New Species of the Genus Richtersius Pilato & Binda, 1989 (Tardigrada: Eutardigrada: Richtersiusidae) from Uzbekistan

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der post with Tajikistan, by Glib Mazepa. The sample was examined for tardigrades using the protocol developed by DASTYCH (1980). Together with the new species, representatives of three other tardigrade genera were found: Echiniscus C. A.S. Schultze, 1840;Hypsibius Ehrenberg, 1848;and Isohypsibius Thulin, 1928. Except for the new species, no other macrobiotid taxa that lay ornamented eggs have been recorded. In order to perform the taxonomic analysis, animals and eggs were extracted from the sample and prepared for a morphological analysis in PCM (for details, see 'Materials examined' below). The specimens were mounted on microscope slides in a small drop of Faure's medium (composition: 30 g of gum arabic, 50 ml of distilled water, 20 ml of glycerol, 150 mg of chloral hydrate; mixed without heating) and secured with a cover slip. The slides were examined under a Leica DMLB light microscope with phase contrast (PCM), associated with a digital camera. All the figures were assembled in Corel Photo-Paint X8. For structures where the focus could not be achieved in a single photograph, a stack of 2 to 10 images was taken with an equidistance of approximately 0.2 ìm and were manually assembled into a single deep focus image.

Morphometrics and morphological nomenclature
All measurements are given in micrometres (ìm). Structures were measured only if their orientation was suitable. The body length was measured from the anterior to the posterior extremity of the body, excluding the hind legs. The types of bucco-pharyngeal apparatuses follow PILATO & BINDA (2010). The terminology used to describe the armature of the oral cavity and the morphology of the eggshells follows GUIDETTI et al. (2016) andSTEC et al. (2020a,b). The macroplacoid length sequence is given according to KACZMAREK et al. (2014). The length of the buccal tube and level of the insertion point of the stylet support were measured according to PILATO (1981). The pt index, which is the ratio of the length of a given structure to the length of the buccal tube, was calculated and expressed as a percentage (PILATO 1981). The width of the buccal tube was measured as the external and internal diameter at the level of the stylet support insertion point. The heights of the claw branches were measured from the base of the claw (i.e. excluding the lunulae) to the top of the branch, including the accessory points. The claw common tract index (cct), which is the proportion of the height of the common tract of the claw (measured from the base of the claw to the separation point between the primary and secondary branch) to the total height of the claw, was calculated and expressed as a percentage (GUIDETTI et al. 2016). The description of the cuticular bars on the legs follows KIOSYA et al. (2021). The distance between the egg processes was measured as the shortest distance between the base edges of the two closest processes. Following STEC et al. (2020a), we measured six additional characteristics: cuticular pore density (PD -the number of pores per 2500 ìm 2 counted within a rectangle in the dorsal cuticle between legs III and IV), pore size (PS -measured as the largest diameter; ten pores per measured specimen), number of teeth in the external and internal lunules III (ExtT and IntT, respectively), and number of teeth in the anterior and posterior lunules IV (AntT and PosT, respectively). The morphometric data were handled using the 'Parachela' ver. 1.8 template, which is available from the Tardigrada Register (MICHALCZYK & KACZMAREK 2013) and is provided as Supplementary Material (SM.01). The tardigrade taxonomy follows BERTOLANI et al. (2014), STEC et al. (2020b and GUIDETTI et al. (2021).

