Chaetonotus (Hystricochaetonotus) luxus, 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 : 49-54

Chaetonotus (Hystricochaetonotus) luxus sp. nov.

urn:lsid:zoobank.org:act:DD747096-A6E0-43F7-B462-349692AAFB65

Figs 28‒30 ; Supp. file 1: Table S6

Morphological diagnosis

Body stocky and around 87–112 µm long. Head wider than neck, separated from trunk by a more or less distinct neck constriction. Cephalion, epipleurae, and hypopleurae clearly demarcated. Trunk widest at ca U55, gradually tapers towards furca base (U83). Mouth ventral, no cuticular teeth. Pharynx without dilatations. Intestine straight, with a marked anterior section. Scales spined, three-lobed, not overlapping, distributed in a minimum of 10 columns, 15 scales per column. Spines with a very short lateral denticle. Scales and spines increase gradually in size in a posterior direction. Dorsal surface covered with: (i) head and upper-neck scales with a small, narrowly rounded anterior lobe and elongated posterior lobes; (ii) lower-neck and anterior trunk scales with a slightly larger anterior lobe and elongated posterior lobes; (iii) posterior trunk scales with an elongated anterior lobe and posterior lobes about as long as anterior lobe; and (iv) posteriormost trunk scales with a tongue-shaped anterior lobe and short posterior lobes. Furca branches shorter than adhesive tubes, lateral margins more or less straight, furcal indentation U-shaped, adhesive tubes long and diverging posteriorly. Dorsal and lateral sides of furca branches covered with narrowly trapezoid, keeled scales with rounded edges and ventral side of furca base and branches covered with (i) a pair of oblong, keeled scales and (ii) three-lobed, spined scales, with an elongated anterior lobe and comparatively short, narrowly rounded posterior lobes, transition between anterior and posterior lobes continuous and indistinct.

Molecular diagnosis

ITS2: 36 T, 61 C, 71 T, 72 G, 126 T, 135 A, 164 C, 171 C, 175 A, 176 A, 177 A. Cytochrome c oxidase subunit I: 69 (205‒207) CTG, 92 (274‒276) TTG, 109 (325‒327) GCG, 156 (466‒468) GCT, 168 (502‒504) CTA, 174 (520‒522) GCC, 176 (526‒528) TTA.

Reference molecules are shown inSupp.file 1: Figs S4 , S 10 ,S 16 .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, 10.16‒34.22% in ITS2, 0.26‒9.61% in 28S, and 5.63‒12.70% in COI. There are 1‒16 CBCs (except for Ch. (H). slavicus sp. nov., Ch. (H). iratus sp. nov., and Ch. (H). arcanus sp. nov. where there are no CBCs) in the 18S rRNA molecule, 1‒4 CBCs (except for Ch. (H). slavicus sp. nov., Ch. (H). gulosus sp. nov., and Ch. (H). arcanus sp. nov. where there are no CBCs) in the ITS2 molecule, and 2‒18 CBCs in the first two domains of the 28S rRNA gene (except for Ch. (H). slavicus sp. nov., Ch. (H). iratus sp. nov., and Ch. (H). arcanus sp. nov. where there are no CBCs).

Etymology

The Latin noun ‘ luxus ’ [m] (‘extravagance, luxury’) refers to the ‘extravagant’ appearance of the new species. The species group name is treated as a noun in the nominative singular standing in apposition to the generic name (Article 11.9.1.2 of the ICZN 1999).

Material examined

Holotype SLOVAKIA • adult (the specimen was destroyed during DNA extraction); Zlaté Piesky lake , municipal recreation area, Ružinov , Bratislava, Podunajská rovina plain; 48°11′17.1″ N, 17°11′24.3″ E.

Paratypes SLOVAKIA • 2 adults (photomicrographs); same collection data as for the holotype; CU-FNS- 17-02-20 / PA-1, CU-FNS- 18-02-20 /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 Fig. 30 .

Type material

A DNA sample of the holotype specimen ZPvs 20 has been deposited in the Natural History Museum, Vajanského nábrežie 2, 810 06 Bratislava, Slovakia (ID Collection Code 01427593).

Type locality

Zlaté Piesky lake, municipal recreation area, Ružinov, Bratislava, Podunajská rovina plain, 48°11′17.1″ N, 17°11′24.3″ 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 ZPvs 20 have been deposited in GenBank under the following accession numbers: OM 421714 , OM 421690 , and OM 424069 , respectively.

