Mixibius felix Lisi 2017, sp. nov.

Mixibius felix sp. nov.

Fig. 1, Table 1

Type locality. Vegetation Island, Victoria Land, Antarctica: 74°47′0″S, 163°37′0″E.

Material examined. (collected by Prof. Salvatore Motta between 1990.12.15 and 1991.1.12): Vegetation Island (2 specimens, holotype and one paratype, slides No. 4535, from debris of the bottom of a small lake, together with Acutuncus antarcticus (Richters, 1904) and Diphascon dastychi Pilato & Binda, 1999; and Mariella Creek (8 specimens, slides Nos. 4540–4541, from debris of the bottom, together with Acutuncus antarcticus and Diphascon dastychi).

Type repository. Holotype and paratypes are preserved in the collection of Binda and Pilato in the Museum of the Department of Biological, Geological and Environmental Sciences (Section of Animal Biology “ Marcello La Greca ” of the University of Catania, Italy).

Specific diagnosis. Colourless, cuticle smooth, eye spots present; bucco-pharyngeal apparatus of the Mixibius type (buccal tube rigid, without ventral lamina and with hook-shaped apophyses for the insertion of the stylet muscles); stylet supports inserted on the buccal tube at 65.7–68.0 percent of its length; pharyngeal bulb with apophyses and two rod-shaped macroplacoids; microplacoid and septulum absent; external claws of Isohypsibius type, internal claw of modified Isohypsibius type.

Description of the holotype. Body 240 µm long, colourless, cuticle smooth, eye spots present in living specimens; bucco-pharyngeal apparatus (Figs. 1 A,B) of Mixibius type; the rigid and narrow buccal tube is 28.4 µm long and 2.5 µm wide (pt = 8.8); the ventral lamina is absent; both the dorsal and ventral apophyses for the insertion of the stylet muscles hook-shaped, slightly asymmetrical with respect to the frontal plane (Fig. 1 B of a paratype, arrows a—ventral and b—dorsal): the dorsal apophysis is slightly more stumpy, with a blunt and swollen caudal apex. A short median cuticular thickening is present caudal to both these apophyses (the ventral less visible) (Fig. 1 B). The buccal armature is not clearly visible but teeth appear to be absent (unlike Acutuncus antarcticus where well developed teeth are clearly visible). The stylet supports are inserted on the buccal tube at 67.8% of its length (pt = 67.8). Pharyngeal bulb with well-developed apophyses and two rod-shaped macroplacoids: microplacoid and septulum absent (Fig. 1 A). The first placoid, with a central narrowing, is 5.9 µm long (pt = 20.8), the second 5.4 µm (pt = 19.0) and the entire macroplacoid row 11.4 µm long (pt = 40.1).

The external claws are of the Isohypsibius type, the internal are modified Isohypsibius type, the angle between the basal portion and the secondary branch being larger than 90° (Figs. 1 C,D). As a consequence of the orientation, it is particularly rare to have specimens with buccal tube and all claws correctly aligned for measurement; in the holotype it is possible to measure the internal claws I, that are 9.3 µm long (pt = 32.7); the external claws III, are 13.4 µm long (pt = 47.2), the main branches of which are the 72.9% of the total claw length; the internal claws of the same pair of legs that are 9.7 µm long (pt = 34.2) and the anterior claws IV, 10.1 µm long (pt = 35.6).

The main branches of all claws have thin accessory points (Fig. 1 C, arrow); the basal extremity of the claws is slightly enlarged but lunules and other cuticular thickening on the legs are absent.

Eggs not found.

Etymology. The specific name felix (felix = happy) expresses the “happiness” of the species for having been finally identified and described.

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

Differential diagnosis. Before comparing Mixibius felix sp. nov. with the other species of the genus, taking into consideration that in some orientations the claws of Mixibius may appear to be of the Hypsibiu s type and could cause some difficulties in distinguishing between Mixibius and Acutuncus, we therefore considered it opportune to insert a comparative figures of Acutuncus (Fig. 2). Acutuncus antarcticus (Fig. 2) has a bucco-pharyngeal apparatus with the characteristic “sharp hooks” for the insertion of the stylet muscles (Figs. 2 A,B arrows) compared with the more stumpy and blunt hook-shaped of Mixibius felix sp. nov. (Fig. 1 B arrows a, b), and the Acutuncus claws are described as ‘external of the Isohypsibius type, internal of the Hypsibius type’ (Figs. 2 C,D), compared with Mixibius type claws (Fig. 1 C, D).

The data for Mixibius pilatoi Wang, 2009 are from the original description of the species.

Nine species are known belonging to the genus Mixibius; Mixibius saracenus Pilato, 1973, Mixibius fueginus Pilato & Binda, 1996, Mixibius ninguidus Biserov, 1999, Mixibius ornatus Pilato, Binda, Napolitano & Moncada, 2001, Mixibius sutirae Pilato, Binda & Lisi, 2004, Mixibius tibetanus Li & Li, 2008, Mixibius pilatoi Wang, 2009, Mixibius schnurae Pilato, Lisi & Binda, 2010 and Mixibius parvus Lisi, Sabella & Pilato, 2014. Only three have a smooth cuticle, therefore it seems necessary to compare Mixibius felix sp. nov. with these species: Mixibius saracenus, Mixibius fueginus and Mixibius pilatoi.

Mixibius felix sp. nov. can be distinguished from Mixibius pilatoi by: lacking lunules; clearly longer placoids (the pt values of the first and second placoids are about 19–22 and 15–19 respectively in the new species, and about 13–14 and 9 respectively in Mixibius pilatoi) (see figures 1A, and 2B in Wang 2009); stylet supports inserted on the buccal tube in a more cephalic position (pt = 65.7–68.0 in the new species, 67.9–73.9 in Mixibius pilatoi according to Wang (2009); and higher values relative to the claw lengths (Table 1).

The new species is very similar to Mixibius fueginus (Fig. 3) and Mixibius saracenus (Fig. 4). However, it differs from Mixibius fueginus by: having slightly narrower buccal tube (pt = 8.0– 10.8 in the new species, 11.1 in the unique known specimen of Mixibius fueginus, luckily of similar body length) (Table 1); the stylet supports inserted on the buccal tube in a more caudal position (pt = 65.7–68.0 in the new species, c. 62 in Mixibius fueginus) (Table 1); and in details of the claw shape (they are less slender in the new species (Figs. 1 C,D and 3B,C) and, in particular, the internal claws have a slightly shorter secondary branch, which narrows more gradually, whereas the internal claws of Mixibius fueginus narrow more abruptly and, as a consequence, the branch appear to have a larger proximal portion and a clearly thinner distal portion (Figs. 1 C,D and 3 B,C).

Mixibius felix sp. nov. differs from Mixibius saracenus by: having eye spots; narrower buccal tube (pt = 8.0– 10.8 in the new species, 13.7–13.9 in Mixibius saracenus (Table 1 and Figs. 1 A,B and 4A); and in the claw shape: the basal portion is shorter and stouter, and both branches are shorter and stouter in the new species (Figs. 1 C,D and 4B,C).