Pseudocrangonyx daejeonensis, Lee, Chi-Woo, Tomikawa, Ko, Nakano, Takafumi & Min, Gi-Sik, 2018

Lee, Chi-Woo, Tomikawa, Ko, Nakano, Takafumi & Min, Gi-Sik, 2018, A new species of the genus Pseudocrangonyx (Crustacea, Amphipoda, Pseudocrangonyctidae) from Korea, ZooKeys 735, pp. 27-44 : 30-36

Pseudocrangonyx daejeonensis sp. n. Figs 2, 3, 4, 5, 6, 7, 8, 9

Material examined.

Holotype: Female (NNIBRIV1, 3.8 mm), Heukseok-dong (36°15.65'N, 127°20.43'E), Daejeon, Korea, collected by Lee CW on 31 May 2017. Paratypes: 1 male (NNIBRIV2, 2.7 mm), 1 female (NNIBRIV3, 2.3 mm), 3 females (KUZ Z1924), data same as for holotype.

Etymology. The specific name is an adjective derived from the type locality name of the new species.

Description.

Female [NNIBRIV1, 3.8mm]. Head (Fig. 2) with short dorsal setae; rostrum reduced; lateral cephalic lobe rounded; antennal sinus shallow with rounded angle; eyes absent. Dorsal margin of pleonites 1-3 and urosomites 1-2 with setae (Fig. 2). Ventral margin of urosomite 1 without setae (Fig. 2). Ventral margin of epimeral plate 1 with seta, posteroventral corner rounded with seta (Fig. 2); ventral and posterior margins of plate 2 each with seta, posteroventral corner rounded with seta (Fig. 2); ventral and posterior margins of plate 3 each with 2 setae, posteroventral corner rounded with seta (Fig. 2).

Antenna 1 (Fig. 3A) 0.38 times as long as body length, peduncular articles 1 to 3 in length ratio of 1.0: 0.5: 0.4; accessory flagellum (Fig. 3B) 2-articulate, terminal article with 3 setae and 1 aesthetasc; primary flagellum 10-articulate, 1 aesthetasc on some articles. Antenna 2 (Fig. 3C) 0.58 times as long as antenna 1; flagellum 0.65 times as long as peduncular articles 4 and 5 combined, consisting of 4 articles; calceoli absent.

Upper lip (Fig. 3D) with rounded anterior margin, bearing fine setae. Mandibles (Fig. 3F, G, H) with left and right incisors with 5- and 4-dentate, respectively; left lacinia mobilis 4-dentate, right lacinia bifid, bearing many teeth; molar process triturative; accessory setal rows of left and right mandibles with 3- and 2- pectinate setae, respectively; palp 3-articulate, article 3 with 1 A-, 7 D-, and 3 E-setae. Lower lip (Fig. 3I) with broad outer lobes with fine setae, mandibular process of outer lobe rounded apically; inner lobes indistinct. Maxilla 1 (Fig. 3J) with inner and outer plates, and palp; inner plate subovate with 2 plumose setae; outer plate subrectangular with 7 serrate teeth apically; palp 2-articulate, longer than outer plate, article 2 with 2 apical robust setae. Maxilla 2 (Fig. 3K) with oblique inner row of 2 setae on inner plate. Maxilliped (Fig. 4A) with inner and outer plates, and palp; inner plate reaching base of palp article 1, with 2 apical robust setae; outer plate not exceeding end of palp article 1, with 2 plumose setae and some medial setae; palp 4-articulate, medial margin of article 2 lined with setae, article 4 with nail.

Gnathopod 1 (Fig. 4B) with subquadrate coxa, bearing setae on its anterodistal and posteroventral corners, width 1.9 times as long as depth; posterior margin of basis with 3 setae; posterodistal corner of carpus with slender setae, some weakly pectinate; propodus stout, subchelate, palmar margin with 3 medial and 3 lateral robust setae; posterior margin of dactylus dentate (Fig. 4C). Gnathopod 2 (Fig. 4D) with subquadrate coxa, bearing setae on its anterodistal and posteroventral corners, width 1.6 times as long as depth; posterior margin of basis with 4 setae; posterodistal corner of carpus with slender setae, some weakly pectinate; propodus stout, subchelate, palmar margin with 7 medial and 2 lateral robust setae; posterior margin of dactylus dentate (Fig. 4E). Pereopod 3 (Fig. 5A) with subquadrate coxa bearing setae on its anterodistal and posteroventral corners, width 1.6 times as long as depth; anterior and posterior margins of basis with 2 and 4 setae, respectively; merus, carpus, and propodus in length ratio of 1.0: 0.9: 0.8; posterior margin and submargin of dactylus each with seta (Fig. 5B). Pereopod 4 (Fig. 5C) with subquadrate coxa bearing setae on its anterodistal corner, width 1.7 times as long as depth; anterior and posterior margins of basis each with 4 setae; merus, carpus, and propodus in length ratio of 1.0: 0.7: 0.8; posterior margin and submargin of dactylus each with seta (Fig. 5D). Pereopod 5 (Fig. 6A) with weakly bilobed coxa bearing setae on anterior and posterior lobes, width 1.7 times as long as depth; anterior and posterior margins of basis with 4 and 6 setae, respectively; merus, carpus, and propodus in length ratio of 1.0: 0.7: 0.8; anterior margin of dactylus with 2 setae (Fig. 6B). Pereopod 6 (Fig. 6C) with coxa bearing concave lower margin, marginally bare; anterior and posterior margins of basis with 5 and 3 setae, respectively; merus, carpus, and propodus in length ratio of 1.0: 0.8: 0.9; anterior margin of dactylus with 2 setae (Fig. 6D). Pereopod 7 (Fig. 6E) with subtriangular coxa, bearing seta on posteroproximal corner; anterior and posterior margins of basis with 3 and 4 setae, respectively; merus, carpus, and propodus in length ratio of 1.0: 0.8: 1.1; anterior margin of dactylus with 2 setae (Fig. 6F).

