Paramoera shakotanensis, Hagihara & Nakano & Tomikawa, 2020

Hagihara, Kumiko, Nakano, Takafumi & Tomikawa, Ko, 2020, A new species of Paramoera (Crustacea: Amphipoda: Pontogeneiidae) from an estuary habitat in Hokkaido, Japan, Journal of Natural History (J. Nat. Hist.) 54 (19 - 20), pp. 1279-1292 : 1280-1290

Paramoera shakotanensis sp. nov.

(New Japanese name: Shakotan-migiwa-yokoebi)

( Figures 1–5 )

Paramoera sp. : Tomikawa et al. 2014: fig 2; Tomikawa et al. 2017: fig. 2.

Paramoera sp. 1 : Nakano and Tomikawa 2018: fig. 4, table 1.

Type material

Holotype. Female (BL 4.6 mm), KUZ Z2041 , collected from Horonaifu River (43.332857°N, 140.410445°E), Shakotan, Hokkaido, Japan, by Masaki Kyono, on 13 May 2012.

Paratypes. In total 3 females: female (BL 5.3 mm), KUZ Z1939 ; female (BL 5.6 mm), KUZ Z2042 ( Figure 1 ); female (BL 5.1 mm), KUZ Z2043 ; data same as for holotype .

Diagnosis

Head with large eyes; peduncular article 2 gland cone of antenna 2 with 2 apical setae; lacinia mobilis of left mandible 5-dentate; mandibular palp article 3 without B-setae; inner plate of maxilla 1 with 3 plumose setae; gnathopod 2, carpus longer than propodus; coxa of pereopod 4 with shallow posterior concavity; coxal gills on gnathopod 2, and pereopods 3–7; peduncles of pleopods 1–3 with facial setae; uropod 2 inner ramus longer than outer ramus; telson longer than wide.

Description

Female [holotype, KUZ Z2041]. Body smooth. Rostrum ( Figure 2 (a)) short, weakly produced; lateral cephalic lobe mammilliform; inferior antennal sinus quadrate, corner of sinus rounded, not incised; eyes sub-oval, large, 0.4 times as high as head. Epimeral plates 1–3 ( Figure 2 (b–d)): lateral surface with many tiny setae; posterior margins without crenulation, with seta; posterodistal corners slightly pointed, with seta; plate 1 with long seta on ventral submargin; plate 3 with 1 or 2 short setae and short bifid seta on ventral submargin.

Antenna 1 ( Figure 2 (e,f)): length 0.5 times as long as body length; peduncular articles progressively shorter, length ratio of peduncular articles 1–3 being 1.0:0.7:0.5; peduncular article 1 with short setae on anterior margin, 2 single and 1 cluster of setae on posterior margin, posterodistal corner with single seta and pair of setae; peduncular article 2 with short seta on anterior margin, 2 clusters of setae on posterior margin, anterodistal and posterodistal corners with cluster of setae; peduncular article 3 with cluster of setae on posterior margin, anterodistal and posterodistal corners with cluster of setae; flagellum 17-articulate, about 1.9 times as long as peduncles, first article with 2 aesthetascs, articles 2, 4, 6, 8, 10, 12, 14 and 16 each with aesthetasc; accessory flagellum 1-articulate, scalelike, with 4 apical setae; calceoli absent. Antenna 2 ( Figure 2 (g,h)): length ratio of peduncular articles 3–5 being 1.0:1.9:1.9; gland cone length 0.8 times that of peduncular article 3, not prolonged, with 2 apical setae; peduncular article 3 with pair of setae on anterior margin, 3 setae on medial face of semi-circular elevation, single seta on lateral face, 3 setae on posterodistal corner; peduncular article 4 with 3 pairs or clusters of setae on anterior margin, 3 pairs of setae on medial surface, and a few short setae on posterior margin; peduncular article 5 with single seta and 2 pairs of setae on anterior margin, short single seta on posterior margin, 2 pairs of setae on medial surface, cluster of setae on anterodistal and posterodistal corners; flagellum with 10+ articles (some distal articles broken); calceoli absent.

