Chaetonotus (Hystricochaetonotus) optabilis, Križanová & Vďačný, 2022

Križanová, Františka Rataj & Vďačný, Peter, 2022, A huge undescribed diversity of the subgenus Hystricochaetonotus (Gastrotricha, Chaetonotidae, Chaetonotus) in Central Europe, European Journal of Taxonomy 840, pp. 1-93 : 33-40

Chaetonotus (Hystricochaetonotus) optabilis sp. nov.

urn:lsid:zoobank.org:act:0B0D89B0-6D24-4D99-A359-18C41CFA1BCE

Figs 18‒22 ; Supp. file 1: Table S4

Morphological diagnosis

Body elongated and about 108‒143 µm long. Head slightly wider than neck, separated from trunk by an inconspicuous neck constriction. Cephalion clearly demarcated, epipleurae and hypopleurae not recognizable. Trunk comparatively narrow, not broader than head, widest at ca U63, narrowest at ca U88. Mouth ventral, no cuticular teeth. Hypostomium bears a rectangular cuticular structure accompanied by two posterior lateral lamellae. Pharynx with anterior and posterior dilatations. Intestine straight, with a marked anterior section. Scales slightly overlapping, distributed in 16–18 columns, 40–45 scales per column. Dorsal surface covered from posterior end of cephalion (ca U1) to furca base (U94) with (i) spined, three-lobed scales with rounded anterior end and (ii) spined, three-lobed scales with truncated anterior end. Spines short, narrowed posteriorly, without denticles. Ventral side carries two broad ciliary bands and two pairs of ciliary fields around mouth and hypostomium. Interciliary filed bears (i) V-shaped scales without keels, (ii) bowl-shaped scales without keels, (iii) obtriangular scales with very minute keels, and (iv) narrowly obovate, keeled scales. Furca base longer than adhesive tubes, lateral margins more or less straight, furcal indentation deeply V-shaped, adhesive tubes diverging posteriorly. Dorsal side of furca base and branches covered with three-lobed scales, ventral side with oblong, spined scales and oval to broadly fusiform, keeled scales.

Molecular diagnosis

18S rRNA gene: 78 C, 148 T, 223 A, 225 A, 228 T, 235 A, 239 T, 240 T, 243 A, 264 A, 267 ‒, 282 T, 283 T, 286 A, 288 A, 336 T, 337 T, 338 C, 340 C, 345 A, 546 A, 639 A, 648 T, 649 A, 652 C, 656 T, 658 A, 664 T, 666 T, 676 A, 678 A, 681 T, 686 T, 688 A, 694 T, 714 T, 722 T, 733 A, 741 A, 1059 A, 1060 A, 1082 T, 1083 T, 1366 A, 1368 T, 1372 G, 1373 A, 1374 A, 1376 T, 1377 T, 1381 ‒, 1383 C, 1384 ‒, 1388 A, 1391 T, 1507 A, 1526 T, 1535 A, 1690 T, 1724 T, 1732 C, 1736 T. 28S rRNA gene: 275 A, 292 A, 309 T, 480 T, 485 T, 487 A, 489 ‒, 500 A, 504 A, 506 A, 520 C, 522 T, 543 A, 588 C, 589 C, 591 ‒, 599 T, 603 T, 608 A, 615 A, 628 C, 634 T, 645 A, 648 T, 652 T, 657 T, 661 A, 662 A, 688 T, 690 A, 698 A, 704 T, 706 A, 713 T, 722 A, 735 G, 742 T, 746 T, 764 A, 777 A, 784 T, 789 C, 791 A, 795 A, 798 A, 810 A, 813 T, 814 A, 819 A, 965 A, 966 C, 996 C. Cytochrome c oxidase subunit I (codon ordinal numbers are followed by the corresponding span of nucleotide positions in parentheses): 20 (58‒60) CTA, 32 (94‒96) AGT, 45 (133‒135) GTC, 49 (145‒147) GCT, 51 (151‒153) GTC, 55 (163‒165) TTC, 57 (169‒171) GTT, 69 (205‒207) CTA, 76 (226‒228) GCG, 77 (229‒231) CCA, 91 (271‒273) TTA, 101 (301‒303) GTG, 109 (325‒327) GCC, 111 (331‒333) ACA, 114 (340‒342) ACG, 133 (397‒399) GCT, 154 (460‒462) ACC, 156 (466‒468) TTA, 184 (550‒552) TTG, 186 (556‒558) TTA, 205 (613‒615) AGG.

