Nihonella, Ballarin & Yamasaki, 2021

Ballarin, Francesco & Yamasaki, Takeshi, 2021, Nihonella gen. nov., a new troglophilic genus of dwarf spiders from Japan with a discussion on its phylogenetic position within the subfamily Erigoninae (Araneae, Linyphiidae), European Journal of Taxonomy 733, pp. 1-18 : 6-8

Nihonella gen. nov.

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Figs 1 A–G, 2–4 ; Table 2

Type species

Nihonella chika gen. et. sp. nov.

Diagnosis

The new genus is distinguished from any other genera belonging to the distal Erigoninae clade by the following unique combination of somatic and genitalic characters: Femur I with 1 prolateral spine; Tibia I with 1 dorsal spine; tibial spine formula: 1.1.1.1; male palp with a well-developed distal suprategular apophysis and a hypertrophic ʻsecondaryʼ DSA ( Figs 2 A–B, 3A–C, 4A–E) (usually presented in Erigoninae as a simple tooth and protruding form a different side of the DSA); a well-developed and uniquely-shaped prolateral tibial apophysis, the same length as the cymbium and partially covering it ( Figs 2 A–C, 3A–B, D, 4E–F). The unusual chaetotaxy and unique shape of the epigyne, with anteriorly converging lateral walls and two distinct, flat, ovoid inflations of the copulatory ducts, also distinguishes the female of this genus from females of any other genera in distal Erigoninae .

Etymology

The generic name is a combination of the word ʻ Nihon ʼ and the Latin suffix ʻ -ella ʼ. The former refers to the country of Japan where the genus is endemic; the latter is the feminine suffix of ʻ -ellus ʼ commonly used in Latin to form the feminine diminutive of a noun. Name in apposition, feminine in gender.

Species included

Only the type species Nihonella chika gen. et. sp. nov.

Taxonomic remarks

The morphology of Nihonella gen. nov. suggests it may be closely related to the species of the Savignia group. However, the presence of numerous differences in genitalia shape and somatic characters does not allow us to include the new genus within this group of species. A close but distinct relationship with the Savignia group is also supported by the molecular analysis (see Fig. 5 ).

Nihonella gen. nov. male palps have a general morphology similar to those found in some genera of the Savignia group (e.g., Araeoncus Simon, 1884 or Diplocephalus Bertkau, 1883 ). They share a similar shape in the embolic division: a long, modified palpal tibia, and a well-developed DSA. However, the new genus shows a distinct hypertrophy of the SDSA, which is extremely long and clearly protruding outside the frontal part of the palpal bulb ( Figs 2 A–B, D, 3A–C, 4A–E). Within the subfamily Erigoninae , some genera belonging to the Savignia group (sensu Millidge 1977) have a large DSA and a tooth-like SDSA (e.g., Alioranus Simon, 1926 , Dactylopisthes Simon, 1884 , Delorrhipis Simon 1885 , Savignia Blackwall, 1833 , etc., see Millidge 1977: figs 128, 135–136, 139). However, none of the Savignia group display a SDSA as strongly developed as in Nihonella gen. nov. Females of Nihonella gen. nov. have an epigyne with two anteriorly converging lateral walls, which resembles the female genitalia, of most of genera within the Savignia group (e.g., Araeoncus Simon, 1884 , Diplocephalus Bertkau, 1883 , Erigonella Dahl, 1901 , Savignia Blackwall, 1833 , etc.). Nevertheless, both males and females of Nihonella gen. nov. have a highly distinctive chaetotaxy which strongly differ from the chaetotaxy usually found in species belonging to the Savignia group (1.1.1.1 vs 2.2.1.1). Although some species included in this group may occasionally have a tibial spine formula of 1.1.1.1, this usually only occurs in males and as a consequence of the reduction of the distal spines in tibia I and II (e.g., Araeoncus crassipes Heimer & Nentwig, 1991 = 1.1.1.1, A. humilis (Blackwall, 1841) = 0.0.1.1: Tanasevitch, in litteris). An exception is the genus Microctenonyx Dahl, 1886 , the female of which has a tibial spine formula of 1.1.1.1. However, Microctenonyx can be easily distinguished from Nihonella gen. nov. by the large genetic distance between the two genera (see Table 2 ), and by the shape of the epigyne and male palp (short SDSA, different shape of epigyne and internal ducts, see Figs 2 A–H, 3A–D vs Millidge 1977: fig. 140 and Bosmans 2007: figs 111–115).

Distribution

Endemic to Western Honshu, Japan. Currently known from three caves only ( Figs 1H , 6 ).

Figures

Fig. 1. Habitus and habitat of Nihonella chika gen. et. sp. nov. A. ♂, holotype (NSMT-Ar 20909), habitus, dorsal view. B. Ditto, ventral view. C. Ditto, lateral view. D. ♀, paratype (NSMT-Ar 20910), habitus, dorsal view. E. Ditto, ventral view. F. Ditto, lateral view. G. Ditto, cephalic region, frontal view. H. Entrance of Anatoyama Cave, type locality of the species. Scale bar = 1 mm.

Fig. 2. Genitalia of Nihonella chika gen. et. sp. nov. A. ♂, holotype (NSMT-Ar 20909), palp, retrolateral view. B. Ditto, prolateral view. C. Ditto, dorsal view. D. Ditto, ventral view. E. ♀, paratype (NSMT- Ar 20910), epigyne, ventral view. F. Ditto, dorsal view. G. Ditto, posterior view. H. Ditto, vulva after being cleared, ventral view. Scale bars: A–D = 0.2 mm; E–H = 0.1 mm.

Fig. 3. Nihonella chika gen. et. sp. nov. A. ♂, holotype (NSMT-Ar 20909), palp, retrolateral view. B. Ditto, prolateral view.C. Ditto, ventral view.D. Ditto, palpal tibia, dorsal view.E. ♀, paratype (NSMT- Ar 20910), epigyne, ventral view. F. Ditto, dorsal view. G. Ditto, vulva, dorsal view. Abbreviations: see Material and methods. Scale bar = 0.1 mm.

Fig. 4. Embolic division and details of the male palp of Nihonella chika gen. et. sp. nov. A. ♂, paratype (NSMT-Ar 20911), embolic division, ventro-retrolateral view. B. Ditto, frontal view. C. Ditto, dorsal view. D. Ditto, ventral view. E. ♂, paratype (NSMT-Ar 20910), palp under SEM microscope, anteroretrolateral view. F. Ditto, detail of the tip of the prolateral tibial apophysis. Abbreviations: see Material and methods.

Fig. 5. Bayesian inference phylogenetic tree of Erigoninae and other subfamilies of Linyphiidae based on the five concatenated genes discussed in the text. Each color denotes a different subfamily while the Savignia group is indicated in blue. The phylogenetic position of the genus Nihonella gen. nov. is highlighted in red. Numbers at each node denote posterior probability support. Branch lengths are scaled in relation to the number of substitutions per site.

Fig. 6. Distribution of Nihonella chika gen. et. sp. nov. in Western Japan. The star refers to the type locality.