Hypogastrura variata, 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

Hypogastrura variata sp. nov. Figs 1-19 , 20-23

Type material.

Holotype Russia, North-East • ♂; Magadan Province, Ola; 59°36.19'N, 151°21.72'E; maritime marsh with Carex subspataceae ; July 2017; M. Bizin and B. Efeykin leg.

Paratypes Russia, North-East • ♀; same data as for holotype • 13 ♀♀, 6 ♂♂ and 10 juveniles; same region, but Tauisk; 59°44.07'N, 149°23.32'E; maritime marsh with Puccinellia phryganodes ; July 2017; M. Bizin and B. Efeykin leg. The types are deposited in MSPU.

Additional material.

more than 300 specimens (alcohol), mainly from the P. phryganodes plant association of the same region. Several specimens from this material were sequenced (Table 1 ). Their partial COI genes were amplified and deposited in the GenBank under the sample ID: KY066780-KY066783.

Diagnosis.

A species of the genus Hypogastrura Bourlet, 1839, with four weakly differentiated, curved, sensory setae (one dorsal and three external) on Ant.4, the relatively clearly differentiated dorsal setae, the tridentate retinaculum, the basal lamella on the unguiculus, one tenent seta on all tibiotarsi, the partly reduced furca with four or five posterior setae, and the highly variable shape of the mucro.

Description.

Length of males 1.2-1.5 mm, females 1.2-1.8 mm, holotype 1.41 mm long. Colour dark, bluish black, not paler ventrally. Granulations fine and uniform, with 14-18 granules between setae p1 on Abd.5. Ant.4 with a simple apical bulb and four weakly differentiated, curved, sensory setae (one dorsal [S3?] and three external [S7, S8, and S9), subapical organite (or) and microsensillum (ms) present as usual (Figs 1 , 2 ). Ant.3 organ typical of the genus, with all usual sensorial elements: two outer guards, two inner sensilla and a lateral microsensillum. Ant.1 and Ant.2 with seven and 13 setae, respectively. Head with 8+8 virtually equal ocelli. PAO slightly smaller than nearest ocelli, usually with four subequal lobes (range 3-6), an accessory boss not developed (Figs 3 , 4 ). Distal edge of labrum with six low papillae, setal formula of labrum, 4/554. Labium typical of the genus, with all common papillae (A-E), 14 guards (a1, b1, b2, d2, and e2 shorter and set on low papillae) and six proximal setae, lateral process (lp) rudimentary (Figs 5 , 6 ). Basomedial field of labium (submentum) with four setae, basolateral field (mentum) with five setae, as usual. Head with 3+3 postlabial setae present along ventral line. Maxillary head unmodified, of general generic type, L.1 hardly longer than maxillary teeth, L.2 and L.3 with short marginal filaments and usually few denticles, all other lamellae densely covered with fine denticles (Fig. 7 ), outer lobe simple, with two sublobal hairs.

Dorsal chaetotaxy typical of the genus (Figs 20-23 ). Most dorsal setae stout and finely serrate, those on abdominal tip (Abd.5 and Abd.6) clearly longer and rougher, sensorial setae thin and not especially long compared to ordinary ones. Main characteristics: detectible differentiation into micro- and meso- or macrosetae on all terga including head in adults (Figs 20-22 ) and, especially, juveniles (Fig. 23 ), usual presence of an additional seta in p-row on head (six p-setae totally) and abnormal variability (unusual shapes, absence or doubling of certain setae). Chaetotaxy of legs 1-3 as follows: upper subcoxae - 1, 2, 3 (among them, one macroseta on each subcoxa); lower subcoxae - 0, 3, 3; coxae - (2)3, 3, 3; trochanters - 7(8), 7(8), 7; femora - 13-14, 13-14, 12-14; tibiotarsi - 19, 19, 18 setae, respectively. Tenent tibiotarsal setae (A1) of moderate length, ~ as long as 1.1-1.5 inner unguis edge, truncate or indistinctly clavate. Unguis slender and usually toothless, rarely an indistinct tooth present in midsection of inner edge. Unguiculus with a clear basal lamella, its apical filament reaching the middle of inner unguis edge or slightly above (Fig. 19 ). Ventral tube with 4+4 distal setae. Retinaculum with 3+3 teeth. Furca short (Figs 8 , 9 ), mucro rudimentary, sometimes completely absent (often asymmetrically), its shape highly variable (Figs 10-18 ). Manubrial field with 10-12+10-12 ventral setae including one or two basolateral macrosetae (Fig. 9 ). Dens usually with four or five posterior setae (whole range 2-6), one of which ~ as long as dens+mucro or even longer (ratio = 0.9-1.2:1). Mucrodens slightly longer than inner edge of hind unguis (1.0-1.3 ×). Anal spines rather strong and slightly curved, set on high contiguous papillae.

Variability.

