Fractonotus Pilato, 1998

Gąsiorek, Piotr, Morek, Witold, Stec, Daniel, Blagden, Brian & Michalczyk, Łukasz, 2019, Revisiting Calohypsibiidae and Microhypsibiidae: Fractonotus Pilato, 1998 and its phylogenetic position within Isohypsibiidae (Eutardigrada: Parachela), Zoosystema 41 (6), pp. 71-89 : 76

Genus Fractonotus Pilato, 1998

Fractonotus Pilato, 1998: 132 .

AMENDED DIAGNOSIS. — Small isohypsibiid (rarely exceeding 200 μm, Fig. 1 ). Cephalic elliptical organs present ( Fig. 7A ). Dorsum and limbs covered with densely arranged, blunt protuberances. Six peribuccal lobes present ( Fig. 6A ). Apophyses for the insertion of stylet muscles (AISM) asymmetrical with respect to the frontal plane – the dorsal apophysis subdivided into two portions: the anterior portion in the shape of a slightly convex longitudinal thickening (and the posterior portion as weakly developed blunt hook); the ventral apophysis in the shape of a mild and long ridge ( Fig. 9A ). Very large pharyngeal apophyses and placoids in the muscle pharynx. Stylet furcae of the Fractonotus - type, i.e. with broad, trapezoid base, thin arms and rounded apices ( Figs 8D , 10A ). Claws of the modified Isohypsibius - type, with triangular bases and strongly curved claw branches ( Fig. 12A, B ). Accessory points symmetrical or occasionally asymmetrical. Smooth eggs laid in exuviae.

DIFFERENTIAL DIAGNOSIS. — Fractonotus shares pronounced cuticular sculpturing with some species of six other parachelan genera: Calohypsibius Thulin, 1928 , some Ramazzottius Binda & Pilato, 1986 , Hypsibius Ehrenberg, 1848 , Pilatobius Bertolani, Guidetti, Marchioro, Altiero, Rebecchi, Cesari, 2014 , Doryphoribius Pilato, 1969 and Isohypsibius Thulin, 1928 , but it can be readily distinguished from these genera by the morphology of the stylet furcae (square/trapezoid in Fractonotus vs narrower and more rectangular in the latter genera; compare Figs 7B, D ; 10 A-C). Furthermore, Fractonotus differs from Ramazzottius , Hypsibius and Pilatobius by having Isohypsibius -like claws (claws of the latter genera are of the Hypsibius or of the modified Hypsibius- type). Moreover, the genus differs specifically from:

– Calohypsibius Thulin, 1928 ( Hypsibioidea : Calohypsibiidae ), by having a different type of cuticular sculpture (roundish or oval tubercles covering the entire dorsum and limbs with smooth dorsal pebble-shaped plaques in Fractonotus , Fig. 5 A-D vs multangular or star-like tubercles and occasional spines arranged less densely in Calohypsibius , Fig. 5E, F ), different structures surrounding the mouth opening (six soft and large peribuccal lobes in Fractonotus , Fig. 6A vs six small well defined papulae in Calohypsibius , Fig. 6B ), a reversed morphology of the dorsal apophysis for the insertion of stylet muscles (an anterior thickening and a tiny posterior hook in Fractonotus , Fig. 7 E-G vs an anterior large blunt hook and a slight posterior thickening in Calohypsibius , Fig. 9A, B ), and by claw morphology (modified Isohypsibius - type claws with pseudolunulae, triangular bases, and elongated, strongly curved branches with conspicuous accessory points in Fractonotus , Figs 11 A-D; 12A, B vs very small, rigid, with the base width equal to the sum of the primary and secondary branch widths, with the vertical septum between the two branches, and without pseudolunulae in Calohypsibius , Figs 11E ; 12C, D ).

– Doryphoribius Pilato, 1969 ( Isohypsibioidea : Isohypsibiidae ), by the presence of elliptical organs on the head (absent in Doryphoribius ), and by the absence of the ventral lamina on the buccal tube (ventral lamina present in Doryphoribius ).