Reference molecules are shown in Supp. file 1: Figs S2 and S 14 . All diagnostic molecular autapomorphies are marked by arrows. Reference alignments with corresponding nucleotide positions are in Supp. file 1: Alignments 1‒4.

The p -distance from species described in the present study is 4.00‒4.39% in 18S, 6.78‒9.79% in 28S, and 10.96‒13.59% in COI. There are 13‒16 CBCs in the 18S rRNA gene and 11‒18 CBCs in the 28S rRNA gene.

Etymology

The Latin adjective ‘ optabil · is, - is, - e ’ [m, f, n] (‘desirable’) refers to the very elegant appearance of the new species.

Material examined

Holotype SLOVAKIA • adult (photomicrographs, hologenophore); temporary pond in the floodplain area of the River Morava , Devín , Bratislava, Podunajská rovina plain; 48°10′29.4″ N, 16°58′35.8″ E; CU- FNS- 05-03-20-1 /HO.

Photomicrographs of the holotype are available at the Department of Zoology, Comenius University in Bratislava at https://fns.uniba.sk/en/gastrotricha/. The holotype is shown in Figs 20B–C , 21C , 22B–C .

Paratypes SLOVAKIA • 2 adults (photomicrographs, both hologenophores); same collection data as for holotype; CU-FNS- 05-03-20-2 /PA-1, CU-FNS- 05-03-20-3 /PA-2 .

Photomicrographs of paratype specimens are available at the Department of Zoology, Comenius University in Bratislava at https://fns.uniba.sk/en/gastrotricha/. Paratypes are shown in Figs 20A , 21A– B, D , 22 .

Type material

A DNA sample of the holotype specimen DB 34 has been deposited in the Natural History Museum, Vajanského nábrežie 2, 810 06 Bratislava, Slovakia (ID Collection Code 01427607).

Type locality

Ephemeral pond in the floodplain area of the River Morava near the foothill of the Devín castle, Bratislava, Podunajská rovina plain, Slovakia, 48°10′29.4″ N, 16°58′35.8″ E.

Gene sequences

The nuclear 18S and 28S rDNA sequences as well as the mitochondrial COI sequence of the holotype specimen DB 34 have been deposited in GenBank under the following accession numbers: OM 421710 , OM 421686 , and OM 424065 , respectively.

Description

HABITUS. Chaetonotus (Hystricochaetonotus) optabilis sp. nov. is about 108–143 µm long and has a slender elongated body, with a head region slightly broader than the inconspicuous neck and a more or less bulbous trunk ( Figs 18A , 19A, E , 20A ). Body width is 18.8–21.7 µm at U10, 22.5–23.7 µm at U50, and 26.1–30.7 µm at U60. The head is relatively wide (11–15 µm at U6), with a plate-like, rounded cephalion. Epi- and hypopleurae are not recognizable. The neck (ca U12–U25) is only inconspicuously marked and smoothly continues to the trunk region. The trunk is about as narrow as the head, gradually dilating from approximately U38 to U63, where it reaches the maximum width. Then it gradually tapers towards U88, where the curved margins of the furca branches start to form. Dorsal sensory bristles were not observed. The furcal indentation is deeply V-shaped. The furca branches are set apart and diverge posteriorly. Well-developed adhesive tubes are approximately 9 µm long, they are straight and almost in parallel ( Figs 18A , 19A, D–E , 21D , 22E ).

HEAD. The cephalion (U1) is rounded, clearly demarcated in the body outline, appears as a lens in the ventral view ( Figs 19A, E , 21A ). Pairs of cephalic ciliary tufts emerge laterally at U4 and U6 ( Figs 19A, E , 21A ). The mouth ring is oval, 5.2–6.2 µm in the largest diameter, and is located subterminally at U2–U5. There are strong but short, rod-like reinforcements lining the walls of the mouth ring and inner delicate structures directed towards the center of the mouth ring. Inner cuticular teeth are not present ( Figs 20A , 21B ). The hypostomium (ca U5–U9) has a complex morphology, i.e., bears a rectangular plate (5.0 ×2.9 µm) with a small central protuberance and two lamellae situated posterior to the plate. The lateral sides of the hypostomium are lined by a relatively wide pair of sensoric ciliary patches from U3 to U8 ( Figs 19A , 20A , 21B ).