Description of the new species
Animals (measurements and statistics are included in Table 1) Body is yellow; all specimens became transparent after the fixation in Faure's medium (Fig. 1A). Eyes were present in five of the 11 specimens mounted in Faure's medium. Body and leg cuticle is without granulation in all life stages and with pores present only in hatchlings (Figs 1A-B, 2A-B). Hatchlings are similar in appearance to adults, except for a smaller body size and roundish pores (0.8-1.6 ìm in diameter) with smooth edges, clearly visible under PCM, scattered randomly throughout the body cuticle, with a mean pore density of 31 ± 5 per 2500 ìm 2 of the dorsal cuticle (Fig. 1B).
Claws are slender, primary branches with distinct accessory points ( Fig. 2A-B) and an internal system of septa as described for Richtersius coronifer s.l. by LISI et al. (2020). The claw common tract index is always below 50%, meaning that the basal portion of the claw is shorter than half the total length of the primary branch. An evident stalk system connecting the claws to the lunulae is visible under PCM ( Fig. 2A-B). The stalk system consists of a thin laminar stalk connecting the claw to the lunula and two posterior lateral extensions, whose distal tips under PCM appear to be connected to the stalk where it contacts the lunula ( Fig. 2A-B). Lunulae are large, with a crown of long, numerous and densely arranged spikes (1.9-3.4 ìm long) ( Fig. 2A-B). All the lunulae are more or less trapezoidal ( Fig. 2A-B). Double muscle attachments in legs I-III and horseshoe structures in legs IV are visible in PCM, whereas cuticular bars are absent ( Fig. 2A-B).
Mouth is antero-ventral. The oral cavity armature is faintly visible under PCM, with only the second band of teeth visible mainly in the larger specimens, and the third band of teeth being sometimes visible in the lateral projection of the anterior portion of the buccal apparatus ( Fig. 3B-C). Under PCM, the second band of teeth is visible as several irregular rows of densely packed and faint dark dots (Fig. 3B-C). The discontinuous third band of teeth is situated between the second band of teeth and the opening of the buccal tube, and is divided into a dorsal and a ventral portion, both in the form of a single large tooth resembling a beak (Fig. 3A). The buccal apparatus is of the Richtersius type (Fig. 3A). The oral cavity is followed by a system of large apophyses that form a buccal crown (Fig. 3A-C). Anteriorly, the system consists of dorso-lateral and ventro-lateral triangular apophyses (Fig. 3A, C). The dorsal and ventral apophyses are composed of anteriorly positioned large cuticular hooks, followed by longitudinal crests (Fig. 3B). The hook in the ventral apophyses is smaller than the dorsal hook (Fig. 3B). The wall of the buccal tube exhib- its a variable thickness, but the internal diameter of the buccal tube is almost uniformly narrow (Fig. 3A). From the mouth opening to the stylet support insertion point, the thickness of the buccal tube wall increases only slightly, while below this point the evident posterior thickness is clearly visible (Fig. 3A). The pharynx is spherical, with bilobed apophyses, three anterior cuticular spikes (typically only two are visible in any given plane) and two granular macroplacoids (2 < 1). The first and second macroplacoids have a faint constriction positioned centrally and subterminally, respectively ( Fig. 3D-E). Table 2)

Eggs (measurements and statistics are included in
The eggs are large, oval, light yellow and laid freely ( Fig. 4A-I). Under PCM, the surface between the processes is smooth, but with evident refracting dots or pores and a crown of thickenings distributed around the bases of the processes (Fig. 4A, D, G). The egg surface between the processes is also covered by irregularly distributed dark dots (Fig. 4A, D, G). The processes are conical, with a wide proximal portion being dome-shaped and a distal portion constituted of a long slender ending (Figs 4A-I, 5A-F). The walls of the egg process in their proximal portion contain light refracting dots that are probably caused by the labyrinthine layer (Fig. 4B, E, H). There are flexible distal portions of the egg processes, sometimes divided into two or three short filaments, and rarely bifurcated into two longer arms ( 125.4-172.3 and 155.6-203.5 ìm in R. ziemowiti), slightly wider bases of the processes (6.6-10.4 ìm in the new species vs. 3.5-6.6 ìm in R. ziemowiti), a larger process base/height ratio (42-83% in the new species vs. 17-40% in R. ziemowiti), a smaller inter process distance (2.1-4.1 ìm in the new species vs. 5.5-13.4 ìm in R. ziemowiti), and a larger pore density count on 2500 ìm 2 of the dorsal cuticle (26-36 in the new species vs. 20-24 in R. ziemowiti); Richtersius tertius due to: the shape of the cuticular pores (roundish pores with smooth edges in the new species vs. roundish pores with wavy edges in R. tertius), the absence of cuticular bars in legs I-III (divided cu-ticular bares are present in R. tertius), a strongly developed thickness of the buccal tube wall posterior to the stylet support insertion point (the thickness is poorly developed and much less obvious in R. tertius), a more posteriorly positioned stylet support insertion point (pt = 71.0-73.9 in the new species vs pt = 65. 3-68.9 in R. tertius), smaller eggs (egg bare and full diameter: 77.6-91.4 and 107.1-129.5 ìm in the new species vs. 117.4-155.3 and 149.8-188.2 ìm in R. tertius), slightly wider bases of the processes (6.6-10.4 ìm in the new species vs. 3.0-6.5 ìm in R. tertius), a slightly larger process base/height ratio (42-83% in the new species vs. 14-42% in R. tertius), a smaller inter process distance (2.1-4.1 ìm in the new species vs. 5.2-13.1 ìm in R. tertius), and a larger pore density count on 2500 ìm 2 of the dorsal cuticle (26-36 in the new species vs. 3-6 in R. tertius).

