Milnesium eurystomum Maucci 1991

Milnesium eurystomum Maucci, 1991

(Figs 1–6, Table 2)

Material examined. Holotype: Female from Southern Greenland, near Narssaq and around the Narssarssuaq airfield, along the road from the airfield to the Kiagtut glacier; Sermiat, on the front of the ‘Inlandsis’, ca. 61°10'N and 45°23'W, ca. 100 m asl, moss and lichen sample from a sun-exposed rock, 1987, coll. Walter Maucci.

Female from Svalbard, Spitsbergen: Northern part of the Revdalen Valley, near the Revvatnet lake and the Revelva river, 77°01'41''N and 15°22'21''E, 67 m asl, moss from rock, 26.06.2010., coll. Łukasz Kaczmarek and Jerzy Smykla.

Description (measurements in Table 2). Body brownish, eyes present (but not visible after preparation). Cuticle smooth, without granulation or pores (Figs 1–2). Two lateral and six peribuccal papillae present (ventral papilla smaller than other papillae).

Spitsbergen of Milnesium eurystomum and a paratype of Milnesium granulatum Ramazzotti, 1962.

CHARACTER M. eurystomum M. granulatum Holotype Spitsbergen Paratype µm pt µm pt µm pt Buccal apparatus of the Milnesium type (Figs 3–4). Six peribuccal lamellae around the mouth opening present. Buccal tube wide and short (buccal tube standard width on average 63% of its length), and funnel-shaped, wider anteriorly (posterior diameter on average 69% of the anterior diameter). Pharyngeal bulb elongated, pear-shaped and without placoids or septulum.

Claws of the Milnesium type, slender (Figs 5–6). Primary branches on all legs with small, but distinct accessory points detaching from the branch at its greatest curvature. Secondary branches with rounded basal thickenings. All secondary branches on all legs with three points (i.e. claw configuration: [3-3]-[3-3]). Single, long transversal, cuticular bars under claws I–III present (Fig. 5).

Although eggs were not described by Maucci (1991) nor found in the Spitsbergen samples, we should probably expect them to be oval, smooth and deposited in exuvium as in all other known Milnesium species.

Remarks. The specimen of M. eurystomum that was collected from Spitsbergen is very similar to the holotype (see Table 1 and Figs 1–6).

In all known Milnesium species lateral papillae are cylindrical in shape, but they were flattened in the holotype and also on the photograph and on the drawing in Maucci (1991). This unusual appearance could however be an artefact caused by the preservation process. An observation of the lateral papillae on the Spitsbergen specimen could solve this problem, but unfortunately in our specimen the mouth was retracted and papillae were squashed, which currently makes it impossible to determine the shape of these structures for this species.

In a more recent paper on Arctic tardigrades Maucci wrote that, “Other specimens [of M. eurystomum] were also found in the Argentinean Andes (the Onelli glacier) and in Chile (Punta Arenas).” (Maucci 1996). However, given that the type locality and these two South American locations lie on different continents and are in different zoogeographical regions, we hypothesise that Maucci could have found a different species that was similar to M. eurystomum. Until the slides from South America are located and re-examined, we suggest the reference to this region should be considered dubious.

Locus typicus. Southern Greenland, near Narssaq and around the Narssarssuaq airfield, along the road from the airfield to the Kiagtut glacier; Sermiat, on the front of the ‘Inlandsis’, ca. 61°10'N and 45°23'W, ca. 100 m asl.

Distribution. Currently confirmed localities are in the Arctic: Southern Greenland and Svalbard Archipelago (Spitsbergen) but see also Remarks above.

Etymology. Walter Maucci named the species ‘ eurystomum ’ (= large-mouthed) because of the high ratio of the buccal tube width to its length.

Type depositories. Holotype is preserved at the Natural History Museum of Verona, Italy.

Differential diagnosis. The ratio of the buccal tube width to length is definitely the most striking of M. eurystomum characters: in the two specimens examined it exceeds 60%. No other described Milnesium species has such a high ratio and the two other species with the highest ratios are: M. jacobi Meyer & Hilton, 2010 (ca. 55%) and M. tetralamellatum Pilato & Binda 1991 (ca. 49%). In the remaining known species the ratio of the buccal tube width to length varies between 21% (M. katarzynae Kaczmarek, Michalczyk & Beasley 2004) to ca. 45% (M. tardigradum s.s.).

M. eurystomum, apart from the buccal tube standard width to length ratio, differs from:

M. jacobi by more cylindrical and broader buccal tube (funnel-shaped in M. eurystomum and ca. 45.9–47.9 μm wide, in M. jacobi 15.3–27.2 μm wide; both standard widths) more anteriorly inserted stylet supports on the buccal tube (pt ca. 60% in M. eurystomum and ca. 88% in M. jacobi), and a different claw configuration ([3–3]- [3–3] in M. eurystomum and [2–3]-[3–3] in M. jacobi).

M. tetralamellatum by having six peribuccal lamellae (four in M. tetralamellatum) and a different claw configuration ([3–3]–[3–3] in M. eurystomum and [2–3]–[3–2] in M. tetralamellatum).