Description

HABITUS. Chaetonotus (Hystricochaetonotus) luxus sp. nov. is about 87–112 µm long and has a stocky body that is more or less tenpin-shaped, with a clearly defined head region, a narrowing neck, and a rather bulbous trunk ( Figs 28A , 29A ). Body height in lateral view is 13.5–14.0 µm at U10, 16.5–16.8 µm at U50, and 18.0–18.2 µm at U60. The head is relatively wide, with a plate-like cephalion. The neck (ca U15–U28) smoothly continues to the trunk, which is significantly wider than the head, gradually dilates from about U37 to U60 where it reaches the maximum width. Then it gradually tapers towards U81, 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. Well-developed adhesive tubes are approximately 11–13 µm long, slightly curved in lateral view, and run almost in parallel ( Figs 28A, H , 29A, D–E , 30A ).

HEAD. The head is five-lobed. The cephalion (U1–U2) is rounded, clearly demarcated in the body outline, and has a free posterior (dorsal) edge. The epipleurae are formed approximately at U3–U5 while the hypopleurae at U6–U13. The latter structures are clearly demarcated in the head outline ( Fig. 28A, H ). Two pairs of cephalic ciliary tufts emerge laterally between the cephalion and the epipleurae edge (ca U3) as well as between the epi- and the hypopleurae edge (U6). The hypostomium (ca U4–U6) is free of structures. The mouth ring is oval, 3.2–6.0 µm in the largest diameter, located subterminally at U2–U6. 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 ( Figs 29A , 30A–B ). Inner cuticular teeth are not present.

INTERNAL MORPHOLOGY. The pharynx extends from ca U5 to U34, is 28–32 µm long and 5.9–8.3 µm wide, sinuous, and has no dilatations ( Figs 29D , 30A–B ). The intestine runs from U35 to U82 and has a separate, well-differentiated anterior section (U35–U38). A pair of protonephridia runs from ca U45 to U60. Transversal bands connected to the base of dorsal scales are well recognizable ( Figs 29D , 30A ). The adhesive gland (ca U80–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 that adhere to the basal cuticle layer along their whole perimeter. Scales are distributed in a minimum of 10 longitudinal rows, with 15 scales in the central row. Their size increases gradually in a posterior direction. Central dorsal and dorsolateral longitudinal rows of scales begin at the level of the anterior edge of the epipleurae (ca U5), while lateral rows start at the posterior end of the hypopleurae (ca U13). Ventral rows are hardly visible due to the thick, elongated dorsal spines (for further explanation, see below) and long locomotory cilia. Five main types of scales could be recognized with respect to the shape of the anterior lobe. (i) The head and upper-neck scales (U5–U25) are 3.2–5.4 ×2.1–5.2 µm in size and have a small, narrowly rounded anterior lobe and elongated posterior lobes, α = 153–162°, and β = 75–89° ( Figs 28B , 30C ). (ii) The lower-neck and anterior trunk scales (U29–42) are 2.6–3.5× 3.7–5.8 µm in size and exhibit a slightly larger anterior lobe and elongated posterior lobes, α = 167–179°, and β = 72–83° ( Fig. 28C ). Both types share an indistinct and continuous transition between anterior and posterior lobes. (iii) Posterior trunk scales (U45–80) are 3.8–7.4× 2.4–4.2 µm in size and display an elongated anterior lobe, posterior lobes about as long as the anterior lobe and distinctly set off from it, α = 153–170°, β = 75–96° ( Figs 28D , 30D ). (iv) Posteriormost trunk scales are 4.3–5.8 ×2.4–3.0 µm in size and have a tongue-shaped anterior lobe and short dilating posterior lobes, α = 144–164°, β = 83–85°. The transition between anterior and posterior lobes is marked ( Fig. 28F ). (v) The dorsal and lateral sides of the furca branches bear three pairs of narrowly trapezoid scales with rounded edges. These scales are keeled and have a size of 4.5– 5.6×1.9–2.7 µm ( Figs 28G–H , 30G ).

SPINES. Spines do not differentiate into various types, only their length increases from 3.4 µm to 13.5 µm in a posterior direction (Supp. file 1: Table S6 ). However, the width of individual spines decreases from 0.7 µm at the base to 0.2 µm at the tip and, hence, spines do not become hair-like terminally. The spine base is situated near the anterior margin of scales. All spines are distinctly curved and bear an inconspicuous lateral denticle ( Figs 28A, E, H , 29A, D , 30D–E ). The lateral denticle emerges comparatively near the spine apex, i.e., the d -value ranges only from 0.9‒1.5 μm, which corresponds to a d ratio of 5.8‒7.4%.

