Halichaetoderma rivale, 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 : 318-322

Halichaetoderma rivale sp.nov.

( Figs 23A–H , 24A–F , 25A–F ; Supporting Information, Table S14)

ZooBank registration: urn:lsid:zoobank.org:act:4A957BC8-A0C2-49E3-88BB-AA1FB43B3A64 .

Morphological diagnosis: Body slender and about 100 µm long. Head narrower than trunk and separated from it by a distinct neck constriction. Cephalion clearly demarcated, epipleurae and hypopleurae conspicuously marked in head outline. Ocellar granules absent. Trunk widest at c. U59, gradually tappers towards furca base (U85). Mouth ventroterminal, without cuticular teeth. Pharynx with reinforcements. Intestine straight, with marked anterior section. Three pairs of dorsal sensory bristles. Scales partially overlapping, distributed in 16–19 longitudinal dorsal alternating columns, 37 scales per central column. Dorsal surface covered from posterior end of cephalion to furca appendages with four types of keeled scales: (i) head and neck scales narrowly oblong, (ii) trunk scales oblong, (iii) upper-furcal and furcal scales narrowly oblong with triangular posterior end, and (iv) a pair of double-keeled, cordiform sensory bristle scales. Hydrofoil scales run along outer side of each ciliary band from edge of hypopleurae to furca base. Ventral interciliary field scales narrowly oblong and keeled. Furca base short, furca indentation broadly helmet-like in shape, furca appendages considerably long, adhesive tubes well developed, slightly narrowing posteriorly. Furca base and furca appendages ventrally covered with a pair of big, very narrowly oblong and keeled scales as well as with small, oblong and keeled scales.

Molecular diagnosis: 18S rRNA gene: 223 C, 232 G, 327 T, 494 C, 662 T, 663 T, 664 T, 665 A, 679 A, 737 T, 938 G, 1064 C, 1066 A, 1251 T, 1262 C, 1733 A. 28S rRNA gene: 164 T, 178 T, 179 C, 253 G, 254 C, 275 A, 276 T, 279 T, 346 A, 444 A, 464 G, 467 G, 480 C, 557 C, 568 T, 578 G, 591 A, 592 A, 593 T, 618 A, 622 C, 629 G, 696 A, 722 G, 727 G, 770 C, 775 T, 798 T, 799 T, 809 C, 876 T, 879 T, 957 T, 1028 A, 1031 A. COI (codon ordinal numbers are followed by the corresponding span of nucleotide positions in parentheses): 28 (82–84) TTA, 65 (193–195) CTA, 69 (205–207) TTC, 96 (286–288) ATC, 119 (355–357) CTC, 121 (361–363) TCA, 155 (463–465) TCA, 175 (523–525) TCT, 192 (574–576) GCT. Reference molecules are shown in Supporting Information, Figs S4 , S 10 . All diagnostic molecular autapomorphies are marked by arrows.

Type locality: Vajspeterský potok creek, Rača , Bratislava, Podunajská rovina plain, Slovakia, 48°12ʹ12.8″N, 17°07ʹ46.9″E .

Material examined: The holotype (adult, hologenophore, CU-FNS-27-02-20/HO) is shown in Figure 25A–F . Photomicrographs of the holotype and the paratype (adult, CU-FNS-02-03-20/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 VP 30 has been deposited in the Natural History Museum, Vajanského nábrežie 2, 810 06 Bratislava, Slovakia (ID Collection Code 01427568) .

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 VP 30, have been deposited in GenBank under the following accession numbers: OQ358144, OQ358133, and OQ354336, respectively .

Etymology: The Latin adjective rival · is, - is, - e (m, f, n) (of brook, living in brook) refers to the habitat where the new species was discovered.

Description: Habitus. Halichaetoderma rivale is about 100 µm long and has a slender tenpin-shaped body. Body width is 11 µm at U10, 14 µm at U50, and 11 µm at U80. The neck (c. U15– U33) is clearly demarcated and smoothly continues to the trunk region. The trunk is bulbous and wider than the head, gradually dilating from about U34 to U59, where it reaches its maximum width. Then it gradually tapers towards U85, where the furca base starts to form. Three pairs of dorsal sensory bristles are inserted at U6, U25, and U83 ( Figs 23A, F , 25B, C ). The furcal indentation is broadly helmet-like in shape. The furca branches are set apart and bear well-developed, relatively short (8 µm), and only slightly diverging adhesive tubes ( Figs 23A, F , 24A, F , 25B–D ).

Head

The head is five-lobed. The cephalion is well marked in the head outline, extends from U1 to U5, and is 14 μm long in a lateral view, with a free posterior (dorsal) edge ( Figs 23F , 25B, D ). Both the epipleurae (c. U3–U5) and hypopleurae (c. U5–U11) are conspicuously marked and well recognizable in the lateral view. The epipleurae are slightly smaller (3.91 × 3.22 μm) than the hypopleurae (5.04–3.83 μm). Two pairs of cephalic ciliary tufts emerge between the cephalion and the epipleurae edge (c. U3), as well as between the epi- and the hypopleurae edge (c. U8) ( Figs 23A, B , 25A ). Ocellar granules are absent. The mouth ring is oval, approximately 3.8–4.4 μm in the largest diameter, and located ventroterminally at c. U1–U3 ( Figs 24A , 25D ). Cuticular teeth are not present. The hypostomium was not observed.