– Isohypsibius Thulin, 1928 ( Isohypsibioidea : Isohypsibiidae ), by the presence of elliptical organs on the head (absent in Isohypsibius ), a different shape of AISM (asymmetrical with respect to the frontal plane in Fractonotus , Fig. 7E vs ridges symmetrical with respect to the frontal plane Isohypsibius , Figs 7H, I ; 9A, C ), and by the claw morphology (modified Isohypsibius - type claws with triangular bases, especially well-marked on the fourth pair of legs, in Fractonotus vs Isohypsibius - type claws with stalk-like bases in Isohypsibius , Figs 11H, I ; 12E, F ).

COMPOSITION AND REMARKS

Currently only three species, Fractonotus caelatus (the nominal taxon), F. verrucosus n. comb. and F. gilvus n. comb., are assigned to the genus. The three species are placed in the single genus because they share a number of taxonomically important traits: AISM shape, the presence of elliptical cephalic organs, two granular macroplacoids in the pharynx, and the type of cuticular sculpturing. On the other hand, Pilato (1998) described the claws of F. caelatus as of the Microhypsibius type, whereas claws in F. verrucosus n. comb. and F. gilvus n. comb. are closer to Isohypsibius type claws. Therefore, given the differences in claw morphology, there is a possibility that F. verrucosus n. comb. and F. gilvus n. comb. belong to a new isohypsibioid genus, and are only delusively similar to Fractonotus due to convergent evolution in the remaining traits. Nevertheless, the majority of traits suggest that all three species should be placed in Fractonotus . Biserov (1986) misinterpreted the AISM of F.gilvus n. comb. ( Fig. 3 therein) as Isohypsibius - type AISM, but our observations of the type material confirm that the species has the AISM of the Fractonotus - type. However, there are more Isohypsibius and Hypsibius species, that exhibit cuticular sculpturing similar to that of Fractonotus . Thus, they may in fact belong to Fractonotus rather than Isohypsibius or Hypsibius . Nevertheless, we refrained from enacting more transfers, as a careful examination of individuals is needed to confirm whether these species, in addition to cuticular sculpturing, also exhibit other characteristics of Fractonotus .

Figures

FIG. 1. — Fractonotus verrucosus (Richters, 1900) n. comb., ontogenetic series (Fig. 1F: differential interference contrast [DIC], the remaining photos are phase contrast microscope [PCM]): A, juvenile habitus (lateral view); B, young adult habitus (dorsal view); C-F, old adult habitus, variability in the development of large tubercles (dorsal views); E, adult habitus with the symmetrical arrangement of plaques (Calohypsibius placophorus sensu da Cunha 1943, dorsolateral view); F, adult habitus (a specimen from Borgholm, Öland from the Thulin collection, lateral view, DIC). Scale bars: 20 μm.

FIG. 7. — A-E, Fractonotus spp., F-G, Calohypsibius spp. and H-I, Isohypsibius spp., elliptical organs and details of the buccal apparatus (PCM):Fractonotus verrucosus (Richters,1900) n. comb.: A, cephalic region,empty arrowheads indicate elliptical organs;B, dorsal view,incised arrowhead indicates buccal tube thickening, and incised empty arrowhead points to the furca;C, ventral view;Fractonotus gilvus (Biserov,1986) n. comb.; D, ventral view,incised empty arrowhead points to the furca;Fractonotus verrucosus (Richters,1900) n. comb.; E, lateral view, arrowhead indicates the posterior portion of dorsal apophysis in the shape of stumpy hook; F, Calohypsibius ornatus (Richters,1900), lateral view, arrowhead indicates the anterior portion of dorsal apophysis in the shape of stumpy hook; G, Calohypsibius schusteri Nelson & McGlothlin, 1996, arrowhead indicates the anterior portion of dorsal apophysis in the shape of stumpy hook, and arrow points to the visible peribuccal papulae; H, Isohypsibius prosostomus (Thulin, 1928); I, Isohypsibius coulsoni Kaczmarek, Zawierucha, Smykla, Michalczyk, 2012. Scale bars: 10 μm.

