Halichaetoderma hexagonale, 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 : 325-330

Halichaetoderma hexagonale sp.nov.

( Figs 29A, B , 30A–I , 31A–E ; Supporting Information, Table S16)

ZooBank registration: urn:lsid:zoobank.org:act:FCB4169B-BDCE-4184-A44C-A048C871786A .

Morphological diagnosis: Body stocky and about 120 µm long. Head narrower than trunk and separated from it by a distinct neck constriction. Cephalion clearly demarcated, epipleurae and hypopleurae marked in head outline. Ocellar granules absent. Mouth ventroterminal, without cuticular teeth. Pharynx with reinforcements. Intestine straight, with marked anterior section. Scales partially overlapping, distributed in 23 longitudinal dorsal alternating columns, 36 scales per central column. Four types of keeled scales on dorsal and lateral body sides: (i) head scales narrowly oblong and posteriorly truncated, (ii) head dorsolateral, neck, and upper-trunk scales oblong, (iii) trunk dorsal scales hexagonal with more or less rounded edges, and (iv) lateral posterior trunk scales oblong with a truncated posterior end. Ventral interciliary field scales narrowly oblong and keeled. Furca base short, furcal indentation narrowly V-shaped, adhesive tubes almost straight. Furca base covered dorsally with (i) a pair of big, very broadly cordiform, keeled scales and (ii) small, oblong, keeled scales, and ventrally with a pair of big, oblong and keeled scales. Furca appendages covered with small, oblong and keeled scales.

Molecular diagnosis: 18S rRNA gene: 121 G, 130 T, 132 A, 297 C, 482 G, 491 T, 495 T, 497 A, 503 C, 507 C, 545 A, 591 T, 687 T, 739 C, 799 T, 824 G, 860 C, 870 C, 1063 A, 1067 A, 1068 T, 1262 C, 1317 C, 1320 G, 1324 G, 1327 T, 1329 A, 1360 T, 1361 T, 1373 A, 1382 A, 1383 A, 1528 G, 1559 C, 1580 G, 1626 T. 28S rRNA gene: 325 G, 444 A, 500 C, 527 G, 570 T, 620 T, 631 C, 687 A, 694 T, 696 G, 807 A, 809 T, 810 G. COI (codon ordinal numbers are followed by the corresponding span of nucleotide positions in parentheses): 65 (193–195) CTG, 96 (286–288) ATT, 126 (376–378) GGT, 139 (415–417) ATT, 175 (523–525) GAA. Reference molecules are shown in Supporting Information, Figs S6 , S 12 . 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-25-02-20/HO) is shown in Figure 31A–E . Photomicrographs of the holotype 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 27 has been deposited in the Natural History Museum, Vajanského nábrežie 2, 810 06 Bratislava, Slovakia (ID Collection Code 01427583) .

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 27, have been deposited in GenBank under the following accession numbers: OQ358146, OQ358135, and OQ354338, respectively .

Etymology: Composite of the Greek numeral hex (ἕξ, six), the thematic vowel · a -, and the Latinised adjective gonal · is, - is, - e (m, f, n) (gonial, angled), referring to the hexagonal scales covering the main portion of the trunk region.

Description: Habitus. Halichaetoderma hexagonale is about 120 µm long and has a stocky, tenpin-shaped body. The head–neck transition is well-marked. The trunk is bulbous and wider than the head, gradually dilating from about U30 to U60, where it reaches its maximum width. Then it gradually tapers towards U85, where the furca base starts to form. Dorsal sensory bristles were not observed. The furcal indentation is very narrowly V-shaped. The furca appendages are set apart and bear well-developed, 12 µm long, and almost straight adhesive tubes ( Fig. 29A, B ).

Head

The head is five-lobed. The cephalion is conspicuous, extends from U1 to U3, and is 11 μm long, with a free posterior (dorsal) edge ( Fig. 29A, B , 31A ). The epipleurae are situated at c. U3– U9 and are about 6.49–7.03 × 5.59–6.23 μm in size, while the hypopleurae are at c. U6–U11 and approximately 5.14–8.84 μm long. Two pairs of cephalic ciliary tufts emerge laterally between the cephalion and the epipleurae edge (c. U3), as well as between the epi- and the hypopleurae edge (c. U6) ( Figs 29A, B , 31D ). Ocellar granules are absent. The mouth ring is oval, approximately 6.3 μm in the largest diameter, and located ventroterminally at U1–U3 ( Figs 29B , 31B ). Cuticular teeth are not present. The hypostomium was not observed.

Internal morphology

The pharynx extends from c. U3 to U28, is 34 μm long and 7.7– 12.1 μm wide. It has two weakly marked anterior and posterior dilatations. The posterior dilatations are wider than the anterior ones. There is a pair of anterior pharyngeal reinforcements, which are visible only at higher magnifications ( Fig. 31A ). 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. Paired adhesive glands (c. U85–U92) are 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 partially overlapping scales that adhere to the basal cuticle layer along most or all of their perimeter. All scales are keeled. Scales are distributed in 23 longitudinal dorsal alternating columns, with usually 36 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 U94. Ventrolateral rows commence at U10 and terminate at the furca appendages at c. U93. There are about seven to eight ventrolateral rows of scales on each body side ( Fig. 29A, B ).