Upper lip ( Figure 2 (i)) ventral margin convex, rounded, with minute setae. Left and right mandibular incisors ( Figure 2 (j,k)) 6-dentate, with left lacinia mobilis 5-dentate and right tridentate; left and right accessory setal rows with 6 and 5 blade setae, respectively; molar process triturative with plumose seta; palp 3-articulate, length ratios of left and right palp articles 1–3 being 1.0:2.7:2.4 and 1.0:2.5:2.2, article 1 bare, article 2 with 10 setae, article 3 with pair of A-, 2 C-, 5 D- and 7 E-setae, lateral surface with many fine setae. Lower lip ( Figure 2 (l)) outer lobes broad, setulose, mandibular lobes narrow; inner lobes indistinct. Maxilla 1 ( Figure 2 (m,n)) inner plate narrow with 3 plumose setae; outer plate rectangular with 10 serrate robust setae; palp 2-articulate; article 1 bare; article 2 with 6 robust setae and 1 slender seta on apical and subapical margins, respectively, outer margin without setae. Maxilla 2 ( Figure 2 (o)) inner plate with oblique inner row of 2 plumose setae; outer plate with about 19–20 slender setae subapically. Maxilliped ( Figure 3 (a)) inner plate not exceeding palp article 1, with 3 robust setae subapically, medial face with oblique row of plumose setae; outer plate exceeding palp article 1, with 2 thick plumose setae and robust setae apically; palp 4-articulate, article 2 oblong, with a row of setae, article 3 unlobate, article 4 shorter than article 3, nail present and not spinose along the inferior margin but with 3 sub-apical setae.

Gnathopod 1 ( Figure 3 (d,e)) shorter than gnathopod 2; coxa subrectangular, left and right coxae with 6 and 4 short setae on ventral margins, respectively; basis length about 3 times longer than wide, with long setae on anterior and posterior margins, and inner surface; ischium with setae on posterodistal corner; merus with long ventral setae; carpus 0.8 times as long as propodus, with finely serrate setae on posterodistal corner; propodus subrectangular, length 1.9 times longer than wide; left propodus with single seta, pair and cluster of setae on anterior margin, single seta and 2 clusters of setae on posterior margin, right propodus single seta and pair and cluster of setae on anterior margin, 2 clusters of setae on posterior margin, palm ( Figure 3 (e)) oblique, about 0.6 times as long as posterior margin, smoothly connected with posterior margin by 2 medial and 2 lateral robust setae; dactylus with seta on anterior margin and 2 short setae subapically, nail indistinct. Gnathopod 2 ( Figure 3 (f,g)) coxa with 6 setae on ventral margin; basis sub-linear, length about 3 times longer than wide, with anterior and posterior marginal setae; ischium with 3 setae on posterodistal corner; merus with long setae distally; carpus 1.2 times as long as propodus; propodus subrectangular, length 2.3 times longer than wide, with 2 pairs and cluster of setae on posterior margin, palm ( Figure 3 (g)) oblique, about 0.5 times as long as posterior margin, smoothly connected with posterior margin by 3 medial and 1 lateral robust setae in left gnathopod 2, by 4 medial and 1 lateral robust setae in right gnathopod; dactyl similar to that of gnathopod 1.

Pereopod 3 ( Figure 4 (a)) coxa ovate, with 6 short setae on ventral margin and robust seta on posterodistal corner; length ratio from basis to propodus 1.0:0.2:0.6:0.5:0.6; basis sub-linear, with long setae on anterior and posterior margins; ischium with 2 setae on posterodistal corner; merus with 2 setae on anterior margin, single seta and cluster of setae on posterior margin; carpus with setae on anterior and posterior margins; propodus with 2 setae on anterior margin, 2 robust setae and 1 slender seta on posterior margin; dactyls 0.4 times as long as propodus, bearing seta on anterior margin and 2 minute setae subapically. Pereopod 4 ( Figure 4 (b)) coxa with shallow posterior concavity, right ventral margins of left and right coxae with 10 and 8 short setae, respectively; length ratio from basis to propodus 1.0:0.3:0.6:0.6:0.6; anterior and posterior margins of basis with long setae; ischium with 1 or 2 setae on posterodistal corner; merus with 1 or 2 setae on anterior margin, 2 setae on posterior margin; anterior and posterior margins of carpus with 1 and 2 setae, respectively; propodus with 2 setae on anterior margin, single seta and pair of robust setae on posterior margin; dactyl 0.3 times as long as propodus, bearing single seta on anterior margin and 2 minute setae subapically. Pereopod 5 ( Figure 4 (c)) coxa bilobed, anterior lobe with small seta, posterior lobe with 2 robust setae and 1 small seta; length ratio from basis to dactylus 1.0:0.3:0.7:0.7:0.8:0.3; basis ovate, posteroventrally lobate; ischium with pair of setae on anterodistal corner; merus with single seta and pair of setae on anterior margin, robust seta on posterior margin; carpus with single seta and pair of robust setae on anterior margin, posterior margin with seta; propodus with 2 pairs of robust setae on anterior margin, single seta and pair of setae on posterior margin; dactylus with seta on posterior margin and 2 small subapical setae. Pereopod 6 ( Figure 4 (d)) coxa bilobate, posterior lobe with 1 robust and 1 slender setae; length ratio from basis to dactylus 1.0:0.3:0.8:0.7:0.8:0.3; basis ovate, posteroventrally lobate; ischium with 2 setae on anterodistal corner; merus with single seta and pair of setae on anterior margin, robust seta on posterior margin; carpus with single seta and pair of robust setae on anterior margin, posterior margin with 2 robust setae; propodus with 2 single and 1 pair of robust setae on anterior margin, single seta and pair of robust setae on posterior margin; dactylus with seta on posterior margin and 2 small subapical setae. Pereopod 7 ( Figure 4 (e)) coxa semicircular with 3 setae on posteroventral margin; length ratio from basis to dactylus 1.0:0.2:0.6:0.6:0.7:0.2; basis ovate, posteroventrally lobate; ischium with 2 setae on anterodistal corner; anterior and posterior margins of merus and carpus with 2 robust setae; propodus with single seta and 2 pairs of robust setae on anterior margin, 2 robust setae and 1 small seta on posterior margin; dactylus with seta on posterior margin and small subapical seta.

