Nipponasellus matsumotoi, Sidorov, 2023

Sidorov, Dmitry, 2023, Trans-Japan Sea land-bridge disjunction: A case of vicariance in the subterranean genus Nipponasellus (Crustacea, Isopoda, Asellidae) in a largescale biogeographical context, Zootaxa 5357 (3), pp. 342-374 : 358-363

Nipponasellus matsumotoi spec. nov.

Figures 4C, D , 5E–H , 10–13 , ESupplement4

Diagnosis (both sexes): Body slender, vermiform-like, but clearly flattened dorsoventrally, lateral margins subparallel, semitransparent in color, spotted with white-coloured muscular prints. Head sub-rectangular, terete, longer than broad. Dorsal surface of head and body covered with minute, leaf-shaped scales. Eyes absent. Antennula length ca. 80% of antennal podomeres length. Antenna length ca. 60% of total body length. Pereopods 1–7 notably increasing in length towards posterior pairs. Pleopod 2 endopodite in males specialized. Uropod long, ca. 12% longer than pleotelson. Body length 3.0– 4.2 mm (♀♀), 5.0 mm (♁♁).

Etymology: Species named after Koichi Matsumoto (Tokyo, Japan), who pioneered the study of the East Asian subterranean animals.

Bionomics (see map, Fig. 1 ): Nipponasellus matsumotoi spec. nov. is a phreatobite that lives in groundwater near the confluence of the Tigrovaya and Partizanskaya Rivers in the foothills of the southern slope of Mount Livadiyskaya (also known as Pidan). Known only from the type locality. Ecology is mostly unknown, but it was accompanied by the following stygobiont fauna: Mackinia sp. ( Isopoda , Janiridae ) and Pseudocrangonyx sp. ( Amphipoda , Pseudocrangonyctidae ).

Material examined: Holotype: male, 5.0 mm, X54659/Cr-2477-FEFU, Russia, Primorsky Krai, Partizansky District, Tigrovaya River (tributary of Partizanskaya River ), channel 3- Tigrovy ( Code : 4AL), near r/bridge, 43º11.401’ N 133º12.660’ E (altitude 103 m a.s.l.), groundwaters, ca. 0.3 m depth, stones, pebbles, sand mix, right side bar, riffle, 12 Jun. 2010, coll. D. Sidorov, K. Semenchenko. Paratypes: 3 females (3.75 mm, 3.4 mm, 3.0 mm), X54660/Cr-2478-FEFU, channel 3-Tigrovy (Code: 12Lazo), 43º11.247’ N 133º12.346’ E (altitude 100 m a.s.l.), 1.0 m depth, 05 Aug. 2009, other data similar to holotype. Material for SEM: Male 5.0 mm, female 4.2 mm, same data as holotype (extra specimen, not included to the type series).

Description (holotype X54659/Cr-2477 and specimen used for SEM): Body ( Figs 4C, D , 5E–H ) slender, vermiform but clearly flattened dorsoventrally, ca. 6.0 times longer as wide, lateral margins subparallel to each other and bearing simple setae; semitransparent, speckled with white-coloured muscular prints in alcohol; head sub-rectangular, terete, with frontal margin concave, without posterolateral expansion, ca. 0.3 longer than broad, post-mandibular lobes pointed, each with 3 or 4 setae; lateral expansion of pereonites 1–7 broadly rounded, each with 2 stiff setae dorsally, pereonites 1–4 with broadly rounded angles directed slightly anteriorly, pereonite 5 trapezoidal, broadly rounded angles directed posteriorly, pereonites 6 and 7 sub-similar, with almost straight or slightly concave distal margins, pereonite 7 the longest; coxopodites moderately developed, not visible from above besides 6 and 7; pleon 2-segmented; pleotelson oblong, tapered distally, ca. 0.3 times longer as wide and 22% of total body length, with median shallow convexity on posterior margin bearing 2 longer setae, lateral margins slightly convex, setiferous; paired genital papilla located sub-laterally on sternite of pereonite 7, small, twice shorter than protopodite of pleopod 1; sternal epithelium (blisters) on ventral surface of pereonites 1–7 absent.

Antennula ( Figs 4C, D , 5E–H , 10 ) length ca. 80% of antennal podomeres length; articles 1–3 length relations: 1: 0.8: 0.5, article 1 strong, article 2 with 2 broom setae at anterodistal angle; flagellum length ca. 0.5 times of podomeres length, of 6 articles, successive articles shorter, distal 3 articles with one minute aesthetasc with seta each. Antenna ( Figs 4C, D , 5E–H , 10 ) length ca. 60% of total body length; articles 2–6 length relations 0.2: 0.2: 0.3: 0.65: 1, article 4 with a group of stiff setae (one of which long) on distal margin, article 5 with a group of short stiff seta along margins, article 6 with cluster of stiff setae of different lengths on distal margin; flagellum length 62% of total antenna length, of 30 articles.