FIG. 6. — Fractonotus verrucosus (Richters,1900) n. comb.and Calohypsibius ornatus (Richters,1900),peribuccal structures (SEM):A, Fractonotus verrucosus n.comb., with six peribuccal lobes typical for Isohypsibiidae;B, Calohypsibius ornatus with six small and roundish peribuccal papulae typical for Calohypsibiidae.Scale bars:1 μm.

FIG. 9. — Schematic comparison of the AISM (lateral view) in: A, Fractonotus Pilato, 1998; B, Calohypsibius Thulin, 1928; C, Isohypsibius Thulin, 1928.

FIG. 8. — Buccal apparatus of Fractonotus verrucosus (Richters, 1900) n. comb. (SEM): A, oral cavity armature, the arrowhead indicates the row of conical teeth; B, the buccal crown and both dorsal and ventral apophyses for insertion of stylet muscles (AISM) in lateral view;C, buccal tube, empty arrowheads indicate lateral thickenings; D, stylet furca; E, pharynx in dorsal view; F, pharynx in lateral view. Scale bars: A, D, 0.5 μm; B, 1 μm; C, E, F, 2 μm.

FIG. 10. — Schematic comparison of stylet furcae in: A, Fractonotus Pilato, 1998; B, Calohypsibius Thulin, 1928; C, Isohypsibius Thulin, 1928.

FIG. 12. — A-B, Fractonotus verrucosus (Richters,1900) n. comb.;C-D, Calohypsibius ornatus (Richters,1900); E-F, Isohypsibius spp., claws (SEM):Fractonotus- type: A, claws III; B, claws IV; Calohypsibius-type; C, claws II; D, claws IV; Isohypsibius-type; E, Isohypsibius prosostomus (Thulin, 1928),claws I; F, Isohypsibius coulsoni Kaczmarek, Zawierucha, Smykla & Michalczyk, 2012, claws II. Scale bars: A, B, 3 μm; C, D, 1 μm; E, F, 4 μm.

FIG. 5. — A-D, Fractonotus verrucosus (Richters, 1900) n. comb.; E, F, Calohypsibius ornatus (Richters, 1900), cuticle morphology (SEM): Fractonotus-type: A, smooth, obtuse tubercles of the smallest specimens (Calohypsibius scabrosus sensu Murray 1911, Calohypsibius verrucosus sensu da Cunha 1947); B, scabrous tubercles occurring in the caudal part of the smallest specimens and on the whole body of adults; C, D, dorsal plaques of adults (Calohypsibius placophorus sensu da Cunha 1943, Calohypsibius verrucosus sensu Thulin 1911,Marcus 1928 and Cuénot 1932), arrowheads indicate coarse portions of the plaques; Calohypsibius-type: E, irregular, multangular tubercles of the cephalic part; F, rows of tubercles of the trunk in close-up. Scale bars: A, B, F, 2 μm; C-E, 20 μm.

FIG. 11. — Comparison of Fractonotus-type (A-D, F), Calohypsibius-type (E), Microhypsibius-type (G), and Isohypsibius-type (H, I) claws (all but Fig. 11F PCM): Fractonotus verrucosus (Richters, 1900) n. comb.: A, claws I, arrowhead indicates weakly developed pseudolunula; B, claws II, interrupted line designates isohypsibioid claw curvature; C, claws IV, arrowhead indicates weakly developed pseudolunula; Fractonotus gilvus (Biserov, 1986) n. comb.; D, claws III, arrowhead indicates weakly developed pseudolunula; Calohypsibius ornatus (Richters, 1900); E, claws IV; Fractonotus caelatus (Marcus, 1928); F, claws IV; Microhypsibius truncatus Thulin, 1928; G, claws III (DIC); Isohypsibius prosostomus (Thulin, 1928); H, claws II; Isohypsibius coulsoni Kaczmarek, Zawierucha, Smykla & Michalczyk, 2012; I, claws II. Scale bars: 10 μm.

FIG. 3. — Fractonotus verrucosus (Richters, 1900) n. comb., habitus (SEM): A, lateral view; B, dorsal view; Calohypsibius ornatus (Richters, 1900), habitus (SEM): C, lateral view; D, dorsal view. Scale bars: A, 30 μm; B-D, 20 μm.