Xerobiotus arenosum, Vincenzi & Cesari & Kaczmarek & Roszkowska & Mioduchowska & Rebecchi & Guideưi, 2024

Vincenzi, Joel, Cesari, Michele, Kaczmarek, Łukasz, Roszkowska, Milena, Mioduchowska, Monika, Rebecchi, Lorena & Guideưi, Yevgen Kiosya and Roberto, 2024, The xerophilic genera Xerobiotus and Pseudohexapodibius (Macrobiotidae; Tardigrada): biodiversity, biogeography and phylogeny, Zoological Journal of the Linnean Society 200 (1), pp. 111-141 : 133-136

Xerobiotus arenosum sp.nov.

( Figs 12 , 13 ; measurements and statistics are in Tables 2 and 3 ; Supporting Information, Table S3 )

ZooBank: urn:lsid:zoobank.org:act:22C9B72E-46AF-4775-983A-04C40E04486D

Type locality: Marina di Bibbona , Livorno, Italy, dunes (43°14ʹ20.13N, 10°31ʹ36.4548E), moss on sandy dunes, coll. R. Guideưi .

Material examined: Thirty-six adults and six eggs.

Type repositories: The holotype (4411s3c), six paratypes and three eggs deposited in Bertolani’s Collection of UNIMORE ( Italy) ; two paratypes and one egg deposited in the Museum of Natural History of Verona ( Italy) . Four paratypes and two eggs were mounted on stubs for SEM observation .

Etymology: The name of the species means sandy coastal hill (dune) in Latin, for the type of environment in which the species was found.

Description: Body whitish; orange eye-spots present. Very small scaưered pores (~0.5 µm in diameter, visible only with SEM) in the dorsolateral cuticle. Very small single granules, distributed almost regularly, present on the entire cuticle (visible with SEM). Legs of the first pair clearly smaller than those of the second and third pairs. Area of the leg cuticle surrounding the claws with a swelling (forming a furbelow-like structure; Figs 12H ) covered with microdigitations (visible with SEM; Fig. 13C–E ).

Buccopharyngeal apparatus of Macrobiotus type, anteroventral mouth with 10 small peribuccal lamellae ( Fig. 13B ). Oral cavity armature of the maculatus type (visible with LM; Fig. 12B–E ), with three dorsal transversal crests (in contact), two ventral lateral crests and medioventral crest split in two granules, a thin anterior band of teeth (visible with SEM; Fig. 13B ). Typically shaped stylet furcae, with oval condyles supported by short branches provided with rounded apophyses. In the pharynx ( Fig. 12B ): large and triangular pharyngeal apophyses overlapping the first macroplacoid; two rod-shaped macroplacoids, length sequence 2 <1 (in lateral view), and evident drop-shaped microplacoid. In frontal view, the first macroplacoid is in the shape of a drop with a medial slight constriction, the second rectangular with rounded corners and with a small terminal slight constriction.

Claws I–III of Xerobiotus type, with thin base (without lunules) ( Fig. 12F ) and with a small cuticular plate connected to the base of the claw (visible with SEM; Fig. 13C ). Claw IV with a longer common tract with a proximal constriction (without lunules), small and short claw branches ( Fig. 12G ), a large cuticular plate connected to the base of the claw (visible with SEM; Fig. 13E ) and a cuticular thickening in the shape of a ring with irregular margins present under the claw base (and connected with it) ( Fig. 12G ). Main branches of all claws with small accessory points.

Spherical eggs laid freely ( Figs 12I , 13F ). Surface between processes of hufelandi type, i.e. covered with a reticulum formed by a mesh of small, densely distributed pores, uniform in size and evenly distributed. Processes of the hufelandi type, small, and a relatively small and convex terminal disc ( Figs 12I , 13G ). The edge of the terminal discs indented, the indentations appearing like a tapered tip (with LM; Fig. 12J ) and like elongated processes ornamented with granules (with SEM; Fig. 13G ). An egg laid by a female of this species was collected.

Reproduction: Parthenogenetic; only females were found. The tetraploid karyotype has 24 chromosomes (4 n) ( Fig. 12K ).

Molecular characterization: Six haplotypes for cox1 (from six specimens; p-distance 0.1%–0.5%), one haplotype for ITS2 (from one specimen), one haplotypes for 18S (from one specimen), and one haplotype for 28S (from one specimen) genes (GenBank accession numbers in Supporting Information, Table S4 ; p-distances in Supporting Information, Table S8). The more similar sequences of X. arenosum belong: for cox1, to X. gretae , Xerobiotus n. sp. 1 and Xerobiotus n. sp. 2, with p-distances of 2.0%–2.9%, 1.6%–10.6% and 2.0%–3.1%, respectively; for ITS2, to X. litus and Xerobiotus n. sp. 1, with p-distances of 2.0% and 2.0%–2.9%, respectively (Supporting Information, Table S8).

Differential diagnosis: Xerobiotus arenosum differs from all other Xerobiotus species by the presence of a tetraploid karyotype with 24 chromosomes (4 n) and by an egg with the shortest processes with respect to all the other species of the genus. Moreover, X. arenosum differs from: X. reductus by the shape of claws IV (with a proximal restriction of the common tract and a cuticular thickening under the claw base), and the shape of distal discs of egg processes (convex and larger in relative diameter) and the higher number of processes on the circumference; X. litus by the presence of a small plate at the base of claws (visible with SEM) and the higher number of processes on the circumference; X. euxinus by the semilunar thickening at the base of the claws I–III (not visible with LM), the absence of lunules in claws IV, absence of males, and convex terminal discs of egg processes; X. gretae by shorter branches of claws I–III, the presence of a reticulated egg surface (with small, rounded meshes) between the processes, and convex terminal discs of egg processes; X. naginae by the shape of claw IV (longer common tract with a proximal constriction) and the higher number of processes on the circumference; and X. xerophilus by the presence of a cuticular thickening at the base of claw IV, and the shape of egg processes (without a funnel-like depression).

Figures

Figure 12. Xerobiotus arenosum. A, animal in toto. B, buccopharyngeal apparatus.C, buccal armature (dorsal view). D, buccal armature (ventral view). E, buccal armature (ventral view).F, claw I. G, hind legs with claw IV (arrowheads indicate thickenings under claw base). H, legs II and III, with the swellings of the furbelow-like structure (arrowheads). I, egg surface and processes.J, terminal discs of egg processes. K, chromosomes (24 univalents, stained with Orcein). A, C–H, PhC; B, I–K, DIC. Scale bars: 20 µm in A, B, H–J; 10 µm in C–G.

Figure 13. Xerobiotus arenosum. A, animal in toto. B, mouth opening. C, legs I, with furbelow-like structure; small plates at the base of the claws (arrows).D, microdigitations of the furbelow-like structure. E, claws IV, with ventral plate (arrows). F, egg. G, egg surface and processes. A–G, SEM. Scale bars:40 µm in A; 3 µm in B, E; 5 µm in C, G; 1 µm in D; 30 µm in F.