Halichaetoderma fluviatile, Križanová & Vďačný, 2024

Križanová, Františka Rataj & Vďačný, Peter, 2024, A Heterolepidoderma and Halichaetoderma gen. nov. (Gastrotricha: Chaetonotidae) riddle: integrative taxonomy and phylogeny of six new freshwater species from Central Europe, Zoological Journal of the Linnean Society 200 (2), pp. 283-335 : 315-318

Halichaetoderma fluviatile sp.nov.

( Figs 21A–J , 22A–F ; Supporting Information, Table S13)

ZooBank registration: urn:lsid:zoobank.org:act:5525D7D1-37AC-4FE5-9E02-A7C56BB05268 .

Morphological diagnosis: Body slender and about 90 µm long. Head narrower than trunk and separated from it by a comparatively long neck. Cephalion clearly demarcated, epipleurae and hypopleurae conspicuously marked in head outline. Ocellar granules absent. Trunk widest at c. U60, gradually tappers towards furca base (U84). Mouth ventroterminal with three cuticular teeth. Pharynx with reinforcements. Intestine straight, with marked anterior section.Two pairs of dorsal sensory bristles. Scales partially overlapping, distributed in 18 longitudinal dorsal alternating columns, 32 scales per column. All scales keeled. Dorsal surface covered from posterior end of cephalion with five scale types: (i) head scales very narrowly rectangular with rounded edges, (ii) upper-neck scales broadly rectangular with rounded edges, (iii) posterior neck and trunk scales roughly hexagonal to obdeltoid with rounded edges, (iv) posterior trunk scales broadly oblong with more or less truncated posterior end, and (v) a pair of double-keeled, cordiform sensory bristle scales. Furca base short, furcal indentation broadly V-shaped, furca appendages rather long, adhesive tubes well developed, thick and narrowing posteriorly with apart posterior tips. Ventral furca base with a pair of narrowly oblong and posteriorly truncated scales.

Molecular diagnosis: 18S rRNA gene: 215 T, 218 A, 220 C, 222 A, 223 T, 224 C, 236 A, 303 T, 499 T, 503 C, 543 C, 663 A, 664 T, 695 T, 706 T, 740 G, 750 A, 778 T, 1061 C, 1062 T, 1064 A, 1065 T, 1067 C, 1074 G, 1262 A, 1263 C, 1328 T, 1417 C, 1491 A, 1550 T, 1618 T. 28S rRNA gene: 293 A, 294 A, 295 C, 302 G, 305 T, 306 T, 328 T, 459 C, 491 C, 494 G, 496 T, 500 C, 524 G, 581 C, 582 A, 583 G, 630 C, 672 T, 673 G, 689 G, 701 T, 704 T, 713 A, 728 C, 753 T, 776 G, 778 C, 781 A, 797 C, 802 C, 948 T, 954 C, 960 T, 1021 T, 1061 T, 1088 G, 1090 C. COI (codon ordinal numbers are followed by the corresponding span of nucleotide positions in parentheses): 32 (94–96) AAT, 68 (202–204) TCT, 96 (286–288) CTC, 103 (307–309) AGA, 126 (376–378) GGT, 128 (382–384) TCG, 144 (430–432) TCG, 189 (565–567) CTG, 190 (568–570) GCG. Reference molecules are shown in Figs 1 , 4 . All diagnostic molecular autapomorphies are marked by thick arrows.

Type locality: A shallow section of the Váh River near the village of Stankovany, Veľká Fatra Mts., Slovakia 49°08ʹ26.3″N, 19°10ʹ14.6″E .

Material examined: The holotype (adult, hologenophore, CU-FNS-24-09-20/HO) is shown in Figure 22A–F . Photomicrographs of the holotype and the paratype (adult, hologenophore, CU-FNS-29-09-20/PA) are available at the Department of Zoology , Comenius University in Bratislava at https://fns.uniba.sk/en/gastrotricha /.

Type material: A DNA sample of the holotype specimen STV 66 has been deposited in the Natural History Museum, Vajanského nábrežie 2, 810 06 Bratislava, Slovakia (ID Collection Code 01427584) .

Gene sequences: The nuclear 18S and ITS1 - 5.8 S-ITS2-28S rDNA sequences, as well as the mitochondrial COI sequence of the holotype specimen STV 66, have been deposited in GenBank under the following accession numbers: OQ358142, OQ358131, and OQ354334, respectively .

