 ).

Comparative material: Scotland, Hill of Foudland (57°23 ʹ 15″N, 2°39 ʹ 39″W; 451 m a.s.l.), old slate quarry, lichen ( Cladonia P. Browne ) from soil, 1 November 2014, Blagden coll. (three specimens) ; Poland, Słowiński National Park (54°42 ʹ 53″N, 17°13 ʹ 24″E; 9 m a.s.l.), the Scots pine ( Pinus sylvestris L.) forest, moss from soil, 3 September 2014, Morek coll. (one specimen) .

Amended description: Body medium-sized to large ( Table 2 ), elongated and massive, of similar width over its entire length. Smaller specimens white, but larger brownish at least in the caudal part, often extending to the whole body ( Fig. 9A ). Cuticle smooth, without pores, wrinkling, or granulation. Head blunt. Cribriform areas not visible under PCM. Legs short and stout. Eyes absent in live animals. Buccopharyngeal apparatus of the Adropion type ( Fig. 9B ). The OCA not visible under PCM ( Fig. 9B ). Furcae of the Hypsibius type. Pharyngeal apophyses absent ( Fig. 9B ). Pharynx oval and broad. Macroplacoid length sequence 1 <2; all macroplacoids elongated and thin, with irregular, deeply serrated margins ( Fig. 9B–D ). The second macroplacoid 2–2.5 times longer than the first, with a clearly swollen terminal part separated from the rest of the structure by a constriction ( Fig. 9C, D ). Microplacoid small and of irregular shape ( Fig. 9B–D ).

Claws of the Hypsibius type, large and robust, with divergent accessory points. Claw septa usually clearly visible, dividing the claw into three portions ( Fig. 9E–G ). Pseudolunulae weakly delineated and only sometimes visible at the base of the internal and anterior claws ( Fig. 9F, G ). Cuticular bars absent; only slightly thickened edges of pulvini may be visible on the internal side of legs ( Fig. 9E ).

Remarks: A more detailed description of A. cf. belgicae was provided by Bernard (1977). Based on claw (stumpy and with distinct septa) and buccopharyngeal apparatus morphology, it is obvious that the specimens collected by Bernard in Michigan represented a similar species, but not A. belgicae s.s. They had three well-developed types of cuticular bars (internal, median, and anterior), as opposed to only pulvini present in A. belgicae . Thus, the American specimens must have belonged to a new species. Likewise, Fujiscon diphasconiellum Ito, 1991 is a different species owing to the dissimilar arrangement of bars; its current status should be Adropion diphasconiellum comb. nov. et stat. rev., by withdrawing it from an incorrect synonymy with A. belgicae ( Maucci 1996) and pending a redescription. Dastych (1988) was the first to mention the brown colour in A. belgicae he collected in Poland. He also illustrated the conspicuous claw septa that are characteristic of the species.

Etymology: Named after the steamship Belgica, used in expeditions to Antarctic (1897–1899) and Arctic (1905, 1907–1909). A noun in genitive singular.

Differential diagnosis: There are only two other Adropion spp. with two macroplacoids in the pharynx: A. diphasconiellum comb. nov. et stat. rev. and Adropion gani ( Sun et al., 2014) (a poorly diagnosed species with macroplacoids arranged rhomboidally; see below for the composition of the genus). Adropion belgicae has no cuticular bars, which distinguishes it sufficiently from A. diphasconiellum comb. nov. et stat. rev., which possesses three types of bars (internal, median, and anterior). Morphometric data would probably also be useful in separation of the two species; however, such data are unavailable owing to the outdated description of the former taxon. Because of general similarities in the morphology of the buccopharyngeal apparatus, A.belgicae canbemisidentifiedasamemberofthegenus Mesocrista Pilato, 1987 ( Gąsiorek et al. 2016). Nevertheless, Mesocrista has a buccal tube that is wider than in Adropion (the thinnest buccal tube measured in Mesocrista had an external diameter of ~3.5 μm, vs. ~2.5 μm at its widest in A. belgicae ), double annulation of the pharyngeal tube (single annulation in Adropion ), and three types of cuticular bars at the claw bases (typically only the edges of external pulvini faintly observable in A. belgicae ).

Figures

Figure 9. Adropion belgicae (Richters, 1911) (topotypes): A, specimen in toto (PCM, ventral view); B, buccopharyngeal apparatus in toto (PCM); C, pharynx in dorsal view (SEM; note irregular margins of placoids); D, pharynx in lateral view (SEM); E, claws II (PCM; white arrowhead indicates the external pulvinus); F, claws II seen at a different angle (PCM); G, claws IV (PCM). Scale bars are in micrometres.