Milnesium minutum, Pilato, Giovanni & Lisi, Oscar, 2016

Pilato, Giovanni & Lisi, Oscar, 2016, Milnesiumminutum and Milnesiumsandrae, two new species of Milnesiidae (Tardigrada, Eutardigrada, Apochela), ZooKeys 580, pp. 1-12 : 2-6

Taxon classification Animalia Apochela Milnesiidae

Milnesium minutum sp. n. Fig. 1, Table 1

Type locality.

Sicily, Moio Alcantara, Contrada Rinazzo 37°54'04"N, 15°03'08"E.

Material examined.

Moio Alcantara: Contrada Rinazzo (holotype and one paratype: (slide No. 4127) from a moss sample on rock collected by Dr. R. Catanzaro (Catania) (April 1986); Noto: Contrada Volpiglia, (one paratype, slide No. 3238) from a moss sample collected on a dry wall by Mr. S. Di Stefano (Catania) (February 1980).

Type repository.

Holotype and two paratypes are deposited in the Binda and Pilato Collection (slides Nos. 4127 and 3238), Museum of the Department of Biological, Geological and Environmental Sciences, University of Catania, Sicily.

Specific diagnosis.

Body of small size (up about 300 µm in the specimens found); colourless; cuticle smooth; eye spots present; six peribuccal and two lateral papillae present; mouth terminal with six triangular peribuccal lamellae with basal stripes; stylet supports inserted on the buccal tube at about 63-66% of its length; claws of the Milnesium type with a [3-3]-[3-3] configuration; primary claws with thin accessory points; secondary claw bases each with a rounded basal thickening (lunule); a long cuticular bar present under claws I–III.

Description of the holotype.

Body colourless, 288 µm long; cuticle smooth without pseudopores, reticulum, tubercles or gibbosities; eye spots present. Six peribuccal and two lateral papillae present. Bucco-pharyngeal apparatus of the Milnesium type (Fig. 1A) (rigid buccal tube without ventral lamina, apophyses for the insertion of the stylet muscles in the shape of very short and flat ridges symmetrical with respect to the frontal plane and without caudal processes; pharyngeal bulb elongated, pear-shaped and without apophyses, placoids or septulum); six triangular peribuccal lamellae present with basal stripes. Wide stylet furcae triangular in shape (Fig. 1A).

Buccal tube cylindrical, 25.7 µm long; the external width at the level of the stylet supports insertion point is 10.9 µm (pt = 42.4). Stylet supports short, inserted on the buccal tube at 65.9% of its length.

Claws of the Milnesium type (Fig. 1), secondary claw branches with three points: configuration [3-3]-[3-3]. Primary claws on legs II, 11.3 µm long (pt = 44.0) and secondary claw, 8.0 µm (pt = 31.1); primary claws on legs III, 11.8 µm long (pt = 45.9); secondary claw, 8.5 µm long (pt = 33.1); primary claws on legs IV, 13.1 µm long (pt = 51.0), secondary claw, 8.6 µm (pt = 33.5). The secondary claw length is 70.8% of the primary claw length on legs II, 72.0% on legs III and 65.6% on legs IV.

Primary claws with thin accessory points (Fig. 1E arrow); each secondary claw base with rounded basal thickening (lunule) (Fig. 1B, arrow a); a long cuticular bar is present under the claws I–III (Fig. 1B arrow b).

Eggs not found.

Remarks.

The paratypes are similar to the holotype in both qualitative and quantitative characters (Table 1).

Etymology.

The specific name minutum (minutus = small) refers to the small body size.

Differential diagnosis.

Eight species of Milnesium with six peribuccal lamellae and a [3-3]-[3-3] claw configuration are known with a smooth cuticle: Milnesium brachyungue Binda & Pilato, 1990; Milnesium eurystomum Maucci, 1991; Milnesium asiaticum Tumanov, 2006; Milnesium antarcticum Tumanov, 2006; Milnesium longiungue Tumanov, 2006; Milnesium zsalakoae Meyer & Hinton, 2010; Milnesium barbadosense Meyer & Hinton, 2012 and Milnesium bohleberi Bartels, Nelson, Kaczmarek & Michalczyk, 2014.