Coxal gills ( Figures 3 (f), 4(a–c,e)) ovate, on gnathopod 2 to pereopod 7. Sternal gill and hump absent. Brood plates ( Figure 3 (h,i)) on gnathopod 2, pereopods 3 and 4 large, brood plate on pereopod 5 small, narrow, strap-like.

Pleopods 1–3 ( Figure 5 (a–c)) peduncles with paired retinacula on inner distal margin, and with facial setae; pleopods 1–3 inner ramui 7-, 8- and 7-articulate, respectively, with bifid plumose setae (clothespin setae) on inner basal margins, terminal setae on rami length 0.7–0.9 times that of rami; outer rami 10-, 10- and 9-articulate, respectively.

Uropod 1 ( Figure 5 (d)) not extending beyond peduncle of uropod 3; peduncle with 2 and 4–5 robust setae along medial and lateral ridges, respectively, basofacial seta absent; inner ramus length 0.7 times that of peduncle, with 1–2 robust setae on inner margin; outer ramus length 0.9 times that of inner ramus, with outer marginal robust seta. Uropod 2 ( Figure 5 (e)) length 0.7 times that of uropod 1; peduncle with 1 medial seta and 2 lateral robust setae dorsally; inner ramus almost as long as peduncle, with 2 robust setae on inner margin; outer ramus length 0.7 times that of inner ramus, with outer marginal robust seta. Uropod 3 ( Figure 5 (f)) length 0.6 times that of uropod 1; both rami equal in length, length 1.2 times that of peduncle, uniarticulate, each ramus with 2 subterminal setae; inner ramus with 5 and 2 robust setae on inner and outer margins, respectively; outer ramus with 2 and 3 robust setae on inner and outer margins, respectively. Telson ( Figure 5 (g)) tapering distally, length 1.3 times longer than wide, cleft for 58%, lateral margin not concave, with several sub-lateral and facial setae, each lobe bearing 1 long seta and 1 short seta sub-apically, apex rounded.

Variation

Antenna 1 of one paratype ( KUZ Z2042 ) length 0.4 and 1.4 times that of body and antenna 2, respectively .

Distribution

Known only from the type locality.

DNA sequences

In total, four sequences of the present paratype (KUZ Z1939) were determined in previous studies ( Tomikawa et al. 2014, 2017; Nakano and Tomikawa 2018): nuclear 28S ribosomal RNA ( AB778502 ; 787 bp), histone H3 ( LC334142 ; 328 bp), mitochondrial 16S ribosomal RNA ( LC334116 ; 418 bp), and cytochrome c oxidase subunit I ( LC146870 ; 658 bp).

