Halichaetoderma, 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 : 312-315

Halichaetoderma gen. nov.

ZooBank registration: urn:lsid:zoobank.org:act:CC06AC83-F400-4AB1-9A8E-9742D710EFB7 .

Diagnosis: Body tenpin-shaped and 88–120 µm long. Head distinctly five-lobed, cephalion, epipleurae, and hypopleurae well marked in the head outline. Two pairs of cephalic ciliary tufts. Mouth ring located ventroterminally, with well-developed rod-like reinforcements and none to three cuticular teeth. Pharynx with anterior and posterior dilatations, as well as with reinforcements. Dorsal, dorsolateral, lateral, and posterior ventral sides of body covered with comparatively small, oval to hexagonal, spineless scales with distinct keels. Scales located close to one another, mostly with overlapping edges. Number of dorsal alternating columns of scales distinctly higher than number of scales in central longitudinal column. Dorsal sensory bristles (setolae) absent or in up to three pairs, posteriormost pair on double-keeled, cordiform scales.Furca base clearly marked.Adhesive tubes thick and tapering toward distal end. Ventral intercilliary field almost naked or scaled. Dorsal upper-furcal region covered by comparatively large scales with keels. Ciliary bands enlarged anteriorly. Freshwater.

Molecular diagnosis: 18S rRNA gene: 649 T, 681 T, 684 A, 1263 C, 1492 A. 28S rRNA gene: 564 C, 804 C, 922 C, 950 C, 951 A, 952 G. COI (codon ordinal numbers are followed by the corresponding span of nucleotide positions in parentheses): 23 (67–69) TTA. All diagnostic molecular synapomorphies were mapped on the reference molecules of the type species, Halichaetoderma fluviatile sp. nov., and are marked by thin arrows.

Type species: Halichaetoderma fluviatile sp. nov. by original designation (Article 68.2 of ICZN 1999).

Etymology: A composite of the stem of the Ancient Greek adjective hálinos [ἅλῐνος (m), of salt], the thematic vowel · i-, the Greek noun khaítē [ΧΑίτη (f), flowing hair], and the Ancient Greek noun dérma [δέΡμᾰ (n), skin], referring to the morphological similarity to the genus Heterolepidoderma and close phylogenetic relatedness to the genus Halichaetonotus . Neuter gender.

Remarks: Morphological diagnostic characters of Halichaetoderma need to be placed in a phylogenetic context to recognize their apomorphic nature. Halichaetoderma does not have any obvious phenotypic autapomorphy with respect to Heterolepidoderma . This is caused by the homoplasious nature of cuticular features, as revealed by the present SIMMAP analyses ( Figs 7–10 ) and by the very broad morphological definition of the latter genus (for further details, see: the ‘Discussion’ section). However, fromthephylogeneticpointofview, themorphological features used to define Halichaetoderma are autapomorphies with respect to Halichaetonotus , the sister-group of the new genus according to the present multi-gene phylogenies and the results of statistical topology testing. Thus, much smaller scales, a higher number of longitudinal rows of scales, the absence of higher and lamella-like keels and spined scales on the dorsal side, as well as the freshwater habitat, are autapomorphies of Halichaetoderma with respect to Halichaetonotus , while they are homoplasious apomorphies with respect to Heterolepidoderma . Finally, it is important to mention that Halichaetoderma is unambiguously defined molecularly and can be distinguished from Halichaetonotus and Heterolepidoderma by a combination of 12 molecular autapomorphies in the nuclear rDNA cistron and the mitochondrial COI gene.

Figures

Figure 7. SIMMAP reconstruction of scale morphologies in chaetonotids based on 10 000 stochastic character maps simulated under the ER model. The present reconstruction analyses indicated that scales became oblong in the last common ancestor of the Halichaetoderma + Halichaetonotus clade as well as three times independently within the Heterolepidoderma s.l. clade. Relative proportions of character states were mapped onto the best scoring ML tree.

Figure 8. SIMMAP reconstruction of spine morphologies in chaetonotids based on 10 000 stochastic character maps simulated under the SYM model. Spines most likely belonged to the ground pattern of the CDN clade. They were lost in the last common ancestor of the Halichaetoderma + Halichaetonotus clade and at least two times convergently within the Heterolepidoderma s.l. clade. Relative proportions of character states were mapped onto the best scoring ML tree.

Figure 9. SIMMAP reconstruction of the absence/presence of a lateral denticle in chaetonotids based on 10 000 stochastic character maps simulated under the ARD model. Although the absence of lateral denticles is conditionally connected with the loss of spines, this side spine structure is not developed in any spined member of the Heterolepidoderma s.l. clade. Relative proportions of character states were mapped onto the best scoring ML tree.

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.