Etymology: The Latin adjective fluviatil · is, - is, - e (m, f, n) (riverine) refers to the river Váh, where the new species was discovered.

Description: Habitus. Halichaetoderma fluviatile is about 90 µm long and has a slender tenpin-shaped body. Body width is 15 µm at U10, 18 µm at U50, and 25 µm at U60. The neck (c. U18–U30) is clearly demarcated (i.e. a distinct neck constriction is formed), comparatively long (c. 14 µm), and smoothly continues to the trunk region. The trunk is bulbous and slightly wider than the head, gradually dilating from about U40 to U59, where it reaches its maximum width. Then it gradually tapers towards U85, where the furca base starts to form. Two pairs of dorsal sensory bristles are inserted at U27 and U82 ( Fig. 21A ). The furcal indentation is broadly V-shaped. The furca branches are set apart and bear well-developed, relatively long (8 µm) and thick adhesive tubes that gradually taper to their posterior tips ( Figs 21A, F, G , 22E ).

Head

The head is five-lobed. The cephalion is well marked in the head outline, extends from U1 to U2, and is 9.5–10.4 μm wide in the dorsal view, with a free posterior (dorsal) edge ( Figs 21A , 22A, B ). Both the epipleurae (c. U2–U5) and hypopleurae (c. U5–U10) are conspicuously marked and recognizable in the head outline ( Fig. 21A, F, G ). Two pairs of cephalic ciliary tufts emerge between the cephalion and the epipleurae edge (c. U2) as well as between the epi- and the hypopleurae edge (c. U5). Ocellar granules are absent. The mouth ring is oval, with short but strong rod-like reinforcements, approximately 4.0–5.0 μm in the largest diameter, and located ventroterminally at c. U2–U4 ( Figs 21G , 22B ). Three cuticular teeth are clearly visible in the mouth opening, two are located laterally and one apically ( Fig. 21E ). The hypostomium (U3–U7) bears a plate that is weakly visible because very thin and partially submerged into the cuticle ( Figs 21E , 22A, B ).

Internal morphology

The pharynx extends from c. U3 to U28, is 22–24 μm long and 5.5–6.7 μm wide, and has anteriorly placed reinforcements (4.4 μm long) ( Fig. 22A ). It has two weakly marked anterior and posterior dilatations. The posterior dilatations are wider than the anterior ones. The pharynx smoothly continues through the pharyngeal–intestinal junction to the differentiated anterior section (U29–U32) of the intestine, which extends from U33 to U87 ( Figs 21F , 22A, D ). Paired adhesive glands (c. U85–U90) are placed right behind the terminal part of the intestine, forming a short dichotomy at the furca base.

Scales

Almost the entire body is covered by partially overlapping scales that adhere to the basal cuticle layer along most of their perimeter. All scales are keeled except for the posterior sensory bristle scales, which are double-keeled. Scales are distributed in 18 longitudinal dorsal alternating columns, with usually 32 scales in the central column. Central dorsal longitudinal rows of scales begin at the level of the posterior edge of the cephalion (c. U3). They run almost along the whole body length till U90. Lateral rows commence at U10 and terminate at the furca base at c. U92 ( Figs 21A , 22C, F ). Five types of dorsal body scales can be distinguished. (i) The head (U2–U15) bears very narrowly rectangular scales (2.50–3.42 × 0.80–1.31 μm) with rounded edges ( Figs 21B , 22C ). (ii) The upper neck region carries broadly rectangular scales (1.61–2.97 × 1.22–1.29 μm) with rounded edges ( Figs 21C , 22C ). (iii) The posterior neck and trunk regions are covered with roughly hexagonal to obdeltoid scales (2.40– 5.60 × 1.41–2.38 μm) with rounded edges ( Figs 21D , 22F ). (iv) The posterior part of the trunk region (U80–U85) bears broadly oblong scales with more or less truncated posterior end ( Fig. 21H ). (v) The dorsal trunk posterior sensory bristle scales are double-keeled, cordiform, and placed at U82 ( Fig. 21J ).