Coxal gills (Figs 4D, 5A, C, 6A, C) on gnathopod 2 and pereopods 3-6; sternal gills absent. Brood plates (Figs 4D, 5A, C, 6A) slender, with numerous setae, on gnathopod 2 and pereopods 3-5.

Peduncle of pleopod 1 (Fig. 7A) with 1 outer marginal and 1 outerdistal seta; peduncle of pleopod 2 (Fig. 7C) with outerdistal seta; peduncle of pleopod 3 (Fig. 7D) lacking marginal and distal setae. Pleopods 1-3 with paired retinacula (Fig. 7B), and lacking bifid setae (clothes-pin setae) on inner basal margin of inner ramus; inner ramus of pleopods 1-3 3-, 3-, and 2-articulate (Fig. 7A, C, D); outer ramus of pleopods 1-3 4-, 3-, and 2-articulate (Fig. 7A, C, D).

Uropod 1 (Fig. 7E) with basofacial seta on peduncle; inner ramus 0.87 times as long as peduncle, inner margin of former with 2 robust setae, outer margin bare, basal part with slender seta; outer ramus 0.63 times as long as inner, marginally bare. Uropod 2 (Fig. 7F) with inner and outer rami; inner ramus 1.10 times as long as peduncle, its inner margin with robust seta, outer margin without setae; outer ramus 0.68 times as long as inner ramus, marginally bare. Uropod 3 (Fig. 7G) with peduncle 0.34 times as long as outer ramus, with 1 robust and 1 slender setae; inner ramus absent; outer ramus 2-articulate, proximal article with robust setae, terminal article 0.32 times as long as proximal article, with 3 distal setae. Telson (Fig. 7H) length 1.3 times as long as wide, cleft for 0.08 times of length, each telson lobe with 2 lateral penicillate setae, 1 apical robust and 1 apical short setae.

Male [NNIBRIV2, 2.7 mm]. Antenna 1 (Fig. 8A, B) 0.46 times as long as body length, primary flagellum 7-articulate, 1 aesthetasc on some articles. Antenna 2 (Fig. 8C) 0.57 times as long as antenna 1; flagellum 0.72 times as long as peduncular articles 4 and 5 combined, consisting of 4 articles, first 2 of which with calceoli (Fig. 8D).

Gnathopod 1 (Fig. 8E) with coxa width 1.84 times as long as depth; palmar margin with 3 medial and 3 lateral robust setae (Fig. 8F). Gnathopod 2 (Fig. 8G) with coxa width 1.66 times as long as depth; palmar margin with 3 medial and 4 lateral robust setae (Fig. 8H).

Uropod 1 (Fig. 9A) with robust seta on inner margin of inner ramus; outer ramus 0.62 times as long as inner. Uropod 2 (Fig. 9B) with 2 serrate and 4 simple robust setae and slender seta at distal part. Uropod 3 (Fig. 9C) with peduncle 0.32 times as long as outer ramus; terminal article of outer ramus 0.5 times as long as proximal article.

Variation. Peduncle of pleopod 1 with or without seta on outer margin.

Distribution.

This species is known only from the type locality.

Molecular phylogenetic position.

The BI tree (mean ln L = −14039.10; Fig. 10) for estimating the phylogenetic position of the new species had an identical topology to that of the ML tree (ln L = −14504.12; not shown). Pseudocrangonyx daejeonensis belonged to a well-supported clade (BS = 98 %, PP = 0.99) containing the three phylogroups known from the western parts of Honshu and Shikoku, i.e., Pseudocrangonyx spp. 3-5. The new species formed a clade (BS = 91 %, PP = 0.99) with Pseudocrangonyx sp. 3 inhabiting the eastern part of Shiga Prefecture, Japan. Monophyly of the present specimens of P. daejeonensis was fully-supported (BS = 99 %, PP = 1.0).