Internal morphology

The pharynx extends from c. U3 to U28, is 26–28 μm long and 5.2–9.3 μm wide, and has anteriorly placed reinforcements (2.6– 2.8 μm) ( Figs 24A , 25D ). 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 23F , 25B, D ). Paired adhesive glands (c. U85–U92) 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 16–19 longitudinal dorsal alternating columns, with usually 37 scales in the central column. Central dorsal longitudinal rows of scales begin at the level of the posterior edge of the cephalion (c. U5). Rows of scales run almost along the whole body length till U94. Lateral rows commence at U10 and terminate at the furca base at c. U93.

The most abundant type of dorsal body scales is oblong with a slight size increase in a posterior direction: head: 2.99– 4.23 × 0.88–1.32 μm, neck: 2.84–4.93 × 1.47–2.27 μm, and trunk: 2.46–6.67 × 1.21–3.66 μm. Not only the size, but also the shape of scales changes from narrowly oblong in the head and neck ( Figs 23C , 25A, C, E ) to oblong in the trunk ( Figs 23D , 25F ). The other type of scales is represented by a pair of cordiform, double-keeled, dorsal posterior trunk sensory bristle scales (1.10 × 1.27 μm) placed at c. U85 ( Fig. 23A, H ). The upper-furcal and furcal region is covered dorsally by narrowly oblong scales with a triangular posterior end ( Fig. 23G ). This fourth type of dorsal scales is about 2.81–3.77 × 1.14–1.61 μm in size.

A ventrolateral row of hydrofoil scales runs along the outer side of each ciliary band from the edge of the hypopleurae to the furca base ( Figs 23A , 24A , 25A, C ). They bear a well-developed lamellar expansion, being 1.55–3.87 × 1.12–2.71 μm in size ( Figs 23E , 24B, F ). Lamellar expansions arise from keels along their entire length ( Fig. 24B ).

Ventral ciliary bands and ventral interciliary field

Ventral ciliary bands commence at U9 and continue to U80. The interciliary field bears only one type of scales, which are very narrowly oblong, keeled, and 1.11–1.81 × 0.64–0.81 μm in size ( Fig. 24C, F ). They form up to seven longitudinal alternating rows. There are three pairs of terminal scales of the ventral interciliary field. The first pair is located centrally at U79–U82 and extends slightly beyond the contour of the furcal indentation. These scales are big (7.79–8.23 × 1.56–1.64 µm), very narrowly oblong and keeled ( Fig. 24E, F ). The second pair of scales is located laterally in relation to the first pair at U81–U82. They are smaller (3.53–3.87 × 0.70–0.84 µm), oblong and keeled ( Fig. 24F ). The third pair of scales is located laterally and posteriorly in relation to the first and second pairs, i.e. at the furca appendages. The scales of the third pair are similar in shape to the scales of the second pair but are slightly smaller.

Figures

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 10. SIMMAP reconstruction of the number of keels in chaetonotids based on 10 000 stochastic character maps simulated under the SYM model.Just a single keel on a dorsal scale most likely belonged to the ground pattern of the CDN clade. This ancestral condition was maintained also in the Halichaetoderma + Halichaetonotus and Heterolepidoderma s.l. clades.Relative proportions of character states were mapped onto the best scoring ML tree.

Figure 23. Halichaetoderma rivale sp. nov.A, dorsal overview showing the keel pattern. The body is about 100 µm long and has a slender tenpinshaped silhouette.The neck is clearly demarcated and smoothly continues to the trunk region.The trunk is bulbous and wider than the head. The furcal indentation is broadly helmet-like in shape.Note the three pairs of dorsal sensory bristles inserted at U6, U25, and U83. B, lateral view showing the cephalic pleurae.C, D, head, neck, and trunk dorsal scales are narrowly oblong to oblong. E, lateral view showing the welldeveloped lamellar expansion of hydrofoil scales. F, internal morphology. G, upper-furcal dorsal scales are narrowly oblong with a triangular posterior end.H, sensory bristle scale is double-keeled and cordiform.ceph, cephalion; das, differentiated anterior section of intestine; epi, epipleurae; hyp, hypopleurae; int, intestine; m, mouth; ov, ovum; ph, pharynx.Scale bar = 30 µm.

Figure 24. Halichaetoderma rivale sp. nov.A, ventral overview showing the general body organization.B, hydrofoil scales have well-developed lamellar expansions arising from keels on their entire length.C, interciliary field scales. D, ventral furca appendages scales are oblong and keeled. E, ventral central upper-furcal scales are very narrowly oblong and keeled.F, detail of the ventral posterior body region. There are three pairs of terminal scales of the ventral interciliary field. These scales are narrowly to very narrowly oblong and keeled.A ventrolateral row of hydrofoil scales runs along the outer side of each ciliary band. Scale bar = 30 µm.

Figure 25. Halichaetoderma rivale sp. nov.A, lateral view of the anterior body region, showing the cephalic pleurae, head scales, and lamellar expansion of hydrofoil scales.B–D, lateral overview showing the general body organization. Note the ventrolateral row of hydrofoil scales, which run along the outer side of the ciliary bands from the edge of the hypopleurae to the furca base. E, F, details showing the head and trunk scales. at, adhesive tube; ceph, cephalion; epi, epipleurae; das, differentiated anterior section of intestine; hfs, lamellar expansion of hydrofoil scales; hyp, hypopleurae; ov, ovum; phr, pharynx reinforcements; s, sensory bristle. Scale bars = 5 µm (E), 10 µm (F), 15 µm (A, C, D), 30 µm (B).