Redescription of Macrobiotus tridigitus Schuster, 1983 and the erection of a new genus of Tardigrada (Eutardigrada, Macrobiotidae)

Based on claw morphology Macrobiotus tridigitus is transferred to the new genus Schusterius. Claws of the genus Schusterius differ from those in the genera Calcarobiotus and Macrobiotus in the unusually well‐developed accessory points which are transformed into very long and thin additional claws, arranged above double claws on each leg. A redescription of Schusterius tridigitus comb. nov. based on the holotype and the paratype is also given.


Introduction
Up to the present 12 genera have been described in the family Macrobiotidae. The largest genus of the family, Macrobiotus, is represented by at least 140 species (Guidetti and Bertolani 2005). Large variability in the claw and buccal apparatus morphology within the genus Macrobiotus (before 1980 the only genus in the family) has led to re-examination of many species and their transfer into new genera in the last years (e.g. Schuster et al. 1980;Pilato andBinda 1987, 1989;Binda and Pilato 1992;Bertolani and Biserov 1996;Claxton 1998;Guidetti and Bertolani 2001;Guidetti and Pilato 2002). Bertolani and Biserov (1996) erected two new genera in the family Macrobiotidae based on the claw structure and stated they are two different phyletic lines. Also, Guidetti and Bertolani (2001) stated that such high variability of claw morphology in Macrobiotidae suggests that in this family more than one phyletic line may be present. In 1993 Dastych described a new genus in Macrobiotidae based exclusively on the claw morphology. These papers show that the claw morphology is very important at the generic level in Macrobiotidae and also in the whole order Eutardigrada. Nevertheless, the species currently assigned to the genus Macrobiotus still vary significantly in the claw and buccal apparatus morphology. It suggests that more genera will be established based on these characters.
After re-examination of the holotype and a paratype of M. tridigitus we have concluded that the claw morphology is clearly different from that of Macrobiotus and decided to transfer this species to a new genus.

Material and methods
Two specimens (holotype and paratype) of M. tridigitus, in very good condition, loaned from Bohart Museum (University of California) were re-examined. Buccal tube length and the level of the stylet support insertion point were measured from the anterior margin of the stylet sheaths. Buccal tube widths were measured as the external diameters at the level of the stylet support insertion point. The pt ratio is the ratio of the length of a given structure to the length of the buccal tube expressed as a percentage (Pilato 1981). Lengths of claws were measured from the base of the claw to the top of the primary branch.
In the description of the holotype, measurements of the paratype are given in parentheses. The pt index is given after the mm value (in square brackets and italics). Due to unfavourable orientation of the holotype only claws of the paratype were measured. All measurements are given in micrometres (mm).
Photomicrographs, drawings and measurements were made using a phase contrast microscope associated with a camera lucida.

Diagnosis
Claws similar to those of the Calcarobiotus type (with primary branches only slightly longer than secondary branches), Y-shaped but with extremely developed accessory points connected to the primary branch by a flexible light-refracting portion. Bucco-pharyngeal apparatus of the Macrobiotus type probably with 10 peribuccal lamellae.

Etymology
The new genus is named after Professor R. O. Schuster who described Macrobiotus tridigitus from Tierra del Fuego (Argentina).
(  Claws similar to Calcarobiotus type claws (primary branches very similar in length) with 2-1-1-2 configuration, Y-shaped but with extremely developed, very long and thin accessory points connected to primary branch by flexible light-refracting portion (Figures 3,  4, 7). Measurements of external claws in paratype: accessory points of I pair 11.0 [33.3]; Eggs unknown.

Type locality
South America, Argentina, Tierra del Fuego, Sierra Martial.

Material examined
Holotype paratype are deposited in the Entomology Collection in Bohart Museum, University of California, Davis, USA.

Discussion
While Schuster (1983) was describing Macrobiotus tridigitus many of the currently existing genera were still assigned to the genus Macrobiotus. In the last few years researchers have started to pay more attention to the claw morphology. It is very likely that if Schuster was describing M. tridigitus today, he would assign it to a new genus. Schusterius gen. nov. is the only known Tardigrada species which has such strongly developed accessory points, which in our opinion justifies the decision of erecting the new genus for it.
The new taxon belongs to the family Macrobiotidae and is closely related to the genus Calcarobiotus Dastych, 1993 by the shape of the double claws (definition of Calcarobiotus type claws amended by Guidetti and Bertolani 2001) (Figures 7, 8) and to Macrobiotus Schultze, 1834 (Figure 9) by its buccal apparatus. Schusterius gen. nov. differs from these two genera in the extremely developed accessory points on the claws. These long accessory points are connected to the primary branches of the external and internal claws by a flexible light-refracting portion. This character seems to be apomorphic and it is not known in other genera of the Macrobiotidae. However, claws with a similar appearance are present in the genus Milnesium Doyére, 1840, Ramazzottius Pilato, 1986, andRamajendas Pilato andBinda, 1990 (Figures 9-12), but in these three genera not just the accessory points but the entire main branches of the claws are thin and elongated. In the genus Milnesium and Ramazzottius the main branch is completely separated and transformed into an additional claw, but in Ramajendas the main branch is still weakly connected with the secondary branch by a light-refracting unit.
At this stage it is difficult to estimate the evolutionary importance of the extreme development of accessory points into additional claws but it seems that it is a true apomorphy. Until now only one species, S. tridigitus, has been ascribed to the genus Schusterius. It is known only from two specimens collected in Tierra del Fuego and no ecological or detailed environmental data are known.