Eremobiotus, Biserov, 1992

Eremobiotus View in CoL claw definition and measurement

Pilato and Binda (2010) gave a summary of all known, at that time, Eutardigrada genera; however, in the definition of the genus Eremobiotus , they did not mention the peribuccal papulae and defined the claws of Eremobiotus species of legs I–III as of “ Isohypsibius - type ”; on page 30, they wrote: “Claws of the typical Isohypsibius type on the first three pairs of legs”; however, that definition did not take into consideration (i) the shape of internal claws I–III, in which the angle between the two branches may reach 180° and (ii) the extremely modified claws of E. ovezovae , obviously different from the typical Isohypsibius type.

Despite the significant differences (especially dimensional) between the claws of E. ovezovae and the other two described species, it is noticeable that in all species of the genus, the internal claws of legs I–III typically have a wider angle compared to a typical Isohypsibius claw (as previously reported by Biserov, 1992). However, these claws cannot be considered of the Eremobiotus type for various reasons (see the discussion below regarding Eremobiotus claws) and therefore should be considered as modified Isohypsibius claws (as reported by Biserov, 1992). The external claws of legs I–III appear to be of the Isohypsibius type.

Claws of the “ Eremobiotus type ” have never been defined formally. Biserov (1992), in describing the genus, mentioned claws of the fourth pairs of legs as “significantly different from those of I– III legs,” having a large basal part of the claws, longer than that of branches, primary branches slightly longer than secondary, and an angle of approximately 180° between the two branches. Upon revising the type material of E. alicatai , E. ginevrae , and E. ovezovae , we observed that (i) the basal portion of the claws I– III is not always longer than the primary/secondary branch, especially undivided distal buccal ring (white indented arrowhead), the circumoral sensory field without evident lobes (white arrowhead) and surrounding area forming the presumed “second band of lobes” (white arrows). Scale bars a 10 μm; b 5 μm

in E. ginevrae ; (ii) the claw branches of the fourth pair of legs do not always exhibit an angle of about 180°, but rather a narrower angle (approximately starting from 140°), varying even among specimens of the same population ( Fig. 5d, e View Fig ). In particular, the anterior claws (homologous to the internal claws of legs I–III) exhibit an angle of 160–180°, while the posterior claws (homologous to the external claws of legs I–III) display a narrower angle, of 140–180°. The branches appear to be less divergent especially in larger specimens ( Fig. 5 View Fig ).

This variability may be dependent also on the claw orientation in the preparation. Despite this variation, the claws of the fourth pair of legs of Eremobiotus still differ from those of every leg pair of all Isohypsibioidea due to the following characters: (i) the primary and secondary branch share a large common portion of almost triangular shape; (ii) internal septa are almost invisible with PCM; (iii) accessory points are short, thick, and asymmetrical with respect to the main axis of the primary branch, appearing to point upwards with PCM. Thus, only those claws, i.e., the claws of the fourth pair of legs, should be treated as the “ Eremobiotus - type ” claws.

Regarding this particular claw type, some problems arise when measurements are taken with the currently used methodology of measurement of the Isohypsibioidea claws, established by Beasley et al. (2008) (see also Fig. 6a, b View Fig ). Until 2002, parachelan claws with an asymmetrical arrangement were measured according to Pilato et al. (1982), i.e., from the base of the claw to the end of the primary branch (including the accessory points), not giving troubles even if Eremobiotus claws were measured. Pilato et al. (2002) introduced a new method but only for the external claws of the Hypsibius type. Beasley et al. (2008) introduced a ◂ Fig. 4 Bucco-pharyngeal apparatus of Eremobiotus alicatai (specimens from the topotypic population) ( a SEM; b–d PCM). a Buccal opening showing the distal buccal ring (white indented arrowhead), a single band of teeth (black indented arrowhead) and the circumoral sensory field with lobes (white asterisk) and surrounding area forming the presumed “second band of lobes” (black asterisk). b Dorsoventral view of the bucco-pharyngeal apparatus. c Lateral view with Isohypsibius - type AISM (arrows). d Detail of the placoids; black arrowheads indicate constriction in the medial portion of the first placoids and in the final portion of the second placoid. Scale bars a 5 μm; b–d 10 μm

new measurement method for claws of the Isohypsibius type (but subsequently applied to other asymmetrical claws as well) to take into consideration also the measurement of the “basal claw height” and “secondary branch length.” Up to date, this has not raised perplexities, but some difficulties are encountered if claws have primary and secondary branch diverging with an angle of approximately 180°, and no septum or flexible connection is present to clearly indicate the border that separates primary and secondary branch bases, as those of the Eremobiotus type. Such a situation makes it impossible to obtain an objective measurement of the structures considered.

Therefore, for those claws ( Eremobiotus type, i.e., the fourth pair of claws, in which septa nor a clear division between primary and secondary branch is observable), we propose to not measure the claws according to Beasley et al. (2008) and reestablish the older method of claw measurement, i.e., the measurement taken from the basal portion of the claw to the end of primary branch according to Pilato et al. (1982), adding the same type of measurement also for the secondary branch (in Pilato et al. (1982) only the primary branch was measured) ( Fig. 6c View Fig ). Moreover, we propose a third measurement for the claws of the Eremobiotus type. This measurement is taken from the tip of the primary branch to the tip of the secondary branch of the same claw, excluding the accessory points ( Fig. 6d View Fig ). Moreover, since the “ Parachela ” template available from the Tardigrada Register (www.tardigrada.net/register, Michalczyk & Kaczmarek, 2013) was used, the line reporting the ratio between the posterior/anterior primary branch and the base was modified. The entry “Anterior base/primary branch (cct)” was replaced with “Anterior branch distance/primary branch (cct),” and the entry “Posterior base/primary branch (cct)” was replaced with “Posterior branch distance/primary branch (cct). This new measurement was not adopted for internal claws of the legs I–III, even if the angle sometimes reaches the 180°, due to the visibility of the insertion of the secondary branch on the primary branch, from which the measurement can be taken ( Fig. 6 View Fig ).

Taxonomic account