Xenylla arnei, Babenko & Efeykin & Bizin, 2020

Babenko, Anatoly, Efeykin, Boris & Bizin, Mikhail, 2020, Three new and one little-known species of Hypogastruridae (Collembola) from Russia's northeast, ZooKeys 1005, pp. 1-20 : 1

Xenylla arnei sp. nov. Figs 36-45 , 46-47

Type material.

Holotype Russia, North-East • ♂; Magadan Province, Tauisk; 59°43.66'N, 149°21.85'E; coastal meadow; July 2017; M. Bizin and B. Efeykin leg.

Paratypes Russia, North-East • 5 ♀♀ and 5 ♂♂, same data as for holotype. The types are deposited in MSPU.

Additional material.

more than 500 specimens (alcohol), mainly from the holotype locality; Several specimens from this material were sequenced (Table 1 ). Their partial COI genes were amplified and deposited in the GenBank under the sample ID: KY066787-KY066789; 9 specimens; same region, AF-51/79: "Geartner Bay, steep slope down to beach with Artemisia , Sedum , Saxifraga , Potentilla , grasses, 20.07.1979"; A. Fjellberg leg.

Diagnosis.

A species from chaetotaxic group II of the genus Xenylla with a general chaetotaxic code of h1rt, characterised by a light brownish colour, non-differentiated dorsal setae, and the absence of a prominent cuticular lobe from the subcoxae of hind legs.

Description.

Length 1.4-1.7 mm. Colour rather light, yellow-brown (chamois), with patches of diffuse darker pigmentation, ocular field and antennal tip dark, ventral side usually paler. Tegument granulation fine and almost uniform. Ant.4 with a simple apical vesicle and four blunt sensilla (one dorsal [S3 or S4?] and three lateral [S7-S9]), relatively short and subequal in size, both a subapical microsensillum and an organite are present. AO typical of the genus, outer sensilla thinner than subapical ones, but not especially short (1:1.2-1.8, Fig. 39 ). Ant.1 and Ant.2 with seven and 12 or 13 setae, respectively.

Head with 5+5 subequal ocelli, as usual. Buccal cone typical of the genus, not elongate. Setal formula of labrum, 4/554, setae of distal row clearly thickened. Labium with all common papillae (A-E), 12 guards (eight long and four short, rhabdoid, papillate) and six proximal setae (Fig. 38 ). Basomedial field of labium (submentum) with four setae, basolateral field (mentum) with five setae, as usual. Maxillary outer lobe simple, with three sublobal setae.

Dorsal setae fine, thin, and barely differentiated (except for those on Abd.6), tergal sensilla clearly longer than ordinary setae (~ 3.2-3.4:1), dorsal and lateral sensilla on Th.2 and Th.3 subequal. Head with a basic set of setae (Fig. 37 ), lateral setae not differentiated. Th.2 and Th.3 also with all usual setae, a2 in a posterior position compared to a1 [h1] only on Th.3, p2 seta set aligned with p1 and p3 (Fig. 36 ). Abdominal terga also with basic set of setae (Fig. 37 ), p-row on Abd.1-3 with p5 present, s = p6 (Fig. 46 ). Ventral chaetotaxy: head with 2+2 setae along midline (p1 absent [r]), m3 present (Fig. 48 ), Th.2 and Th.3 without axial setae [t]. Abdominal sterna as in Fig. 41 : Abd.2 with p1 but without p2, Abd.3 without medial unpaired seta above retinaculum. General chaetotaxic code as h1rt.

Ventral tube with 4+4 setae. Retinaculum with 3+3 teeth. Furca complete, both dens and mucro thin, long, and clearly separated ventrally. Mucrodens/U3 ratio as 2.6-2.9:1. Dens with two dorsal setae (Fig. 43 ), ventral side of dens and mucro completely smooth, without primary granulations (Figs 44 , 45 ). Mucro shorter than dens (0.7-0.8:1), with a low outer lamella, ventral thickening neither prominent nor with a clear tooth. Chaetotaxy of legs 1-3 usually as follows: upper subcoxae - 1, 3, 3; lower subcoxae - 0, 3, 3; coxae - 3, 8, 8; trochanters - 6, 6, 6; femora - 13, 12, 11; tibiotarsi - 19, 19, 18 setae, respectively. Tibiotarsal setae A1 and A7 on all legs as long as 1.5-1.7 inner edge of unguis, clearly clavate. Unguis with a pair of lateral teeth and usually with a small tooth in upper third of inner edge. Anal spines short, usually curved and set on tiny cuticular papillae.