INTERNAL MORPHOLOGY. The pharynx extends from ca U5 to U29, is 28.0–33.4 µm long and 7.3–7.8 µm wide, sinuous, and has marked anterior and posterior dilatations ( Fig. 19E ). The posterior dilatation (ca U20–U25) is larger than the anterior one (ca U7–U9) (Supp. file 1: Table S4 ). The intestine runs from U29 to U85 and has a separate, well-differentiated anterior section (U29–U33). Transversal bands connected to the base of dorsal scales are well recognizable ( Fig. 19E ). The adhesive gland (ca U85– U87) is placed right behind the terminal part of the intestine forming a short dichotomy at the subtle furca base. The organ X has a lemniscate shape and is situated ahead of adhesive glands ( Figs 19E , 21D ).

SCALES. Almost the entire body is covered by slightly overlapping three-lobed scales that adhere to the basal cuticle layer along either all or most of their perimeter. Scales are distributed in 16–18 longitudinal rows, with 40–45 scales in the central row. Their size increases slightly in a posterior direction. Central dorsal and dorsolateral longitudinal rows of scales begin at the level of the anterior edge of the cephalion (ca U1), while lateral rows start at ca U6. They run almost along the whole body length (till U94). Ventrolateral rows commence at U23 due to the highly developed and wide anterior part of ciliary rows and terminate in the furcal region (at ca U91). Ventral rows start at U10, are staggered, and exhibit a horizontal zonation pattern. Given the shape of the anterior scale lobe, two main types were recognized: (i) head and neck scales (U1–U30), with a rounded anterior lobe and comparatively short posterior lobes, α = 180–196°, β = 47–68° ( Figs 18B, D , 20B , 22C ) and (ii) trunk and dorsal furca scales (U31– U93), with a truncated anterior lobe and longer posterior lobes, α = 162–188°, β = 58–85° ( Figs 18C, E , 22B ). Both types share an indistinct and continuous transition between the anterior and posterior lobes. Rounded structures (1.0–2.4 µm across) irregularly interspersed within type 1 and 2 scales were observed along the whole dorsal side ( Figs 18A–C , 20B–C ). As they were clearly recognizable only in a single specimen, we cannot exclude that these structures are artifacts. The V-shaped furcal indentation carries the fourth type of dorsal scales that are three-lobed and have a tongue-shaped anterior lobe, relatively narrow posterior lobes, α = 156–176°, β = 56–80°, and a marked transition between the anterior and posterior lobes ( Fig. 18G ).

SPINES. Dorsal, dorsolateral, lateral, and ventrolateral scales bear simple, slightly curved spines that gradually narrow towards the distal end ( Figs 18D–G , 20A ). The spine emerges rather close to the anterior margin of scales. Spines do not differentiate into various types, only their length slightly increases in a posterior direction (Supp. file 1: Table S4 ). A lateral denticle is not formed. Ventral scales are spineless.

VENTRAL CILIARY BANDS AND VENTRAL INTERCILIARY FIELD. Ciliary bands start at U10, where they are conspicuously broad ( Figs 19A , 21B , asterisks). However, they begin to narrow from U25 and terminate at U88. The ciliary bands are accompanied by two ventrolateral rows of three-lobed scales that extend from U22 to U91. They are 3.6–4.9× 2.6–3.6 µm in size and have an indistinct and continuous transition between the anterior and posterior lobes ( Fig. 19A ). The ventral interciliary field bears four types of horizontally distributed scales: (i) minute (0.7–0.9 ×0.8–1.2 µm), V-shaped, double-edged scales without keels (U10–U30) ( Fig. 19B ); (ii) minute (0.8–1.1× 0.8–1.3 µm), bowl-shaped, double-edged scales without keels (U31–U36) ( Fig. 19C ); (iii) small (1.3–2.0×1.0–1.5 µm), obtriangular, doubleedged scales with a minute keel (U38–U81) ( Figs 19D , 22A ); and (iv) larger (1.7–3.4 ×1.0–1.5 µm), narrowly ovate, double-edged, keeled, and slightly overlapping scales arranged in three distinct rows (U82–U88) ( Fig. 19D, F ). The furcal region (U87–U93) carries two types of scales: (i) a pair of oblong, comparatively large (7.3–7.6× 2.4–2.6 µm), spined scales, with a broadly rounded posterior end ( Figs 19D, H , 22F ) and (ii) oval to broadly fusiform, rather small (1.0–4.1×1.0–1.8 µm), keeled scales ( Figs 19D, G , 22E ).