Etymology

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

Remarks

Paramoera shakotanensis sp. nov. is characterised by the antennal sinus without sharp incision and the small number of setae on maxillae 1 and 2 (up to 3 on the inner plate of maxilla 1 and up to 2 on the oblique inner row of the inner plate of maxilla 2). The present species shares these features with P. austrina ( Bate, 1862) , P. (H.) crassicauda Staude, 1995 , P. hermitensis Barnard, 1932 , and P. tristanensis Barnard, 1932 . Paramoera shakotanensis sp. nov. can be distinguished from these four species by the following features (features of comparison species in parentheses) ( Bate 1862; Barnard 1932; Staude 1995): from P. austrina by antennal sinus deep (shallow), epimeral plate 3 moderately (broadly) expanded posteriorly, and ventral margin of coxa of pereopod 4 rounded (almost straight); from P. crassicauda by eyes large (small, reduced) and pereopod 7 with coxal gill (lacking); from P. hermitensis by posterior margin of epimeral plate 3 smooth (posterior margin weakly serrate), telson length 1.3 (1.8) times longer than wide with almost straight (concave) lateral margins, and telson with sub-lateral and distal setae (distal setae only); and from P. tristanensis by posterior margin of epimeral plate 3 smooth (posterior margin slightly crenulate) and lateral margins of telson almost straight (convex) with (without) setae.

The original descriptions of P. fasciculata ( Thomson, 1880) , P. fissicauda ( Dana, 1852) and P. litoralis ( Oldevig, 1959) lack information on antennal sinus and setal numbers of maxillae 1 and 2 ( Dana 1852; Thomson 1880; Oldevig 1959). However, P. shakotanensis sp. nov. is distinguished from P. fasciculata and P. litoralis by the following features: palm of propodus of gnathopods 1 and 2 with 4 and 5 robust setae, respectively, on posteroproximal corners (2–3 and 3 robust setae, respectively, in P. litoralis ), telson tapering distally (lateral margins parallel in P. fasciculata ) with sub-lateral setae (lacking lateral setae in P. litoralis ).

The insufficient description of P. fissicauda makes it difficult to compare with the present new species. Bellan-Santini and Ledoyer (1974) described P. fissicauda based on materials from the Kerguelen and Crozet islands. Paramoera fissicauda as described by them has the sharply incised antennal sinus and the maxillae 1 and 2 with many setae, and thus obviously differs from P. shakotanensis sp. nov. However, Bellan-Santini and Ledoyer’s P. fissicauda has been considered an undescribed species ( De Broyer and Jazdzewski 1993). Accordingly, the taxonomic relationship between P. shakotanensis sp. nov. and P. fissicauda remains subject to future study.

Paramoera shakotanensis sp. nov. is also similar to P. anivae Labay, 2012 and P. erimoensis Kuribayashi and Kyono, 1995 in having an inferior antennal sinus lacking an incision, pereopod 7 with coxal gill, and rami of uropod 3 without plumose setae. However, P. shakotanensis can be distinguished from these two species by the following features ( Kuribayashi and Kyono 1995; Labay 2012) (features of P. anivae and P. erimoensis in parentheses): from P. anivae by peduncular article 2 gland cone of antenna 2 with 2 setae (with 6 setae), inner plate of maxilla 1 with 3 plumose setae (5 plumose setae), outer margin of palp article 2 of maxilla 1 without seta (with seta), and inner plate of maxilla 2 with oblique inner row of 2 plumose setae (3 plumose setae); and from P. erimoensis by inner plate of maxilla 1 with 3 plumose setae (5 plumose setae), inner plate of maxilla 2 with oblique inner row of 2 plumose setae (3 plumose setae), and carpus of female gnathopod 2 longer than propodus (shorter than propodus).

Paramoera shakotanensis sp. nov. possesses large eyes despite this species inhabiting interstitial habitats. Unlike the epigean species, the previously known hypogean Paramoera amphipods can be characterised by their eyes, which are vestigial or completely lacking, among other characteristics; and, thus, four subgenera have been erected for those species ( Staude 1995; Sidorov 2010). Of the four described subgenera, three subgenera, Moanamoera Staude, 1995 , Humilomoera Staude, 1995 and Rhithromoera Staude, 1995 , were erected by Staude (1995) for the subterranean and/ or interstitial species distributed in the North Pacific. The subgenus Moanamoera was established for the Paramoera amphipod inhabiting brackish lava ponds in the Hawaiian Islands ( Barnard 1977; Staude 1995), and the remaining two subgenera, Humilomoera and Rhithromoera , were erected for the interstitial subtidal and/or brackish pool species inhabiting the eastern coast of the North Pacific ( Staude 1995). Recently, an additional subgenus, Ganigamoera Sidorov, 2010 , was described for the stygobitic Paramoera species inhabiting inland freshwater habitats in the Russian Far East ( Sidorov 2010). The remaining species not included in those four subgenera have been assigned to the subgenus Paramoera ( Staude 1995) .

