Grevenius granulifer

Morphotype of Grevenius granulifer View in CoL and its distribution

The short cuticular bars under internal claws I–III were first reported by Pilato and Binda (1977) and later consolidated as species-specific ( Bertolani, 1982). However, these bars are often poorly visible or even entirely/asymmetrically lacking in some specimens. It is advisable not to use their presence as one of the main criteria in identifying G. granulifer . In contrast with the well-established notion that the ventral side is also sculptured, as the dorsum ( Bertolani, 1982; Ramazzotti & Maucci, 1983), it is in fact smooth besides the usual wrinkling ( Fig. 1B View Fig ). The dorsal sculpturing gradually disappears laterally and is poorly developed on legs I–III ( Fig. 1A View Fig ). Interestingly, already Bertolani (1975) underlined that there are differences in dorsal sculpturing between Italian populations: one exhibited a reticulum, whereas another classical granular tubercles. Given that the sculpturing of the topotypic population can be described as a reticulum with tubercles positioned at the corners of polygons throughout most of the trunk, except for the cephalic region, which is wrinkled ( Fig. 2 View Fig ), it may be assumed that cuticular sculpturing can be variable in the genus. This was also revealed in the case of G. asper ( Dastych, 2016) . Such variability could explain potential misidentifications with species that seemed to be distinct in the form of sculpturing from G. granulifer (e.g., Tumanov, 2003) and suggests a need for re-evaluating their conspecificity. Grevenius baldii ( Ramazzotti, 1945) was proposed as a likely sister species of G. granulifer based on the similarity of sculpturing ( Bertolani, 1982; Ramazzotti & Maucci, 1983), but it attains much smaller body size (<200 μm in body length) and has sculptured ventral side ( Bertolani & Balsamo, 1989).

Since the year of its description ( Thulin, 1928), the species was reliably recorded from many Palaearctic localities, with a detailed drawing/microphotographic documentation ( Cuénot, 1932, Marcus, 1936, Rudescu, 1964, Greven & Blom, 1977, Betolani, 1982, Ramazzotti & Maucci, 1983, Maucci, 1986, Dastych, 1988). The Nearctic record by Schuster et al. (1978) was published with a SEM picture of the cuticle, which indicated that the identification was correct. Also, the record from Korea ( Moon et al., 1989) seems to truly represent G. granulifer . Therefore, assuming that a biogeographic structuring is present among limnic tardigrades as in their cryptogam-dwelling relatives (Gąsiorek, 2023), the species should be tentatively regarded as Holarctic until new evidence shows otherwise. Non-Holarctic records ( McInnes, 1994) are thus treated as uncertain and not considered herein. The first comprehensive DNA barcodes for G. granulifer were published in Møbjerg et al. (2007), and when cross-checked with new data, they positively verified the presence of this species in Denmark (see also above).

Recently, Massa et al. (2024) introduced a division of Grevenius into four morphogroups to ease species identification. Grevenius granulifer was included within the asper morphogroup (three macroplacoids in the pharynx and no microplacoid), which is clearly separate from the annulatus morphogroup (two macroplacoids in the pharynx and no microplacoid). The latter should be probably separated from Grevenius s.s. as an independent genus. However, the species recorded by Massa et al. (2024) from the British Columbia does not belong to Grevenius , but to Isohypsibius s.s., as it was found in moss ( Grevenius is primarily aquatic) and represents a classical Isohypsibius prosostomus morphotype ( Dastych, 1988; Thulin, 1928) with sculpturing in the form of reticulum. A precise diagnosis of I. prosostomus , and the more precise diagnosis of Isohypsibius s.s., is still lacking due to the unavailability of modern redescription.