Chaetonotus (Hystricochaetonotus) arcanus, 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 : 65-69

Chaetonotus (Hystricochaetonotus) arcanus sp. nov.

urn:lsid:zoobank.org:act:ACDC5C8E-E066-4BA7-B360-E4EC7549790B

Figs 37‒38 ; Supp. file 1: Table S9

Morphological diagnosis

Body stocky and around 100 µm long. Head wider than neck, separated from trunk by a distinct neck constriction. Clearly demarcated cephalion, epipleurae and hypopleurae inconspicuously marked. Trunk widest at ca U37, gradually tapers towards furca base (U82). Mouth almost apical, no cuticular teeth. Hypostomium bears a single small boomerang-like cuticular structure. Pharynx with delicate dilatations. Intestine straight, with a marked anterior section. Scales spined, three-lobed, not overlapping, distributed in about 12 columns, 14 scales per column. Spines with a short lateral denticle. Scales and spines increase gradually in size in a posterior direction. Dorsal surface covered by scales from posterior end of cephalion (ca U3) to furca branches (ca U73). Furca branches slightly longer than adhesive tubes, lateral margins more or less straight, furcal indentation deeply U-shaped, adhesive tubes well-developed.

Molecular diagnosis

ITS2: 163 G, 172 T. 28S rRNA gene: 562 A, 693 C. Cytochrome c oxidase subunit I (codon ordinal numbers are followed by the corresponding span of nucleotide positions in parentheses): 58 (172‒174) ATA, 97 (289‒291) ACG, 122 (364‒366) GTA, 167 (499‒501) CGG, 174 (520‒522) GCG, 181 (541‒543) CTG, 184 (550‒552) TTA, 185 (553‒555) TCA, 193 (577‒579) ATC.

Reference molecules are shown in Supp. file 1: Figs S7 , S 12A, S 19 . 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 0.00‒4.33% in 18S, 5.88‒28.88% in ITS2, 0.51‒7.12% in 28S, and 8.44‒12.59% in COI. There are 1‒16 CBCs (except for Ch. (H). iratus sp. nov., Ch. (H). luxus sp. nov., and Ch. (H). slavicus sp. nov., where there are no CBCs) in the 18S rRNA molecule, 1‒3 CBCs in the ITS2 molecule (except for Ch. (H). gulosus sp. nov., Ch. (H). luxus sp. nov., and Ch. (H). slavicus sp. nov., where there are no CBCs), and 1‒18 CBCs in the first two domains of the 28S rRNA molecule (except for Ch. (H). luxus sp. nov. and Ch. (H). slavicus sp. nov., where there are no CBCs).

Etymology

The Latin adjective ‘ arcan · us, - a, - um ’ [m, f, n] (‘hidden’) refers to the morphological similarity of the new species with C. (H.) superbus sp. nov.

Material examined

Holotype SLOVAKIA • adult (photomicrographs, hologenophore); shallow section of the River Váh , Stankovany, Veľká Fatra Mts; 49°08′26.3″ N, 19°10′14.6″ E; CU-FNS- 29-09-20 /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 Fig. 38 .

Type material

A DNA sample of the holotype specimen STV 67 has been deposited in the Natural History Museum, Vajanského nábrežie 2, 810 06 Bratislava, Slovakia (ID Collection Code 01427571).

Type locality

Shallow section of the River Váh near the village of Stankovany, Veľká Fatra Mts, Slovakia 49°08′26.3″ N, 19°10′14.6″ E.

Gene sequences

The nuclear 18S and ITS 1-5.8S-ITS2-28S rDNA sequences as well as the mitochondrial COI sequence of the holotype specimen STV 67 have been deposited in GenBank under the following accession numbers: OM 421723 , OM 421699 , and OM 424078 , respectively.

Description

HABITUS. Chaetonotus (Hystricochaetonotus) arcanus sp. nov. is about 100 µm long and has a stocky, tenpin-shaped body, with a clearly defined head region, a narrowing neck, and a rather bulbous trunk ( Figs 37A, H , 38B ). Body width is ca 19.8 µm at U10, ca 15.6 µm at U50, and ca 17.8 µm at U60. The head is relatively wide (19.8 µm at U10), with a plate-like, rounded cephalion. The neck (ca U17–U27) is rather inconspicuously marked and smoothly continues to the trunk. The trunk gradually dilatates from about U35 to U61, where it reaches the maximum width that is only slightly narrower than the maximum width of the head. Then, the trunk gradually narrows towards U82, where curved margins of the furca branches begin to emerge. Dorsal sensory bristles were not observed. The furcal indentation is deeply U-shaped. The furca branches are set apart and diverge posteriorly. Well-developed adhesive tubes are approximately 9.4 µm long and straight ( Figs 37A, H , 38D ).