One of the most characteristic features of the new species is its high-level variability of such important morphological traits as the number of PAO lobes and dental setae, as well as the shape and presence of a mucro (Table 2 ). This may be assumed as a direct consequence of rather severe conditions of boreal maritime marshes.

Etymology.

The name of the new species is intended to reflect the morphological variability.

Affinities.

Apart from the new species, there are only four congeners known in the world fauna that are characterised by a shortened furca with five or fewer dental setae, coupled with a tridentate retinaculum and only one tenent seta on each leg: H. oreophila Butschek, 1948, H. exigua Gisin, 1958, H. mongolica (Nosek, 1976), and H. magistri Babenko, 1994. The first two species are from high-montane habitats in the European Alps, and both have been recently redescribed ( Skarżyński 2011). They are much smaller than H. variata sp. nov. (0.8 mm vs. 1.2-1.8 mm) and have short, undifferentiated, dorsal setae and a longer furca (dens+mucro/U3 with ratio ~ 2 vs. 1.0-1.3 in H. variata sp. nov.). In addition, H. oreophila is characterised by the presence of m-setae on Abd.5, a broadened maxillary L.1, an inner tooth on the unguis and a hook-like mucro with a broad outer lamella, whereas H. exigua shows a larger PAO (~1.5 ocellus) and more numerous setae on VT (5+5 vs. 4+4 in H. variata sp. nov.).

The two other similar species, H. mongolica and H. magistri , are known from mountainous regions of Central Asia (northern Mongolia and western Tuva). Of these, H. magistri can easily be distinguished due to the presence of six or seven curved sensilla on Ant.4 (vs. four in H. variata sp. nov.) and the presence of additional setae on Abd.4 and Abd.5. As regards H. mongolica , its comparison with H. variata sp. nov. is impossible, because the holotype, the only known specimen, of H. mongolica was immature ( Skarżyński 2011). According to the original description ( Nosek 1976) and redescription of the type ( Skarżyński 2011), H. mongolica differs from H. variata sp. nov. in being smaller (0.6 mm long) and lighter in colouration, the "body clothed sparsely with short setae", long tergal sensilla and an inner tooth on the unguis, but at least some of these characters may reflect its immature status. Nevertheless, H. mongolica and H. variata sp. nov. are unlikely to be synonymous from an ecological point of view alone, because their habitat preferences are drastically different: litter of a mountain forest vs. a saline maritime marsh.

There are another four known Palaearctic congeners which may be related to the above group: H. capitata Cassagnau & Delamare, 1955 (Lebanon), H. verruculata Rusek, 1967 (China), H. ramia Lee & Choe, 1979 (South Korea), and H. pizzoci Fanciulli & Dallai, 2008 (Italy). All of them are also characterised by the presence of a single tenent seta on each leg, and the unguiculus with a basal lamella and a tridentate retinaculum, but they all share a complete, less strongly reduced furca with six posterior setae.

Molecular data.

Unfortunately, the GenBank does not contain sequences of any of the above-mentioned species. Therefore, the isolated position of H. variata sp. nov. among fifteen taxonomic units of Hypogastrura present in the GenBank is not particularly surprising and may well serve as an additional confirmation of its independent status. Molecular data have shown that the divergences between all units considered are rather high (Table 3 ). The average interspecific divergence between all species is 26.3% (ranging from 15.9 to 36.4%), while it is 26.7% for H. variata sp. nov. and the other fourteen species (ranging from 23.0 to 31.4%). Nevertheless, it seems noteworthy that the molecular trees obtained fail to fully reflect the relationships within the genus Hypogastrura based on morphological evidence alone. The most apparent assumption explaining this fact is that the molecular data are still too scant to realistically construct reliable trees that would adequately reflect the real phylogenetic relationships within the genus.

Distribution and ecology.

Hypogastrura variata sp. nov. was collected in two neighbouring sites located on the northern shore of the Sea of Okhotsk, both in the vicinity of Magadan. It seems to inhabit a narrow belt of mudflat maritime marshes, i.e., a monodominant plant association Puccinellietum phryganodis , where it achieves very high abundance levels and is the most common collembolan species. Its occurrence in all other types of marsh in the study area was sporadic.

Figures

Figures 1 - 19. Hypogastrura variata sp. nov. 1 sensorial equipment of Ant. 3 and Ant. 4 2 tip of Ant. 4 3 PAO and nearest ocelli 4 PAO, different specimen 5 labial palp 6 labial papilla E 7 maxillary head 8 furca and retinaculum, lateral view 9 sternum of Abd. 4 10 - 18 mucro, different specimens 19 tip of leg 3. Scale bars: 0.1 mm (9), 0.01 mm (1 - 8, 10 - 19).

Figures 20 - 23. Hypogastrura variata sp. nov. 20 chaetotaxy of head and Th. 1 21 chaetotaxy of Th. 2 and Th. 3, lateral view 22 chaetotaxy of Abd. 2 - 6, lateral view 23 chaetotaxy of juvenile, I instar. Scale bars: 0.1 mm.