The head carries small (2.30–4.49 × 1.44–1.71 μm), narrowly oblong, and posteriorly truncated scales, which commence dorsally at c. U3 and extend up to U23 ( Figs. 30A , 31D ). The dorsolateral head region as well as the neck and upper-trunk bear small (2.86–3.15 × 1.27–1.56 μm) and oblong scales ( Figs. 30B, C , 31B ). The main trunk portion is covered dorsally with hexagonal scales with slightly rounded edges, which start at c. U35 and continue till c. U87 ( Figs. 30D , 31C ). Posterior dorsolateral trunk region (from c. U88) carries very narrowly oblong (2.54–4.60 × 1.48–1.56 μm) scales with a truncated posterior end ( Fig. 30H ). These scales are arranged in several transversal rows, each with three or four scales on the right and left body sides. The transition between the central posteriormost trunk section and the furca base (c. U92) bears relatively small (2.92 × 1.85 μm) and oval scales ( Fig. 30E ). The furca base is dorsally covered with a pair of big (5.21– 5.34 × 3.64–3.84 µm) and very broadly cordiform scales ( Figs 30I , 31E ). Central and lateral parts of the furca base as well as the furca appendages carry both dorsally and ventrally oval scales, resembling those on the central posteriormost trunk region ( Fig. 30F ).

Ventral ciliary bands and ventral interciliary field

Ventral ciliary bands commence at U7 and continue to U85. The interciliary field bears only one type of scales, which are very narrowly oblong, keeled, and 2.90–3.88 × 1.25–1.79 μm in size ( Fig. 30G ). They form up to seven longitudinal alternating rows. There is a pair of big oblong terminal scales of the ventral interciliary field. These scales are located centrally at U86–U90 ( Fig. 29B ).

Figures

Figure 6. Phylogenetic tree of chaetonotids based on 18S, 28S, and COI sequences.Only the highlighted part of the inset phylogram is shown. The upper half of the phylogram is presented in Figure 5. Bootstrap values for maximum likelihood conducted in IQ-TREE and posterior probabilities for Bayesian inference conducted in MrBayes were mapped onto the best scoring maximum likelihood tree. Fully statistically supported nodes are marked with black solid circles.GenBank accession numbers can be found in Supporting Information, Table S1. The scale bar denotes four substitutions per one hundred nucleotide positions.

Figure 12. Heterolepidoderma kolickae sp. nov. A, ventral overview showing the scale and spine pattern. B, ventrolateral scales are keeled and cordiform. C, trunk lateral scales are spined and narrowly lanceolate. D, upper-furcal ventral scales are relatively big, oblong, and keeled. E, mouth region. F, interciliary field scales are minute, bowl-shaped, and spineless. G, internal morphology. ceph, cephalion; ct, cuticular teeth; das, differentiated anterior section of intestine; epi, epipleurae; hs, hypostomium; hyp, hypopleurae; int, intestine; ph, pharynx. Scale bar = 30 µm.

Figure 29. Halichaetoderma hexagonale sp. nov.A, dorsal overview showing the keel pattern.B, ventral overview showing the keel and ciliary pattern.ceph, cephalion; epi, epipleurae; hyp, hypopleurae. Scale bar = 30 µm.

Figure 30. Halichaetoderma hexagonale sp. nov. A, head dorsal scales are narrowly oblong with a posterior truncated end. B, C, head dorsolateral, neck and upper-trunk dorsal scales are oblong to narrowly oblong. D, trunk dorsal scales are hexagonal with more or less rounded edges.E, F, trunk dorsal posterior central scale and furca lateral scales are oblong. G, interciliary field scales are narrowly oblong and keeled. H, trunk dorsolateral posterior scales are oblong with a truncated posterior end. I, furca base dorsal scale is very broadly cordiform.

Figure 31. Halichaetoderma hexagonale sp. nov. A, detail showing the pharynx reinforcements. Note the well-developed cephalion with a free posterior edge. B, ventrolateral view of the head, showing the mouth and the narrowly oblong scales with a posterior truncated end. The mouth ring is oval, approximately 6.3 μm in the largest diameter, and located ventroterminally at U1–U3. C, trunk dorsal scales are hexagonal with more or less rounded edges.They are keeled, partially overlap, and adhere to the basal cuticle layer along most or all of their perimeter. This dominant scale type covers the main trunk portion from approximately U35 till almost U87. D, dorsal view of the head, showing the epipleurae and the narrowly oblong scales with a posterior truncated end. E, dorsal view of the posterior trunk region, showing hexagonal scales and a big very broadly cordiform scale (arrowhead). ceph, cephalion; epi, epipleurae; m, mouth; phr, pharynx reinforcements.Scale bars = 5 µm (E), 10 µm (A, C, D), 15 µm (B).