Chaetonotus (Hystricochaetonotus) avarus, 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 : 41-46

Chaetonotus (Hystricochaetonotus) avarus sp. nov.

urn:lsid:zoobank.org:act:7B146EA8-7E66-46A3-A4C2-8707A9C9EB56

Figs 23‒27 ; Supp. file 1: Table S5

Morphological diagnosis

Body elongated and about 130 µm long. Head slightly wider than neck, separated from trunk by an inconspicuous neck constriction. Cephalion, epipleurae, and hypopleurae clearly demarcated. Trunk comparatively narrow, not broader than head, widest at ca U62, narrowest at ca U87. Mouth ventral, three cuticular teeth. Hypostomium bears a pentagonal cuticular structure with two protuberances. Pharynx without dilatations. Intestine straight, with a marked anterior section. Scales slightly overlapping, distributed in 14 columns, 30–32 scales per column. Dorsal surface covered with: (i) head scales small, with a subtle, well-delimited anterior lobe and comparatively long and diverging posterior lobes; (ii) neck scales boomerang-like, with a broadly rounded anterior lobe, elongated posterior lobes; (iii) dorsolateral neck scales with a well-delimited, broadly rounded anterior lobe and distinct transition between anterior and posterior lobes; (iv) main trunk scales with a rounded anterior end, posterior lobes comparatively long, narrowly rounded distally, and diverging; (v) posterior trunk scales with a conspicuously prolonged, tongue-shaped anterior lobe and relatively short posterior lobes tapering distally; and (vi) furca base scales with a narrower and more elongated anterior lobe and short posterior lobes very narrowly rounded or almost acute distally. Spines slightly increasing in length from head to posterior trunk region, gradually tapering to become hair-like terminally, without denticles. Ventrolateral spines shorter than dorsal ones. Ventral side carries two broad, anteriorly connected (U9–U27) ciliary bands. Interciliary field covered by (i) minute, tongue-like, keeled scales; (ii) small, roughly rectangular, keeled scales; (iii) oblong scales with a short spine; and (iv) a pair of big, elongated oval, keeled scales. Furca branches slightly longer than adhesive tubes, lateral margins more or less straight, furcal indentation deeply V-shaped, adhesive tubes short. Dorsal side of furca branches covered with elongated oval, keeled scales and ventral side of furca branches covered with (i) two pairs of three-lobed, spined scales, having an elongated, tongue-shaped anterior lobe and comparatively short, narrowly rounded to acute posterior lobes and (ii) cordiform, short-spined scales.

Molecular diagnosis

18S rRNA gene: 268 A, 1739 G. 28S rRNA gene: 128 C, 136 G, 447 G, 691 A, 737 C, 764 T, 765 A, 767 A. Cytochrome c oxidase subunit I (codon ordinal numbers are followed by the corresponding span of nucleotide positions in parentheses): 84 (250‒252) CTA, 91 (271‒273) CTA, 99 (295‒297) CTA, 102 (304‒306) TCG, 120 (358‒360) TCC, 154 (460‒462) ACT, 192 (574‒576) GGA, 213 (637‒639) GGG.

Reference molecules are shown in Supp. file 1: Figs S3 and S 15 . 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 0.38‒4.17% in 18S, 2.49‒9.44% in 28S, and 8.44‒13.15% in COI. There are 1‒14 CBCs in the 18S rRNA molecule and 2‒18 CBCs in the first two domains of the 28S rRNA molecule.

Etymology

The Latin adjective ‘ avar · us, - a, - um ’ [m, f, n] (‘avaricious, covetous, greedy’) refers to the three cuticular teeth of the new species.

Material examined

Holotype SLOVAKIA • adult (the specimen was destroyed during DNA extraction); Vajspeterský potok creek , Rača , Bratislava, Podunajská rovina plain; 48°12′12.8″ N, 17°07′46.9″ E.

Paratypes SLOVAKIA • 2 adults (photomicrographs); same collection data as for holotype; CU-FNS- 25-02-20 / PA-1, CU-FNS- 26-02-20 /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 Figs 25–27 .

Type material

A DNA sample of the holotype specimen VP 32 has been deposited in the Natural History Museum, Vajanského nábrežie 2, 810 06 Bratislava, Slovakia (ID Collection Code 01427566).

Type locality

Vajspeterský potok creek, Rača, Bratislava, Podunajská rovina plain, Slovakia, 48°12′12.8″ N, 17°07′46.9″ E.

Gene sequences

The nuclear 18S and 28S rDNA sequences as well as the mitochondrial COI sequence of the holotype specimen VP 32 have been deposited in GenBank under the following accession numbers: OM 421713 , OM 421689 , and OM 424068 , respectively.