Discussion
Richtersius mazepi sp. nov., discovered in this study, is the fourth species to be formally described within the genus Richtersius. Its peculiar egg ornamentation morphology clearly distinguishes the new species from its congeners. In their recent paper in which the type species was redescribed, STEC et al.
(2020a) expressed the hope that any further descriptions of the Richtersius taxa would be integrative, with phenotypic data tightly linked to the DNA sequences of the given species. In this case, we have gone against those recommendations by providing a classical taxonomic description based on morphology and morphometry. We argue that all species are working hypotheses with assigned names (DE QUEI-ROZ 2007). To define and delineate a species, multiple forms of evidence can be used, such as morphological, genetic, ecological, reproductive and geographical analyses, as well as their combinations. In our case, the establishment of a new species is supported solely by morphological and morphometric data that explicitly shows Richtersius mazepi sp. nov. to be different from all other species within the genus. In other words, the line of phenotypic evidence is sufficient to delineate the species from the other known taxa. Nevertheless, we strongly support integrative studies in tardigrade taxonomy and recommend that these be preferred over classical morphology-based taxonomic studies where possible, especially when cryptic or pseudocryptic species complexes are involved. Importantly, however, situations in which some types of evidence are omitted from the taxonomic descriptions (e.g. DNA, SEM, physiology) will surely occur. In itself, the inability to include all possible forms of evidence does not automatically invalidate the establishment of a new species. The prohibition of sufficient and reliably formulated species hypotheses based only on some types of evidence, as was proposed in a recent paper by G¥SIOREK et al.
(2021), is not warranted. Indeed, inaccurate and outdated species descriptions constitute a considerable obstacle in taxonomical studies; however, such obstructions cannot be overcome through the establishment of a single rule that indicates some integrative configurations of data as the only correct solution. Furthermore, revisions and redescriptions constitute readily available tools for appropriate actions when species descriptions turn out to be insufficient. Prohibiting species descriptions that do not include some arbitrarily chosen types of data could seriously hamper our understanding of biodiversity. Taking this study as an example, without the new species description, knowledge about the morphological diversity of egg ornamentation in the genus Richtersius would remain limited. Naming a species allows it to be catalogued and ensures that it will be considered in future taxonomic studies (SEIFERT 2017). The danger of impeding the completion of the inventory of living biota is even more alarming now in the era of the so-called sixth mass extinction and the very slow tempo of new species descriptions (FONTAINE et al. 2012). As there is no one universal solution and rule, the responsibility for pinning a name to a given organism lies solely with the authors, who should always try to provide the best possible evidence when testing species hypotheses. The issue is not trivial, as species play a central role in biology, and species names greatly influence how we evaluate these elements of biodiversity, their conservation and their evolution.