Ventral ciliary bands and ventral interciliary field

Ventral ciliary bands commence at U6 and terminate at U85. With respect to the arrangement of basal bodies of locomotory cilia, three regions can be distinguished: a clavate anterior region (U6–U9), a central roughly rectangular patch (U9–U15) situated between the two ciliary bands, and the main portion (U8–U85) of the ciliary band ( Figs 21E , 22B ). Basal bodies are irregularly placed in the whole anterior region and central patch as well as in the beginning (U8–U15) of the main portion of the ciliary bands. Locomotory cilia then become regularly arranged in more or less equidistantly spaced horizontal rows in the rest of the ciliary bands ( Fig. 22B ). Only the posterior part of the central patch is connected to the ventral ciliary bands via an isthmus on both sides.

The ventral interciliary field is almost naked but for one pair of scales, which is located on the ventral side of the furca base ( Fig. 21G ). These scales are big (12.99–14.75 × 4.99–6.43 μm), narrowly oblong, and posteriorly truncated ( Figs 21I , 22E ).

Figures

Figure 1. Secondary structure of the 18S rRNA molecule of Halichaetoderma fluviatile sp. nov.Thick arrows mark species-specific diagnostic molecular characters, while thin arrows denote diagnostic characters of the genus Halichaetoderma. Thus, there are five genus-specific molecular characters (thin arrows) localized in the double-stranded regions of helix 21es6c from the variable V4 region, helix 33 from the V6 region, and helix 41 from the V8 region. On the other hand, there are as many as 31 species-specific nucleotide characters (thick arrows) scattered over the whole 18S molecule both in the single- and double-stranded regions. Note the long-range tertiary contacts between the V2 and V4 regions (highlighted in grey). The helix number system follows Lee and Gutell (2012) and Petrov et al. (2014). The reference 18S secondary structure map of Saccharomyces cerevisiae Meyen ex E.C.Hansen (inset) is from http://apollo.chemistry.gatech.edu/ RibosomeGallery (Petrov et al. 2014).

Figure 4. Secondary structure of the 5.8S rRNA molecule and a part of the 28S rRNA molecule (covering the entire domain I and partially domains 0 and II) of Halichaetoderma fluviatile sp. nov. Thick arrows mark species-specific diagnostic molecular characters, while thin arrows denote diagnostic characters of the genus Halichaetoderma. The helix number system follows Petrov et al. (2014). The reference secondary structure map of the 5.8S and 28S rRNA molecules of Tetrahymena thermophila Nanney and McCoy, 1976 (inset) is from http://apollo. chemistry.gatech.edu/RibosomeGallery (Petrov et al.2014).

Figure 21. Halichaetoderma fluviatile sp. nov. A, dorsal overview showing the keel pattern. B, C, head and neck dorsal scales are very narrowly to broadly rectangular. D, H, dorsal scales are roughly hexagonal to obdeltoid with rounded edges in the upper and central trunk region (D), while broadly oblong with a truncated rear end in the posterior trunk region (H). E, mouth and under-mouth region. Grey dots represent basal bodies of cephalic sensory cilia, while grey areas the ventral ciliary bands. F, internal morphology.G, ventral overview. I, upper-furcal ventral central scales are narrowly oblong with a truncated posterior end. J, posterior sensory bristle scale is double-keeled and cordiform.car, clavate anterior region; ceph, cephalion; crp, central rectangular patch; ct, cuticular teeth; das, differentiated anterior section of intestine; epi, epipleurae; hs, hypostomium; hyp, hypopleurae; int, intestine; ph, pharynx; sc, sensory cilia; vcb, ventral ciliary bands. Scale bar = 30 µm.

Figure 22. Halichaetoderma fluviatile sp. nov. A, B, details of the anterior body region, showing the pharynx reinforcements and ventral ciliary bands. Note that basal bodies are very narrowly spaced in the whole anterior region and central patch as well as in the beginning (U8–U15) of the main portion of the ciliary bands. Locomotory cilia then become regularly arranged in more or less equidistantly spaced horizontal rows in the rest of the ciliary bands. C, F, dorsal view showing the head and trunk scale patterns. D, optical section of the trunk showing the internal morphology. E, ventral view of the posterior body region.Arrowheads mark the narrowly oblong scales with a truncated posterior end.ceph, cephalion; crp, central rectangular patch; das, differentiated anterior section of intestine; ov, ovum; phr, pharynx reinforcements; pij, pharyngeal‒intestinal junction; vcb, ventral ciliary bands.Scale bars = 5 µm (F), 10 µm (C, D, E), 15 µm (A, B).