Remarks.

Pseudocrangonyx daejeonensis is morphologically similar to P. coreanus in having 1) relatively small body size (smaller than 6 mm), 2) eyes completely absent, 3) carpus of gnathopod 2 without serrate robust setae on posterodistal corner, 4) outer margin or outer distal corner of pleopods 1 and 2 with setae, 5) inner basal margin of inner ramus of pleopods without bifid setae, and 6) small number of articles (less than 5) of rami of pleopods. However, the former is distinguished from the latter by the following features (features of P. coreanus in parentheses): 1) antenna 1 shorter (longer) than 0.4 times as long as body length, 2) antenna 2 of female without calceoli (with calceoli), 3) uropod 1 not exceeding (slightly exceeding) tip of uropod 2, and 4) outer ramus of uropod 2 without robust seta (with robust seta).

Pseudocrangonyx daejeonensis is also similar to P. febras Sidorov, 2009 and P. gudariensis Tomikawa and Sato in Tomikawa et al. (2016) in having 1) relatively smaller body size, 2) eyes completely absent, and 3) urosomite 1 without basal setae. However, P. daejeonensis is distinguished from these two species by the following features: from P. febras (features of P. febras in parentheses), 1) antenna 1 shorter than 0.4 times as long as body length (longer than 0.7 times), 2) peduncular article 2 of antenna 1 0.5 (0.7) times as long as article 1, 3) palp article 2 of mandible with 3 (7) setae, 4) carpus of male gnathopod 2 without serrate robust setae on posterodistal corner (with serrate robust setae), 5) fewer articles of pleopodal rami, up to 4 (more, up to 6), 6) inner ramus of uropod 1 with 2 inner marginal robust setae (5 inner and 3 outer marginal robust setae), 7) outer ramus of uropod 1 without setae (with 2 robust setae), and 8) inner ramus of uropod 2 with inner robust seta (3-4 inner and 2-3 outer marginal robust setae); from P. gudariensis (features of P. gudariensis in parentheses), 1) basal part of inner ramus of uropod 1 with 1 slender setae (with 3 slender setae), 2) outer ramus of uropod 1 without setae (with 2 setae), 3) inner margin of inner ramus uropod 2 with 1 robust setae (with 4 robust setae), and 4) telson lobe with 1 robust seta apically (with 2 robust setae apically).

Although the phylogenetic position of P. coreanus remains uncertain, the results of the previous molecular phylogenetic studies ( Tomikawa et al. 2016; Zhao and Hou 2017) and our phylogenetic analyses showed that P. daejeonensis and the two morphologically similar species, P. febras and P. gudariensis , did not form a monophyletic lineage with large genetic divergences. Because these three species inhabit interstitial waters, not subterranean habitats, morphological similarities observed among them may reflect their similar habitat preferences.

The phylogenetic position of P. daejeonensis also sheds light onto the complex faunistic relationships between the Pseudocrangonyx species inhabiting the Japanese Archipelago and those inhabiting the Far Eastern continental area. Common ancestors of the Japanese Pseudocrangonyx species were considered to have migrated from the continental part to the Japanese Archipelago ( Sidorov and Holsinger 2007). Previous systematic studies revealed that the Pseudocrangonyx amphipods distributed in northern Japan and the western tip of Honshu, Japan, i.e., P. yezonis and Pseudocrangonyx sp. 2, are phylogenetically close to the continental species ( Sidorov and Holsinger 2007; Tomikawa et al. 2016; Zhao and Hou 2017). As P. daejeonensis formed a well-supported clade with Pseudocrangonyx sp. 3, which is indigenous to the central part of Honshu, their phylogenetic relationship suggested that the species diversity of the Japanese Pseudocrangonyx has been increased as a result of multiple continental-origins. It is also feasible that P. daejeonensis diverged from a common ancestor indigenous to the Japanese Archipelago. To clarify the biogeographical history of Pseudocrangonyx amphipods, further faunistic surveys along with molecular phylogenetic analyses are essentially needed.

The uncorrected p-distance of 15.0 % for COI, calculated using MEGA7.0.16 ( Kumar et al. 2016) between P. daejeonensis and Pseudocrangonyx sp. 3 is equivalent to sequence divergence thresholds for discriminating amphipod species ( Witt et al. 2006; Rock et al. 2007; Hou et al. 2009). The former is distinguished from the latter in having the following features (features of Pseudocrangonyx sp. 3 in parentheses): 1) outerdistal corner of peduncle of pleopod 3 without seta (with seta), 2) each of inner and outer ramus of pleopod 3 2-articulate (3-articulate), 3) outer rami of uropods 1 and 2 without marginal robust setae (with marginal seta), and 4) robust setae on distal part of proximal article of uropod 3 short, not reaching tip of terminal article (long, exceeding tip of terminal article) (Tomikawa pers. observation).