Figures

Fig. 18. Chaetonotus (Hystricochaetonotus) optabilis sp. nov. A. Dorsal overview showing the scale and spine pattern. B–C. Details showing the distribution of head and trunk scales. D–E. Head and trunk scales, the schematic drawing of the scale base is shown above that of spined scales. F. Lateral view of a trunk spined scale. G. Dorsal furcal scales. Scale bar = 30 µm.

Fig. 19. Chaetonotus (Hystricochaetonotus) optabilis sp. nov. A. Ventral overview showing the scale and ciliary pattern. B. V-shaped scales without keels. C. Bowl-shaped scales without keels. D. Detail of the ventral posterior body region. E. Internal morphology. F. Narrowly obovate keeled scale. G. Fusiform keeled scale. H. Oblong spined scale. Scale bars: A, E = 30 µm; D = 10 µm.

Fig. 20. Chaetonotus (Hystricochaetonotus) optabilis sp. nov., differential interference contrast.A. Paratype (CU-FNS-05-03-20-3/PA-2). B–C. Holotype (CU-FNS-05-03-20-1/HO). A. Overview showing the general body organization. Asterisks mark the beginning of the ventral ciliary bands. B‒C. Surface views, showing the head and trunk scale patterns. Scale bars: A = 30 µm; B = 5 µm; C = 10 µm.

Fig. 21. Chaetonotus (Hystricochaetonotus) optabilis sp. nov., differential interference contrast. A–B, D. Paratype (CU-FNS-05-03-20-3/PA-2). C. Holotype (CU-FNS-05-03-20-1/HO). A. Detail of the anterior body region. B. Ventral view of the head region. Asterisks mark the beginning of the ventral ciliary bands, arrows mark the cuticular structures of the hypostomium. C‒D. Details of the posterior body region, showing the ovaries and the organ X. Scale bars = 10 µm.

Fig. 22. Chaetonotus (Hystricochaetonotus) optabilis sp. nov., differential interference contrast. A, D–F. Paratype (CU-FNS-05-03-20-3/PA-2). B–C. Holotype (CU-FNS-05-03-20-1/HO). A. Scales in the ventral interciliary field. B. Trunk scales. C. Head scales. D‒E. Details of the ventral posterior body region. F. Oblong spined scales. Scale bars: A‒E = 5 µm; F = 2 µm.

Fig. 2. Sampling sites of nine new gastrotrich species discovered in Slovakia. A. One locality (STV) was situated in northern Slovakia, while five localities were concentrated in the capital city of Bratislava (marked by the red rectangle). B. Detail of the Bratislava district showing the localization of five sampling sites. Map tiles by ©Stamen Design, under a Creative Commons Attribution (CC BY 3.0) license. Data by OpenStreetMap, under ODbL.

Fig. 14. Chaetonotus (Hystricochaetonotus) mirabilis sp. nov., differential interference contrast. A–B, E. Paratype (CU-FNS-25-10-19/PA-1). C–D, F–G. Paratype (CU-FNS-02-10-19/PA-2). A. Dorsal view showing the scale and spine pattern. B. Head scales. C. Neck and upper trunk dorsolateral scales. D. Terminal dorsal trunk scales with a lateral denticle (arrows). E. Ventral ciliary bands. White dashed lines mark upper-furcal scales. F. Lateral trunk scales. G. Dorsal trunk spines with a membrane. White double arrowheads mark the highly refractive bodies in the intestine. Scale bars: A = 30 µm; B–F = 5 µm; G = 10 µm.