Etymology.

The new species is named after the famous Norwegian collembologist, Arne Fjellberg, who discovered it in the Magadan Region more than 40 years ago.

Affinities.

Using the most recent key to the Asiatic species of the genus ( Jia and Skarżyński 2019), X. arnei sp. nov. keys out to X. humicola (Fabricius, 1790), because their general chaetotaxic codes [h1rt] are identical. Moreover, both species are similar in many other important morphological traits, namely the labial palp is with four e-guards (cf. Fig. 38 and Fig. 48 ), which is not typical of the genus [see Fjellberg (1999)], the virtually identical maxillae (cf. Fig. 40 and Figs 49 , 50 ), three sublobals on the maxillary outer lobe, a retinaculum with 3+3 teeth, a long dens with two setae and a narrow straight mucro. Nevertheless, several fine morphological differences between X. arnei sp. nov. and X. humicola are traceable. Apart from the different colouration, the non-differentiated setae l1 and l3 on the head (vs. similar in size, but spine-like setae in X. humicola ) and the absence of subcoxal lobes from Th.3 in X. arnei sp. nov. (variable in shape, but always present in X. humicola ) seem to be the most sound. In addition, the species status of X. arnei sp. nov. is well confirmed by molecular evidence (Table 4 ).

Xenylla arnei sp. nov., together with the widespread X. humicola and the Japanese X. brevispina Kinoshita, 1916 [h1rtsv], represent the only known Holarctic members of the species group II ( Stebaeva and Potapov 1994). There is also one more form, X. convexopyga Lee, Park & Park, 2005, from the Korean Peninsula, in which the chaetotaxy is rather similar. The chaetotaxic code given for this species in the original description is as follows: krtsv. This code is likely to be not fully correct. The absence of a m3 seta from Th.2 [k] contradicts to fig. 1A in Lee et al. (2005), where this seta is present, albeit marked as m4 (a situation unknown within the genus). In our view, the species rank of X. convexopyga and its separation from X. brevispina need confirmation.

All other species of this group, viz. X. yukatana Mills, 1938 [h1tiqa3a4], X. gamae Cardoso, 1968 [h1tiq], X. nigeriana Gama & Lasebikan, 1976 [h1tiqsoa3], X. brasiliensis Gama, 1978 [h1rtlq], and X. nirae Gama & Oliveira, 1994 [h1rtlqiomu], are characterised by more strongly reduced chaetotaxy patterns and inhabit various tropical regions.

Molecular data.

GenBank currently contains COI sequences for only eleven species of the genus (of 140 species known worldwide). The obtained interspecific divergences (Table 4 ) between all of them, together with X. arnei sp. nov., range between 18.8 to 34.9% (mean 26.5%). Analogous data for X. arnei sp. nov. are 22.2-29.1% (mean 24.9%), which can be considered as an additional argument in favour of its specific status. Nevertheless, such a scant amount of primary data does not allow for any serious statements to be made, but a trend to the absence of parallelisms in molecular and morphological evolution can be traced quite clearly.

Distribution and ecology.

Xenylla arnei sp. nov. was collected in two neighbouring sites located on the northern shore of the Sea of Okhotsk, in the vicinity of Magadan. In this region, it was mainly found in various herbaceous meadows at some distance off the coastal line, where its abundance may be very high. Its occurrence in all other types of coastal plat associations in the study area was rather sporadic.

Figures

Figures 36 - 45. Xenylla arnei sp. nov. 36 dorsal chaetotaxy of head and thorax 37 dorsal chaetotaxy of Abd. 4 - 6 38 labial palp 39 sensorial equipment of Ant. 3 and Ant. 4 40 maxillary head 41 ventral chaetotaxy of Abd. 2 and Abd. 3 42 tip of leg 3 43 dens and mucro, lateral view 44 mucro, dorsal view 45 dens and mucro, ventral view. Scale bars: 0.1 mm (36, 37, 41), 0.01 mm (38 - 40, 42 - 45).

Figures 46 - 50. Xenylla arnei sp. nov. (46, 47) and X. humicola (48 - 50) 46 dorsal chaetotaxy of Abd. 1 47 ventral chaetotaxy of head 48 labial palp 49 maxillary head, specimen from Kola Peninsula 50 maxillary head, specimen from Chukotka. Scale bars: 0.1 mm (46, 47), 0.01 mm (48 - 50)