Labrum ( Fig. 10E ) sub-rounded with fine setae on apex. Labium ( Fig. 10F ) sub-triangular, split almost one-third, outer lobes rounded and tapered proximally. Mandibles ( Figs 10C, D ) sub-equal, with 5-cuspidate pars incisiva (incisor) and 5-cuspidate left lacinia mobilis, setal row of 5 serrate and 3–5 plumose stiff setae; palp 3-segmented, ca. 0.3 times shorter than mandibular corpus, article 1 with 1 short plumose distal seta, article 2 ca. 30% longer than article 1, with a row of 6 simple medial setae, article 3 curved, with a row of 7 simple setae. Maxillula ( Fig. 10G ) inner plate with 5 apical pappose setae; outer plate with 14 serrate robust setae, 1 plumose seta, 1 simple thin seta, 1 simple seta sub-apically, 1 simple seta on ventral face. Maxilla ( Fig. 10H ) inner plate longer than outer plate and as long as middle plate; broadly rounded, sub-triangular, with longer row of 14 simple setae apically, and with oblique row of 18 simple setae; lateral and middle plates with 12 and 16 simple setae, respectively. Maxilliped ( Fig. 10I ) basal endite broad, apex broadly rounded and reaching a half of palp article 3, about twice longer as wide, with 4 denticulate coupling hooks at distomesial margin, as broad as palp article 2, with mix of 19 simple and 6 pappose stiff setae distally, 6 or 7setae along distomesial margin, 8 simple slender setae of oblique row and with numerous short simple setae sub-apically on ventral surface; palp article 1 shortest, ca. 2.0 times as wide as long; article 2 strongly bulging medially, ca. 0.3 times shorter than article 4; articles 3–5 length relations: 0.5: 1: 0.5, with a few setae along outer margin, densely setose along inner margin; apex of epipodite broadly rounded, with a row of setae along lateral margin.

Pereopods 1–7 ( Figs 11A–G ) sub-similar in construction, notably increasing in length towards posterior pairs; pereopods 2–7 with 1 denticle and 1 seta along dactylus inner margin. Pereopod 1 length ca. 28% of body length; basis moderate, a little shorter than propodus, with single setae on superior and inferior margins, respectively; ischium length 65% of basis length; merus trapezoidal, dilated distally with moderately curved superior margin bearing 2 stiff setae on distal-superior angle, merus inferior margin 3 setae; carpus poorly developed, triangular, with 2 short stiff setae on inferior margin; propodus unicuspidate, slightly broad, oblong, ca. 2.3 times as long as wide, superior margin somewhat longer than palm, both margins convex, palmar margin armed with 4 strong, notched setae and 5 simple setae, ca. 0.7 of total length of palmar edge with pilosity pattern; dactylus nearly straight, ca. 0.2 times shorter than palm, with 6 denticles (one of which strong) along inferior margin and with 5 simple setae along superior margin, claw length 35 % of total dactylus length, with 1 sub-basal seta. Pereopod 2 ca. 17% longer than pereopod 1; basis–dactylus length relations: 0.9: 0.7: 0.6: 0.7: 1.0: 0.5; merus subrectangular, dilated distally with moderately curved superior margin bearing 3 stiff setae (2 of them very long) on distal-superior angle; propodus 10 % longer than carpus, with biungulate dactylus; claw about one-third as long as dactylus length. Pereopods 3–7 slender, similar to pereopod 2.

Pleopod 1 ( Figs 12A, B ) protopodite sub-rectangular, tapered distally, 1.2 times as long as wide, about 63% of exopodite length, with 1 or 2 weakly denticulate coupling hooks on medial margin; exopodite sub-ovoid, tapered proximally, ca. 2.0 times as long as wide, without concavity on lateral margin, with sparse short simple setae on terminal margin. Pleopod 2 ( Figs 12C , 13A–D ) protopodite sub-quadratic, slightly tapered distally, as long as wide, with a rarefied pattern of cuticular scales; exopodite ca. 0.26 times shorter than protopodite, article 1 trapezoidal, naked, terminal article elongated, catch lobe poorly developed, with 1 long and 2 short simple setae on medial and lateral margins, respectively; endopodite prominent, slightly curved, ca. 0.3 times longer than protopodite, tapered distally, ornamented with sparse finger-like (or ctenoid) cuticular scales close to terminal end, basal spur weak (or vestigial), ‘crypte’ weakly expressed, extended ca. 1/3 of total endopodite length, ‘cannula’ obscure, completely immersed in ctenoid cuticular scales and transformed into modified structure characterized by some specific accessories as ‘hook’, ‘heel’ and several minute outwards opening pores with filiform ‘sensilla’. Pleopod 3 ( Fig. 12D ) exopodite semicircular, somewhat tapered distally, with almost rectilinear medial and rounded lateral margins, 2.0 times as long as wide, divided by oblique line into two almost equal parts, terminal and lateral margins fringed with sparse simple setae; endopodite oblong, length ca. 70% of exopodite length. Pleopod 4 ( Fig. 12E ) exopodite sub-ovoid, 1.8 times as long as wide, terminal and lateral margins fringed with numerous minute hairs, respiratory area surface ca. 25% of exopodite surface; endopodite length ca. 90% of exopodite length. Pleopod 5 ( Fig. 12F ) exopodite elongate, ovoid, 2.1 times as long as wide, respiratory area surface ca. 50% of exopodite surface, line area as straight suture transverse from internal to lateral margin; endopodite elongate, ovoid, longer than wide. Uropod ( Fig. 12G ) long, ca. 12% longer than pleotelson; peduncle armed with stiff setae, distinctly longer than both rami, length relations: 1: 0.7: 0.3.