Milnesium minutum sp. n. differs from all these species in having a smaller body size, and other character detail indicated in the following comparisons. We noticed that the three specimens we attributed to Milnesium minutum sp. n. are in particular very similar to Milnesium asiaticum and, considering the body size, it was necessary to determine whether they were three young specimens of Milnesium asiaticum or belonged to a different species. Three facts have to be stressed: a) we collected the specimens attributed to the new species in two different localities. b) We examined and measured specimens of the 15 species of Milnesium present in the collection of Binda & Pilato, and we noticed that for each species in all cases the buccal tube width pt index values for smaller specimens were lower than larger specimens. Specimens of the new Sicilian species with 300 µm body length have buccal tube width pt values that are similar to (or slightly higher than) those of Milnesium asiaticum , which have a body length more than twice as long (Tables 1 and 2). c) Milnesium minutum sp. n. differs from Milnesium asiaticum in having wider buccal tube with respect to the body length; a lower posterior primary claw pt ratio, and a slightly higher percent ratio between the secondary claw and primary claw lengths on legs III and IV (Tables 1-2; Figs 1C, D and 2A). These facts led us to conclude that the three Milnesium minutum sp. n. specimens were not young examples of Milnesium asiaticum but, independent of body size, belonged to a distinct species.

In addition to the body size, the new species differs from Milnesium eurystomum and Milnesium bohleberi by having a cylindrical (not funnel-shaped) buccal tube; from Milnesium eurystomum by having a higher pt of the insertion point of the stylet supports (pt = 63-66 in Milnesium minutum sp. n. vs 58-61 in Milnesium eurystomum ); and from Milnesium bohleberi in having lower percent ratio between the secondary claw and the primary claw lengths on all legs (the percent ratio is 69.7-72.6 in the claws I–III of Milnesium minutum sp. n. and 77.9-84.9, for Milnesium bohleberi (according to Bartels et al. 2014); in claw IV the values are 65.6-68.9 in Milnesium minutum sp. n. and, 78.9-80.4 for Milnesium bohleberi (see: Bartels et al. 2014)).

Milnesium minutum sp. n. differs from Milnesium brachyungue by having slightly lower pt of the stylet supports insertion point (63-66 in the new species vs 67-70 in Milnesium brachyungue ), by higher pt of the primary and the secondary claw lengths, and by higher values of the percent ratio between the secondary claw and primary claw lengths (Tables 1 and 2, Figs 1 and 2B).

The new species differs from Milnesium longiungue by having accessory points as well as lower pt of the primary claw lengths and higher values of the percent ratio between the secondary claw and primary claw lengths (Tables 1 and 2, Figs 1 and 2C).

The new species differs from Milnesium antarcticum by having a higher pt of the buccal tube width (38.6-42.4 in Milnesium minutum sp. n., 25.9-31.8 in Milnesium antarcticum according to Tumanov 2006); lower pt of the insertion point of the stylet supports on the buccal tube (63.0-66.0 in the new species, 70.0-73.7 in Milnesium antarcticum according to Tumanov 2006); higher pt of the primary claw lengths on legs I-III (Tables 1 and 4, Figs 1 and 2D).

Milnesium minutum sp. n. differs from Milnesium zsalakoae by the more anterior insertion of the stylet supports on the buccal tube (pt = 63-66 in Milnesium minutum sp. n., 68.2-71.1 in Milnesium zsalakoae , according to Meyer and Hinton 2010). The new species also differs by having accessory points and by having a higher percent ratio between the secondary claw and primary claw lengths on legs IV where the values are 65.6-68.9 in Milnesium minutum and 47.2-48.6 for Milnesium zsalakoae (see: Meyer and Hinton 2010).

The new species clearly differs from Milnesium barbadosense by having eyes and by having the stylet supports inserted on the buccal tube in a more anterior position (pt = 63-66 in the new species, about 73 for Milnesium barbadosense according to Meyer and Hinton 2012) (Tables 1 and 4).