Description

HABITUS. Chaetonotus (Hystricochaetonotus) avarus sp. nov. is about 130 µm long and has a slender elongated body, with a head region slightly broader than the neck and trunk ( Figs 23A, K , 24A , 25A‒C ). Body width is22.0–23.0 µm at U10, 18.0–19.0 µm at U50,and 20.0–21.0 µm at U60.The head is relatively wide (11–15 µm at U6), with a plate-like cephalion. Epi- and hypopleurae are clearly demarcated in the head outline ( Figs 23A, K , 24A , 25A‒C , 26A ). The neck (ca U14–U27) is only inconspicuously marked and smoothly continues to the trunk. In comparison with the head, the trunk is comparatively slender, gradually dilatating from about U35 to U55, where it reaches the maximum width. Then, the trunk gradually narrows towards U87, where more or less straight margins of the furca branches start to form. Dorsal sensory bristles arise from the cuticle in two pairs at U28 and U78 ( Fig. 23A, L ). The furcal indentation is deeply V-shaped. Well-developed adhesive tubes are approximately 6–12 µm long, they are straight and short ( Figs 23A, K , 24A, H , 27E ).

HEAD. The cephalion (U1) is clearly demarcated in the body outline, surrounds the mouth ventrally like a bib, and is distinctly flattened and lenticular in the dorsal view ( Figs 23A, K , 24B , 25A‒C , 26A, C ). Two pairs of cephalic ciliary tufts emerge at U3 and U7. In addition, inverted V-shaped streaks of basal bodies extend from the level of the mouth to the posterior level of the hypostomium ( Figs 24A , 26C ). The mouth ring is oval, approximately 5.5 µm in the largest diameter, and located subapically 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 ( Fig. 26C ). Three inner cuticular teeth are clearly visible, two are located laterally and one apically ( Fig. 25A ). The hypostomium (ca U5– U9) is rich in structures, i.e., it is composed of a pentagonal cuticular plate bearing two protuberances ( Figs 24B , 26C ).

INTERNAL MORPHOLOGY. The pharynx extends from ca U5 to U29, is almost 35 µm long and 7.4-9.1 µm wide, sinuous, and without dilatations ( Figs 25A , 26A ). 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. The adhesive gland is placed right behind the terminal part of the intestine (ca U85–U87), forming a short dichotomy at the subtle furca base ( Figs 23K–L , 25C ).

SCALES. Almost the entire body is covered by slightly overlapping, mostly three-lobed scales that adhere to the basal cuticle layer along either all or most of their perimeter. Scales are distributed in 14 longitudinal rows, with 30–32 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 U14 at the posterior edge of the hypopleurae. They run almost along the whole body length (till U92) ( Figs 23A , 27B ). Ventrolateral rows commence at U13 due to the highly developed hypopleurae and terminate in the furcal region at ca U85. Ventral rows commence rather far away from the anterior body end at U29, because of the strongly developed and apically completely fused bands of locomotory cilia ( Figs 24A , 25C ). Seven main types of dorsal scales were recognized. (i) Head scales (U2–U17) are small (3.1–4.1×1.6–3.0 µm) and have a subtle, well-delimited anterior lobe, comparatively long and diverging posterior lobes, α = 156–173°, β = 55–87° ( Figs 23B–C , 27A ). (ii) Dorsal neck scales (U17–U27) are boomerang-like with a broadly rounded anterior lobe, elongated posterior lobes, and an indistinct and continuous transition between the anterior and posterior lobes, α = 166–173°, β = 55–87° ( Fig. 23F ). (iii) Dorsolateral neck scales possess a well-delimited, broadly rounded anterior lobe, i.e., the transition between anterior and posterior lobes is distinct, α = ~171°, β = ~70° ( Fig. 23D ). (iv) The main trunk region bears distinctly bigger scales (5.3–9.3 ×3.1–6.1 µm) whose anterior end is rounded, posterior lobes are comparatively long, narrowly rounded, and diverging, the transition between anterior and posterior lobes is well marked, α = 146–176°, β = 86–102° ( Fig. 23G ). (v) Posterior trunk scales are 8.1–8.9 ×4.1–4.8 µm in size, their anterior lobe is conspicuously prolonged becoming tongue-shaped, while their posterior lobes are relatively short and taper towards the distal end, the transition between anterior and posterior lobes is slightly marked, α = 140–170°, β = 53–68° ( Figs 23H , 27C ). (vi) Furca base scales are 4.5–8.4×2.8–3.6 µm in size, their anterior lobe is slightly narrower and more elongated, posterior lobes are short and very narrowly rounded or almost acute distally, the lobe transition is less distinct than in the previous type, α = 149–163°, β = 69–78° ( Fig. 23J ). (vii) Furca branches carry elongated oval, keeled scales measuring 5.7–6.0 ×1.7–1.8 µm ( Fig. 23I ). These scales reach up to the apical margin of the adhesive tubes.

SPINES. Dorsal, dorsolateral, lateral, and ventrolateral scales bear simple, slightly curved spines that gradually narrow towards the distal end ( Figs 23A, E, K–L , 25A–B ). The spine emerges rather close to the anterior margin of scales. Spines do not differentiate into various types, only their length slightly increases from 3.5 µm to 10.9 µm in a posterior direction (Supp. file 1: Table S4 ). A lateral denticle is not developed.

