Halichaetoderma aureum, 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 : 323-325

Halichaetoderma aureum sp.nov.

( Figs 26A, B , 27A–H , 28A–E ; Supporting Information, Table S15)

ZooBank registration: urn:lsid:zoobank.org:act:70C22347-0A53-4236-BDC0-7A443FB534E9 .

Morphological diagnosis: Body stocky and about 105 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 20 longitudinal dorsal alternating columns, 30 scales per central column. Four types of keeled scales on dorsal and lateral body sides: (i) head scales elongated oval, (ii) neck and trunk scales oblong with truncated posterior end, (iii) dorsolateral trunk scales roughly hexagonal with rounded edges, and (iv) lateral scales broadly tongue-shaped. Ventral interciliary field scales narrowly oblong and keeled. Furca base short, furcal indentation U-shaped, adhesive tubes long and almost straight. Upper-furcal region dorsally covered with oblong, anteriorly bluntly acute, and posteriorly truncated scales. Upper-furcal region ventrally covered with a pair of big, keeled, claviform, and posteriorly truncated scales.

Molecular diagnosis: 18S rRNA gene: 121 G, 130 T, 132 A, 298 C, 483 G, 492 T, 496 C, 498 A, 499 C, 504 C, 508 C, 546 A, 592 T, 688 T, 740 C, 800 T, 825 G, 861 C, 871 C, 1064 A, 1068 A, 1069 T, 1210 G, 1263 C, 1318 C, 1321 G, 1325 G, 1328 T, 1330 A, 1361 T, 1362 T, 1374 A, 1383 A, 1384 A, 1506 T, 1528 G, 1559 C, 1580 G, 1626 T. 28S rRNA gene: 325 G, 444 A, 513 T, 527 G, 532 T, 570 T, 628 A, 631 C, 656 T, 688 A, 695 T, 697 A, 722 A, 777 A, 808 G, 810 T, 811 G. COI (codon ordinal numbers are followed by the corresponding span of nucleotide positions in parentheses): 47 (139–141) GCA, 65 (193–195) TTA, 75 (223–225) GGC, 96 (286–288) ATT, 97 (289–291) ATT, 126 (376–378) GGT, 139 (415–417) ATT. Reference molecules are shown in Supporting Information, Figs S5 , S 11 . All diagnostic molecular autapomorphies are marked by arrows.

Type locality: Zlaté Piesky Lake , municipal recreation area, Ružinov , Bratislava, Podunajská rovina plain, 48°11ʹ17.1″N, 17°11ʹ24.3″E .

Material examined: The holotype (adult) was destroyed during DNA extraction. Two paratypes (adults, CU-FNS-14-02-20/ PA-1 and CU-FNS-17-02-20/PA-2) are shown in Figure 28A–E . Photomicrographs of paratypes 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 ZPvs 21 has been deposited in the Natural History Museum, Vajanského nábrežie 2, 810 06 Bratislava, Slovakia (ID Collection Code 01427589) .

Gene sequences: The nuclear 18S and ITS1-5.8S-ITS2-28S rDNA sequences, as well as the mitochondrial COI sequence of the holotype specimen ZPvs 21, have been deposited in GenBank under the following accession numbers: OQ358145, OQ358134, and OQ354337, respectively.

Etymology: The Latin adjective aure · us, - a, - um (m, f, n) (golden) refers to the type locality, Zlaté piesky (Arēnae aureae, golden sands), as well as to the golden appearance of the intestine content caused by ingested algae.

Description: Habitus. Halichaetoderma aureum is about 105 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 U27 to U56, where it reaches its maximum width. Then it gradually tapers towards U80, where the furca base starts to form. Dorsal sensory bristles were not observed. The furcal indentation is U-shaped. The furca branches are set apart and bear well-developed, relatively long (8.5–10.7 μm), and almost parallel adhesive tubes ( Figs 26A, B , 27H ).

Head

The head is five-lobed. The cephalion is conspicuous, extends from U1 to U3, and is about 9.4 μm long, with a free posterior (dorsal) edge ( Figs 26A, B , 27H , 28C ). The epipleurae and hypopleurae are situated approximately at U3–U6 and U7– U11, respectively. The hypopleurae are more developed than the epipleurae. 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 26A, B , 28B ). Ocellar granules are absent. The mouth ring is oval, approximately 5.5 μm in the largest diameter, and located ventroterminally at U1–U4 ( Figs 26B , 27H , 28C ). Cuticular teeth are not present. The hypostomium was not observed.

Internal morphology

The pharynx extends from c. U3 to U28, is 33 μm long and 6.5–11.6 μm wide, and has anteriorly placed reinforcements. It has two weakly marked anterior and posterior dilatations. The posterior dilatations are wider than the anterior ones ( Figs 26B , 27H , 28A, C ). 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 27H , 28A ). 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 20 longitudinal dorsal alternating columns, with usually 30 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 ( Fig. 26A ). Ventrolateral rows commence at U10 and terminate at the furca base at c. U93. There are about four to five ventrolateral rows of scales on each body side ( Fig. 26B ).