As described above, the new species bears morphological characteristics mostly consistent with the diagnosis of Humilomoera as defined by Staude (1995). However, the new species possesses large eyes and a coxal gill on pereopod 7, two characteristics that are at odds with Staude’s (1995) definition of this subgenus. Eye reduction is a character that can differ even among two populations of the same species of amphipod if, for example, one population lives underground and the other inhabits surface waters ( Culver et al. 1995). Therefore, the absence of eyes in the previously known Humilomoera species is deemed to be a highly derived character that is related to their interstitial habitats. Moreover, another subgenus, Rhithromoera , also contains species with and without a coxal gill on pereopod 7 ( Staude 1995).

It was stated that the subgenus-level classification of Paramoera remained unresolved, when the two subgenera Humulomoera and Rhithromoera were erected ( Staude 1995). Moreover, it was implied that the other subgenus, Ganigamoera , might not be a monophyletic taxon ( Sidorov 2010). The subterranean P. relicta was once classified within the genus Relictomoera Barnard and Karaman, 1982 , of which the type species is P. relicta . According to a systematic revision of P. relicta ( Nakano and Tomikawa 2018) , however, P. relicta was genetically close to the epigean P. koysama , synonymising Relictomoera with Paramoera . Since the precise phylogenetic relationships among P. relicta and the other hypogean species classified within the three subgenera remained unresolved, Relictomoera was not treated as a valid subgenus by Nakano and Tomikawa (2018). Therefore, the new species from Hokkaido is not assigned to the subgenus Humilomoera nor other subgenera, to avoid additional taxonomic confusion. To clarify the subgeneric assignment of P. shakotanensis sp. nov., as well as to test the validity of the subgenus-level classification of Paramoera , a molecular phylogenetic study should be conducted, along with an evaluation of the morphological characteristics of all Paramoera species.

A few Paramoera species show sexual dimorphism in their pleopods ( Kuribayashi and Kyono 1995; Sidorov 2010). The outer ramus of pleopod 2 in their males is modified to be more or less shortened and broadened with thickened short setae distally, whereas that of the females shows the usual form common in Paramoera ( Kuribayashi and Kyono 1995; Sidorov 2010). However, it remains unclear whether P. shakotanensis sp. nov. exhibits sexual dimorphism in its pleopods, since no males of the new species have been collected. The morphological characteristics of the males of P. shakotanensis sp. nov. should be documented by a future taxonomic study; the DNA sequences provided from the paratype of the new species will greatly help identify male individuals of the new species.

Figures

Figure 1. Paramoera shakotanensis sp. nov., paratype, female, KUZ Z2042, lateral view. Scale bar: 1.0 mm.

Figure 2. Paramoera shakotanensis sp. nov., holotype, female, KUZ Z2041. (a) Head, lateral view. (b–d) Epimeral plates 1–3, lateral views. (e) Antenna 1, medial view. (f) Accessory flagellum of antenna 1, medial view. (g) Antenna 2, medial view. (h) Gland cone of peduncular article 2 of antenna 2, medial view. (i) Upper lip, anterior view. (j), (k) Left and right mandible, medial views. (l) Lower lip, ventral view. (m) Right maxilla 1, anterior view. (n) Palp of left maxilla 1, anterior view. (o) Maxilla 2, anterior view. Scale bars: 0.1 mm.

Figure 3. Paramoera shakotanensis sp.nov., holotype,female,KUZ Z2041. (a) Maxilliped, anterior view. (b,c) Inner and outer plates of maxilliped, anterior views. (d) Gnathopod 1, medial view. (e) Palmar margin of propodus and dactylus of gnathopod 1, medial view. (f) Gnathopod 2, medial view. (g) Palmar margin of propodus and dactylus of gnathopod 2, medial view. (h,i) Brood plates of gnathopod 2 and pereopod 5, medial views. Scale bars: 0.1 mm.

Figure 4. Paramoera shakotanensis sp. nov., holotype, female, KUZ Z2041. (a–e) Pereopods 3–7, lateral (a,c) and medial (b,d,e) views. Scale bars: 0.1 mm.

Figure 5. Paramoera shakotanensis sp. nov., holotype, female, KUZ Z2041. (a–c) Pleopods 1–3, lateral (a,c) and medial (b) views. (d–f) Uropods 1–3, ventral views. (g) Telson, ventral view. Scale bars: 0.1 mm.