VENTRAL CILIARY BANDS AND VENTRAL INTERCILIARY FIELD. Unfortunately, the ventral side could not be observed in detail due to the thickness and stiffness of dorsal spines, which precluded turning over and squeezing the worms. Moreover, the ventral locomotory cilia also hampered detailed observations of the ventral side. Despite these difficulties, the following observations were conducted. The longitudinal ciliary bands begin at ca U10 and run backward to ca U84. The ciliary bands are accompanied from U13 to U75 by two ventrolateral rows of small (1.4–4.4 ×1.4–2.8 µm in size), three-lobed scales (α = 160–167°, β = 79–85°) equipped with relatively long spines (6.6–9.6 µm) ( Fig. 29A–B ). The furcal region (U81–U91) carries two types of scales ( Figs 29A, C, E , 30F ): (i) a pair of oblong, keeled scales being 6.8–7.0×2.8–3.0 µm in size and (ii) three-lobed, spined scales (1.9–3.8 ×1.5–2.3 µm), with an elongated anterior lobe and comparatively short, narrowly rounded posterior lobes, the transition between the anterior and posterior lobes is continuous and indistinct, α = 158–172°, and β = 73–84°.

Figures

Fig. 28. Chaetonotus (Hystricochaetonotus) luxus sp. nov. A, H. Dorsal and lateral overviews showing the scale and spine pattern. B. Head scale. C. Neck and upper-trunk scale. D–E. Frontal and lateral views of a trunk spined scale. F. Posterior trunk scale. G. Dorsal furcal scale. Scale bars = 30 µm.

Fig. 29. Chaetonotus (Hystricochaetonotus) luxus sp. nov. A. Ventral overview showing the scale and ciliary pattern. B. Ventrolateral scale. C. Ventral furcal scale. D. Internal morphology. E. Detail of the ventral terminal body region. Scale bars: A = 30 µm; D = 20 µm; E = 10 µm.

Fig. 30. Chaetonotus (Hystricochaetonotus) luxus sp. nov., differential interference contrast. A, E. Paratype (CU-FNS-17-02-20/PA-1). B–D, F–G. Paratype (CU-FNS-17-02-20/PA-2). A. Lateral overview showing the internal morphology. B. Anterior body region. C. Head scales. D. Dorsolateral trunk scales. E. Lateral view of dorsal trunk spined scales carrying a minute lateral denticle (arrows). F. Ventral furcal scales. G. Dorsolateral oblong scales lining the furca sides. White stars in (A) and (B) mark the cerebral ganglion. Scale bars: A = 15 µm; B, E = 10 µm; C–D, F–G = 5 µm.

Fig. 4. Histograms showing intra- and interspecies p-distances of four molecular markers studied in nine new gastrotrich species. A. Barcoding analysis of 18S rRNA gene sequences. The black arrow marks the overlap between intra- and interspecies p-distances. B. Barcoding analysis of ITS2 molecules. Sequences are identical within species (red arrow), while interspecies p-distances range from 5.88% to as much as 34.76%. C. Barcoding analysis of the first two domains of 28S rRNA gene. Although there is no overlap between intra- and interspecies p-distances, no distinct barcoding gap is recognizable. D. Barcoding analysis of COI showing the distinct gap between intra- and interspecies p-distances (red arrow).

Fig. 10. Chaetonotus (Hystricochaetonotus) mirabilis sp. nov. A‒B. Dorsal and lateral overviews showing the scale and spine pattern. C. Detail showing the cuticular teeth protruding from the mouth ring. D. Elongated dorsal spines with a membrane. E. Terminal dorsal spines. F. Internal morphology of a strongly squeezed specimen (body shape hence not representative). G. Ventral overview. Scale bars: A = 30 µm; F–G = 20 µm.

Fig. 16. Chaetonotus (Hystricochaetonotus) superbus sp. nov., differential interference contrast. A. Holotype (CU-FNS-15-06-20/HO), lateral overview showing the scale and spine pattern. B. Paratype (CU-FNS-22-06-20/PA), optical section showing the internal morphology in lateral view. Black stars in (A) mark the epipleurae and hypopleurae. White star in (B) marks the cerebral ganglion. Arrows mark the lateral denticle of spines. Scale bars = 30 µm.