Hypsibius scabropygus Cuénot, 1929

Species: Hypsibius scabropygus Cuénot, 1929 View in CoL

Body cylindrical, with weakly delineated and blunt cephalic region ( Fig. 3A View Figure 3 ). Eyes present as a group of pigment granules, which dissolve in Hoyer’s medium. Entire dorsal cuticle wrinkled ( Figs 4A View Figure 4 , 5A, B View Figure 5 ) and with marked muscle attachment points in the middle of the trunk; posteriorly, the wrinkling changes into distinct sculpturing in the form of large, plaque-like protuberances in the caudal body portion ( Figs 4B–D View Figure 4 , 5A, B, E–H View Figure 5 ). The protuberances are opalescent under stereomicroscope and in PCM. Most often, the sculpturing is arranged in two belts: more anterior with smaller protuberances (rarely shaped in a rectangular, plate-like manner; Fig. 4C View Figure 4 ), and posteriormost with large, multangular protuberances ( Figs 4B–D View Figure 4 , 5B View Figure 5 ). In SEM, the dorsoposterior sculpturing can be characterized as locally thickened wrinkling ( Fig. 5E–H View Figure 5 ). Moreover, flattened structures are identifiable in SEM (not visible in PCM) and interpreted as rudimentary elliptical sensory organs ( Fig. 5C, D View Figure 5 ).

A pair of cribriform muscle attachment sites ( Kihm et al. 2023) flanking the mouth opening present ( Fig. 6F View Figure 6 ). Six peribuccal lobes present around the mouth ring ( Fig. 6E View Figure 6 ), lamellae or papulae absent. Bucco-pharyngeal apparatus of the Ramazzottius type ( Figs 3B–D View Figure 3 , 6A–D View Figure 6 ). OCA absent ( Fig. 6E View Figure 6 ). Apophyses for the insertion of stylet muscles (AISMs) asymmetrical with respect to the frontal plane: dorsal apophysis is shorter, higher, and stumpy ( Fig. 3D View Figure 3 ), with a prominent caudal apex ( Figs 6A, B, D View Figure 6 , 7A, B View Figure 7 ). Ventral apophysis slenderer ( Fig. 3D View Figure 3 ), with more developed caudal processes ( Fig. 6C View Figure 6 ). Lateral porous areas present in the buccal crown ( Figs 6E View Figure 6 , 7B View Figure 7 ). Furcae unmodified, of the Hypsibius type, with well-developed condyles ( Figs 3D View Figure 3 , 6G, H View Figure 6 ). Buccal tube narrow, with a clear dorsal longitudinal wall thickening ( Fig. 6A, B, D View Figure 6 ), posteriorly bent and terminating with pharyngeal apophyses ( Figs 3D View Figure 3 , 6A–D View Figure 6 ). Pharynx circular ( Fig. 3C, D View Figure 3 ), with only two granular macroplacoids ( Figs 3B–D View Figure 3 , 6A–D View Figure 6 , 7C–F View Figure 7 ). Macroplacoids are of similar size or the first is only slightly longer than the second. Both macroplacoids with a central or subcentral constriction ( Fig. 7C–F View Figure 7 ), but only the first constriction visible in PCM ( Fig. 3B–D View Figure 3 ).

Claws Ramazzottius View in CoL -like (sensu Tumanov 2020), with elongated primary branches of external/posterior claws and thinned connective portions in the form of light-refracting units ( Fig. 8C–L View Figure 8 ). SEM microphotographs revealed that these units are particularly slender and flexible ( Fig. 8E, G, J–L View Figure 8 ). Posterior claws are higher than external claws I–III. Accessory points on all primary branches evident and clearly divergent ( Fig. 8C–L View Figure 8 ). Pseudolunulae at claw bases present ( Fig. 8D, H, I View Figure 8 ) but often poorly visible. Typically, pseudolunulae are most prominent under claws IV. Pulvini absent. Almost none of the bar types defined for hypsibiids ( Gąsiorek et al. 2024) are present (as described by Cuénot 1929, 1932), but in some specimens, transverse basal bars on the legs I–III ( Fig. 8D View Figure 8 ) are developed asymmetrically and irregularly. These structures must lie under the cuticle as they are unidentifiable in SEM, and probably serve as muscle attachment points for retractor leg muscles.

Eggs laid in exuviae (up to five eggs per exuvia were found), but not smooth. Chorion with densely granulated surface ( Fig. 3E View Figure 3 ), but without any processes. The granulation is developed already within the ovary during chorion formation in the female’s body. No specimens with spermatozoa were found.

Comparison with related species

Hypsibius sp. nov., the new species represented only by one population from the vicinity of Kolding (Jutland), is similar to H. scabropygus in most characters, but differently developed caudal sculpturing, which is less sclerotized, without protuberances, and resembles folding of the cerebral cortex ( Fig. 4F, G View Figure 4 ), allows for a quickdelimitationunderalightmicroscope.Itsclawlight-refracting units seem to be weakly developed ( Fig. 8A View Figure 8 ) when compared with those of H. scabropygus ( Fig. 8C–L View Figure 8 ). Due to the scarcity of material, we refrain from formal description of this species. Importantly, paratypes of H. camelopardalis also have faint light-refracting units ( Fig. 8B View Figure 8 ). The population of H. cf. camelopardalis from Italy shows that the sculpturing can be formed as typical multangular plaques ( Fig. 4E View Figure 4 ), weakly elevated above the epicuticle.

Among the better-documented literature records, H. cf. scabropygus from the Baltic States ( Zawierucha et al. 2014) exhibits posterior bars and certainly represents another new species. Also H. ragonesei Binda and Pilato, 1985 and H. stiliferus Abe, 2004 show posterior bars; all other bar types are absent in the H. scabropygus complex of spp. Given the morphological oddities of H. scabropygus and several other spp. ( Table 2 View Table 2 ), a new hypsibiin genus is erected.