Variation: Not observed.

Sexual dimorphism: Females are generally similar to males except for being slightly smaller. Collected females did not have developed marsupial plates (oostegites). Pereopod 1 propodus is not sexually dimorphic; male and female pereopods are similar. Pereopod 4 not sexually dimorphic. Female pleopod 2 broadly sub-triangular, tapered distally, bearing 2 minute setae.

Taxonomic comments: Nipponasellus matsumotoi spec. nov. has a vermiform-like habitus with free pereonites, clearly stygomorphic, most similar to N. takefuensis (Matsumoto, 1961) . However, it can be distinguished from the latter by (characters of N. takefuensis in brackets): mandibular palp 3-articulated (reduced, 1-articulated); antenna length in males about 60% of total body length (up to 100%) (see Table 2 ). In contrast to the remaining species of Nipponasellus and all other East Asian asellids, N. matsumotoi spec. nov. has a unique morphology of the ‘appendix masculina’. Neither the sickle-shaped endopodite of pleopod 2 in males of the remaining Japanese Nipponasellus nor the extremely diverse and divergent North American Caecidotea -related taxa resemble the ‘cannula’ completely immersed in ctenoid cuticular scales. Nevertheless, N. matsumotoi spec. nov. should be considered only as a highly specialised member of the genus Nipponasellus , since these unique external morphological features are not sufficient to justify the establishment of a new genus. Moreover, considering that the species is only known from the type locality, it is hard to make any reliable judgement about its origin.

Figures

FIGURE 1. Height map of the Japanese continental margin arc system showing distribution of the genus Nipponasellus in East Asia, with position of the phreatic sampling sites. Each point may represent 1–3 closely adjacent localities (for further explanation, see text).

FIGURE 4. Habitus of examined specimens: (A, B) Nipponasellus sudzukhensis spec. nov., holotype X54657/Cr-2475-FEFU, male, 5.5 mm, dorsal view (A); lateral view (B). (C, D) Nipponasellus matsumotoi spec. nov., holotype X54659/Cr-2477-FEFU, male, 5.0 mm, dorsal view (C); lateral view (D).

FIGURE 5. Head features in dorsal, lateral, ventral and anterior oblique views (maxilliped removed): (A–D) Nipponasellus sudzukhensis spec. nov., paratype X54658/Cr-2476-FEFU, female, 5.0 mm; (E–H) Nipponasellus matsumotoi spec. nov., paratype X54660/Cr-2478-FEFU, female, 4.2 mm.

FIGURE 10. Nipponasellus matsumotoi spec. nov., holotype X54659/Cr-2477-FEFU, male, 5.5 mm: (A) antennula; (B) antenna; (C) mandible, left; (D) mandible, right; (E) labium; (F) labrum; (G) maxillula, with enlarged outer plate; (H) maxilla, with enlarged inner plate; (I) maxilliped.

FIGURE 11. Nipponasellus matsumotoi spec. nov., holotype X54659/Cr-2477-FEFU, male, 5.5 mm: (A) pereopod 1, with enlarged palm; (B) pereopod 2; (C) pereopod 3; (D) pereopod 4; (E) pereopod 5; (F) pereopod 6; (G) pereopod 7.

FIGURE 12. Nipponasellus matsumotoi spec. nov., holotype X54659/Cr-2477-FEFU, male, 5.5 mm: (A) pleotelson, ventral view, with enlarged section of coxopodite 7 (g.p., genital papilla); (B) pleopod 1; (C) pleopod 2, with schematic outline of details of appendix masculina; (D) pleopod 3; (E) pleopod 4; (F) pleopod 5; (G) uropod. Paratype X54660/Cr-2478-FEFU, female, 4.2 mm: (H) pleopod 2.

FIGURE 13. Scanning electron micrographs of pleopod 2 of Nipponasellus matsumotoi spec. nov., adult male: (A) pair of pleopods 2, ventral view; (B) endopodite of pleopod 2, right; (C, D) appendix masculina enlarged, right and left.