The head carries elongated, considerably narrow scales (2.84–3.60 × 1.01–1.63 μm) from c. U3 up to U23 ( Figs 27A , 28B ). The neck and the trunk bear oblong, posteriorly truncated scales that increase in size in a posterior direction (neck: 3.32– 4.63 × 1.49–1.68 μm, trunk: 4.30–5.50 × 2.00–2.89 μm) ( Fig. 27B ). Dorsolateral sides of the trunk carry roughly hexagonal scales with rounded edges ( Figs 27D , 28E ), which start at the edge of the cephalion and continue till c. U94. Trunk lateral rows consist of broadly tongue-shaped scales with a truncated posterior end ( Fig. 27C ). The furca base (U90) is dorsally covered with a pair of big (4.6–4.9 × 1.4–1.7 μm), oblong, anteriorly bluntly acute and posteriorly truncated scales ( Fig. 27F ).

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 that are very narrowly oblong, keeled, and 3.05–3.42 × 0.91–1.14 μm in size. They form up to six longitudinal and regularly arranged rows ( Figs 26B , 27E , 28D ). The central part of the upper-furcal region is ventrally covered with a pair of big (5.41–6.58 × 2.28–3.14 µm), keeled, claviform, and posteriorly truncated scales ( Fig. 27G ).

Figures

Figure 5. Phylogenetic tree of chaetonotids based on 18S, 28S, and COI sequences.Only the highlighted part of the inset phylogram is shown. The lower half of the phylogram is presented in Figure 6. 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.Sequences in bold were obtained during this study.The scale bar denotes four substitutions per one hundred nucleotide positions.

Figure 11. Heterolepidoderma kolickae sp. nov. A, dorsal overview showing the spine and keel pattern. B, head dorsal scales are spined and three-lobed with a subtle anterior lobe and a well-marked transition between the anterior lobe and the posterior lobes. C, upper-neck dorsal scales resemble head scales but the transition between the anterior lobe and posterior lobes is indistinct. D, trunk dorsal scales are keeled and oblong with a truncated posterior end. E, trunk dorsolateral scales are spined and narrowly lanceolate. ceph, cephalion; epi, epiplaeurae.Scale bar = 30 µm.

Figure 26. Halichaetoderma aureum sp. nov. A, dorsal overview showing the keel pattern. B, ventral overview showing the keel and ciliary pattern. The body is about 105 µm long and has a stocky tenpin-shaped silhouette.The head is five-lobed.The head-neck transition is wellmarked.The trunk is bulbous and wider than the head, gradually dilating from about U27 to U56, where it reaches its maximum width. Then it gradually tapers towards U80, where the furca base starts to form.Almost the entire body is covered by keeled scales that are distributed in 20 longitudinal dorsal alternating columns, with usually 30 scales in the central column. Central dorsal longitudinal rows of scales begin at the level of the posterior edge of the cephalion.They run almost along the whole body length till U94. Ventrolateral rows commence at U10 and terminate at the furca base.There are about four to five ventrolateral rows of scales on each body side.ceph, cephalion; epi, epipleurae; hyp, hypopleurae.Scale bar = 30 μm.

Figure 27. Halichaetoderma aureum sp. nov. A, head dorsal scales are narrowly oblong. B, C, trunk dorsal and lateral scales are broadly tongueshaped with a truncated posterior end. D, trunk dorsolateral scales are roughly hexagonal with rounded edges. E, interciliary field scales are narrowly oblong.F, upper-furcal dorsal scales are oblong with a bluntly acute anterior end and a truncated posterior end. G, upper-furcal ventral scales are relatively big, claviform, and posteriorly truncated.H, internal morphology. ceph, cephalion; das, differentiated anterior section of intestine; int, intestine; m, mouth; ph, pharynx.

Figure 28. Halichaetoderma aureum sp. nov. A, optical section showing the internal morphology. The pharynx extends from c. U3 to U28, is 33 μm long and 6.5–11.6 μm wide, and smoothly continues through the pharyngeal-intestinal junction to the differentiated anterior section (U29–U32) of the intestine, which extends from U33 to U87. Note the golden appearance of the intestine content caused by ingested algae. B, surface view showing the head narrowly oblong scales. C, optical section of the head, showing the pharynx and the cephalion with a free posterior edge.The pharynx has two dilatations, the posterior dilatation is wider than the anterior one. D, interciliary field scales are narrowly oblong and keeled. Note that the interciliary field scales are regularly arranged, forming up to six longitudinal rows. E, dorsal trunk scales are oblong with a truncated posterior end. ceph, cephalion; int, intestine; m, mouth; ph, pharynx; sc, sensoric cilia. Scale bars = 5 µm (D), 10 µm (B, E), 20 µm (A, C).