HEAD. The cephalion (U1) is rounded, clearly demarcated in the body outline ( Fig. 37A ). The epipleurae (U3‒U6) and hypopleurae (U6‒U10) are only inconspicuously marked. Pairs of cephalic ciliary tufts emerge laterally at U3 and U5. The mouth ring is oval, ca 5.2 µm in the largest diameter, and 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. The hypostomium (ca U5–U9) carries a small cuticular boomerang-like structure. The lateral sides of the hypostomium are lined by a relatively wide pair of basal body patches (from U3 to U8) ( Figs 37G , 38E ).

INTERNAL MORPHOLOGY. The pharynx extends from ca U5 to U34, is 28–29 µm long and 4.6–7.2 µm wide, sinuous, with inconspicuously marked anterior and posterior dilatations ( Figs 37H , 38B ). The cerebral ganglion appears as a mass surrounding the pharynx along its whole length. The intestine runs from U35 to U82. The adhesive gland (ca U82–U91) is placed right behind the terminal part of the intestine, forming a short dichotomy at the subtle furca base.

SCALES. Almost the entire body is covered by not overlapping three-lobed scales (U3–U73) that adhere to the basal cuticle layer along their whole perimeter. Scales are distributed in about 12 longitudinal rows, with 14 scales in the central row. Their size increases gradually from 3.0–4.5 ×1.9–2.2 µm to 4.9–5.6×4.1–4.3 µm in a posterior direction (Supp. file 1: Table S9). Central dorsal and dorsolateral longitudinal rows of scales begin at ca U3, while lateral rows start at ca U13. Two main types of scales could be recognized concerning the shape of the anterior lobe. (i) The head, neck, and upper-trunk scales (U3–U37) have a narrowly rounded anterior lobe and elongated posterior lobes, α = 156–163°, and β = 54–85°. The transition between the anterior and posterior lobes is indistinct and continuous, providing the scales with an A-shaped appearance ( Figs 37B, D , 38C ). (ii) The posterior trunk scales (U50–73) exhibit a broadly rounded anterior lobe and elongated posterior lobes, α = 153–167°, and β = 75–96°. The transition between the anterior and posterior lobes is marked unlike in the first scale type ( Fig. 37F ).

SPINES. All spines bear a distinct lateral denticle and gradually narrow towards their distal end. Keels start comparatively close to the anterior margin of scales. Spines are not straight but slightly curved. They do not differentiate into various types, only their length increases in a posterior direction ( Figs 37A, C, E, H , 38A–B , Supp. file 1: Table S9). More specifically, the length increase is rather inconspicuous and gradual from the head (3.1 µm) to the posterior neck region (5.8 µm). The most pronounced length change occurs at the beginning of the trunk (ca U50), where dorsal spines increase significantly from 5.8 µm to 15.2 µm ( Figs 37A, H , 38B ). The lateral denticle is comparatively distant from the spine apex, i.e., d -value ranges from 1.2–3.0 μm, which corresponds to a d ratio of 16.7‒23.3%.

VENTRAL CILIARY BANDS AND VENTRAL INTERCILIARY FIELD. Unfortunately, the ventral side was not observed in detail.

Figures

Fig. 37. Chaetonotus (Hystricochaetonotus) arcanus sp. nov. A. Dorsal overview showing the scale and spine pattern. B–C. Dorsal and lateral views of a head scale. D. Dorsal view of a neck scale. E–F. Lateral and dorsal views of a trunk scale. G. Detail showing the mouth region. H. Internal morphology. Scale bars = 30 µm.

Fig. 38. Chaetonotus (Hystricochaetonotus) arcanus sp. nov., holotype (CU-FNS-29-09-20/HO), differential interference contrast. A. Dorsal trunk scales and spines. Arrows mark lateral denticles. B. Body overview. C. Head scales. D. Adhesive tubes curved underneath trunk. E. Mouth and boomeranglike hypostomial plate. Scale bars: A, D = 10 µm; B = 30 µm; C, E = 2 µm.

Fig. 7. Secondary structure of the first two domains of the 5.8‒28S rRNA molecule of Chaetonotus (Hystricochaetonotus) mirabilis sp. nov. Diagnostic molecular autapomorphies are marked by red arrows. The reference 28S secondary structure map of Saccharomyces cerevisiae Meyen ex E.C.Hansen (inset) is from http://apollo.chemistry.gatech.edu/RibosomeGallery (Petrov et al. 2014).

Fig. 12. Chaetonotus (Hystricochaetonotus) mirabilis sp. nov., holotype (CU-FNS-11-09-19/HO), differential interference contrast. A‒B. Optical sections showing the general body organization. C. Lateral overview. Arrows in (B) and (C) mark the lateral denticle of spines. Scale bars = 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.