VENTRAL CILIARY BANDS AND VENTRAL INTERCILIARY FIELD. Ventral ciliary bands commence almost right behind the hypostomium. Their anterior region is conspicuously broad, causing both bands to be completely fused from U9 to U27 ( Figs 24A , 26B–C ). However, the middle distal part of the merged ciliary bands more or less follows the course of the posterior margin of the hypostomium, i.e., it is concave from ca U10 to U13. Basal bodies are densely packed and do not form regular rows in the anterior fused region of the ciliary bands. The anterior field of basal bodies splits into two distinct ventral bands about at U27. Locomotory cilia then become regularly arranged in more or less equidistantly spaced horizontal rows. Each row typically consists of seven narrowly spaced basal bodies from ca U28 to U83. Rows of basal bodies are lined up with horizontal rows of ventral scales. This regular pattern changes about at U84 where distances between horizontal ciliary rows gradually decrease. Also, the number of basal bodies per row gradually decreases and ciliary bands terminate at U88 ( Figs 24A , 25C ). The ciliary bands are accompanied by two ventrolateral rows of three-lobed scales, with an indistinct and continuous transition between the anterior and posterior lobes ( Fig. 24A ). The inner row starts at U22 and consists of smaller scales (2.4–4.3 ×1.2–2.8 µm) ( Fig. 24E ), while the outer row begins at ca U40 and is built up from bigger scales (4.8–5.3 ×2.2–3.6 µm) ( Fig. 24D ). The ventral interciliary field bears four types of horizontally distributed scales: (i) minute (1.5–1.8 ×1.3–1.8 µm), tongue-like, double-edged scales with a minute keel (U28–U32); (ii) small (1.9–2.3 ×1.6–2.1 µm), roughly rectangular, double-edged scales with a keel (U33–U83) ( Figs 24C , 27D ); (iii) slightly bigger (3.9–4.4 ×1.0–1.5 µm), oblong, doubleedged scales with a short spine (U83–U87); and (iv) a pair of big (6.1–7.8× 2.0–3.2 µm), elongated oval scales with a keel ( Figs 24H , 27E ). All ventral interciliary field scales are anteriorly merged into the cuticle. The furca branches carry two types of scales (U88–U95): (i) two pairs of three-lobed, spined scales (3.2–5.0 ×1.1–2.0 µm), with an elongated, tongue-shaped anterior lobe and comparatively short, narrowly rounded to acute posterior lobes ( Fig. 24F ) and (ii) smaller (0.7–3.2 ×0.4–1.8 µm), cordiform, short-spined scales ( Figs 24G–H , 27E ).

Figures

Fig. 23. Chaetonotus (Hystricochaetonotus) avarus sp. nov. A. Dorsal overview showing the scale and spine pattern. B–C. Head scales. D. Neck dorsolateral scale. E. Lateral view of a trunk spined scale. F. Neck central scales. G. Trunk scales. H. Posterior trunk scales. I. Dorsal furca branches scales. J. Dorsal furca base scales. K. Internal morphology. L. Lateral overview. Scale bars = 30 µm.

Fig. 24. Chaetonotus (Hystricochaetonotus) avarus sp. nov. A. Ventral overview showing the scale and ciliary pattern. B. Mouth region. C. Detail showing the transition region of two scale types in the interciliary field. D–E. Ventrolateral three-lobed scales. F. Ventral furcal three-lobed scales. G. Furcal cordiform scales. H. Detail of the terminal ventral region. Scale bars: A = 30 µm; H = 10 µm.

Fig. 25. Chaetonotus (Hystricochaetonotus) avarus sp. nov., paratype (CU-FNS-26-02-20/PA-2), differential interference contrast. A–B. Optical section showing the internal morphology. C. Ventral overview. Scale bars = 30 µm.

Fig. 26. Chaetonotus (Hystricochaetonotus) avarus sp. nov., paratype (CU-FNS-26-02-20/PA-2), differential interference contrast. A. Anterior body region. B. Anteriorly merged ventral ciliary bands. C. Detail of the mouth region. Hypostomium bears a pentagonal cuticular plate with two central protuberances. Scale bars = 10 µm.

Fig. 27. Chaetonotus (Hystricochaetonotus) avarus sp. nov., paratype (CU-FNS-26-02-20/PA-2), differential interference contrast. A. Dorsal head scales. B. Overview showing the dorsal scale and spine pattern. C. Posterior trunk scales. D. Scales in the ventral interciliary field. E. Detail of the ventral posterior region. Scale bars: A, C–D = 5 µm; B = 30 µm; E = 15 µm.

Fig. 3. Terminology and measurement scheme for scales and spines. A. Frontal view. B. Lateral view.

Fig. 15. Chaetonotus (Hystricochaetonotus)superbus sp. nov.A, H. Dorsal and lateral overviews showing the scale and spine pattern. B. Head scale. C. Neck scale. D. Trunk scale. E. Ventral scales. F. Upperfurcal dorsal scales. G. Lateral view of a trunk spined scale. I. Lateral view showing the connection of a transversal band to the base of a scale. J. Lateral overview showing the internal morphology. K. Lateral view of the anterior body region. Scale bars = 30 µm.