Molecular phylogenetic analyses
The monophyly of Jesogammarus was inferred with maximum (100% bootstrap support [BS]) and relatively low (0.85 posterior probability [PP]) support values in the maximum likelihood (ML) and Bayesian inference tree (BI) trees, respectively (Fig. 2). Although, Jesogammarus formed a sister group with Barrowgammarus Bousfield, 1979 (87% BS), their relationship was not supported by BI analyses. The new species collected in this study, J. (J.) acalceolus, was nested within Jesogammarus and clustered with J. (J.) hinumensis Morino, 1993 and J. (J.) ikiensis Tomikawa, 2015. In this study, the phylogenetic position of J. (A.) koreanus was also clarified for the first time: this species formed a sister group with J. (A.) debilis, with high support values (98% BS, 1.0 PP). Of the 22 species of Jesogammarus, 20, excluding Jesogammarus (Jesogammarus) fontanus Hou & Li, 2004 and Jesogammarus (Jesogammarus) ilhoii Lee & Seo, 1992, were included in the molecular phylogenetic analyses of this study.
Ancestral state reconstruction
The likelihood reconstruction (Fig. 2) demonstrated that the calceolus on male antenna 2 was an ancestral character state of the most recent common ancestor (MRCA) of the Jesogammarus species, with 0.96 proportional likelihood (PL). The character state of the MRCA of J. (J.) acalceolus sp. nov. + J. (J.) hinumensis and J. (J.) acalceolus sp. nov. + J. (J.) hinumensis + J. (J.) ikiensis was the presence of calceolus, with 0.92 and 0.99 PL, respectively. The character state of the MRCA of Barrowgammarus + Eogammarus + Jesogammarus + Ramellogammarus was the presence of calceolus, with 0.52 PL.
Taxonomic account
Family Anisogammaridae Bousfield, 1977
Genus Jesogammarus Bousfield, 1979
Type species. Anisogammarus jesoensis Schellenberg, 1937
Diagnosis. Pleonites not carinate dorsally, with slender and robust setae (robust setae often lacking). Dorsal margins of urosomites with 4 (3), 4 (2), 2 (4) clusters or single robust setae; urosomite 2 without prominent median tooth. Antenna 1 longer than antenna 2; article 1 of peduncle subequal to or slightly longer than article 2. Male antenna 2, flagellum with or without calceoli. Maxilla 1, palp article 1 without setae. Female gnathopods 1 and 2 strongly dissimilar. Coxal gills on gnathopod 2 and pereopods 3–7, gills 2–5 each with 2 accessory lobes, gills 6 and 7 each with 1 accessory lobe. Uropods 1 and 2, rami extending beyond peduncle of uropod 3. Uropod 3, inner ramus not longer than 0.4 times of that of outer ramus; terminal article distinct. Brood plate 2 of female broadly expanded anteroproximally.
Remarks. The presence of a calceolus on the flagellum of male antenna 2 is a major diagnostic feature of Jesogammarus, which distinguishes it from Ramellogammarus [21,22]. However, the discovery of the new species, J. acalceolus, which lacks a calceolus, indicated that the calceolus was not critical for diagnosis. The genus Jesogammarus is distinguishable from Ramellogammarus by the dissimilar female gnathopods 1 and 2 and the expanded brood plates of the female. The genus Jesogammarus shares a similar coxal gill type with marine Locustogammarus Bousfield, 1979 and Spasskogammarus Bousfield, 1979 but differs from these two genera in terms of the following features (features of Locustogammarus and Spasskogammarus in parentheses): from Locustogammarus, in terms of longer antenna 1 than antenna 2 (subequal), dissimilar female gnathopods 1 and 2 (similar), uropods 1 and 2 with rami extending beyond the peduncle of uropod 3 (not extending), and a distinct terminal article of uropod 3 (very small); from Spasskogammarus, in terms of dorsal margins of pleonites with slender setae (lacking), longer antenna 1 than antenna 2 (subequal), and slender pereopods 5–7 (short).
Jesogammarus (J.) acalceolus sp. nov.
[New Japanese name: Shitsuko-yokoebi]
Figures 1B, 3–5
Material examined. Holotype: male (7.4 mm, NSMT-Cr 29003), Haguro Shrine Spring, Hirosaki, Aomori Prefecture, Japan (40.6153°N, 140.3854°E), collected by A. Ohtaka, N. Kimura, and K. Tomikawa on 10 December 2020. Paratypes: two females (7.3 mm, NSMT-Cr 29004; 6.7 mm, NSMT-Cr 29005 [G1845]), two male (7.7 mm, NSMT-Cr 29006; 7.5 mm, NSMT-Cr 29007 [G1844]), data same as for the holotype; male (6.8 mm, NSMT-Cr 29008 [G1625]), same locality of the holotype, collected by A. Ohtaka on 23 December 2018; 3 males (7.3–7.6 mm, NSMT-Cr 29009) and three females (6.4–7.3 mm, NSMT-Cr 29009) , same locality of the holotype, collected by A. Ohtaka on 17 June 2018; 3 males (5.8–8.0 mm, NSMT-Cr 29009) and three females (5.3–6.4 mm, NSMT-Cr 29009) , same locality of the holotype, collected by N. Kimura on 23 December 2018; seven males (7.6–8.8 mm, NSMT-Cr 29009) and three females (5.6–6.6 mm, NSMT-Cr 29009) , same locality of the holotype, collected by N. Kimura on 10 December 2020; 10 males (6.9–9.9 mm, NSMT-Cr 29009) and 11 females (5.9–8.3 mm, NSMT-Cr 29009) , same locality of the holotype, collected by A. Ohtaka on N. Kimura on 12 December 2020.
Description. Male [7.4 mm, NSMT-Cr 29003]. Head (Fig. 3), rostrum short; lateral cephalic lobe with ventral margin weakly concave; antennal sinus rounded; eyes small, subreniform, major axis 0.3 × head height. Pereonites, dorsal surfaces smooth (Fig. 3). Pleonites 1–3 (Fig. 3B–D), dorsal margins each with 3, 2, and 2 setae. Epimeral plate 1 with rounded posterior margin bearing seta, seta on posteroventral corner (Fig. 3A); epimeral plate 2 with posterior margin almost straight bearing 3 setae, seta on weakly produced posteroventral corner, 2 and 1 robust setae on ventral margin and submargin, respectively (Fig. 3A); epimeral plate 3 with posterior margin almost straight bearing 3 setae, seta on quadrate posteroventral corner, 3 robust setae on ventral margin (Fig. 3A). Urosomite 1 (Fig. 3E) with dorsal margin bearing a pair of lateral robust setae and a middle cluster of robust setae; urosomite 2 (Fig. 3F) with dorsal margin bearing a pair of lateral robust setae and clusters of robust setae; urosomite 3 (Fig. 3G) with dorsal margin bearing a pair of robust setae.
Antenna 1 (Fig. 3H) 0.6 ×length of body; length ratio of peduncular articles 1–3 1.0 : 0.9 : 0.6; peduncular article 1 with posterodistal corner lacking robust seta, posterior margin with 3 pairs of setae and single seta; peduncular article 2 with posterior margin bearing 6 clusters of setae; peduncular article 3 with posterior margin bearing 4 clusters of setae; accessory flagellum comprising 4 articles; primary flagellum comprising 20 articulate, aesthetasc on each article. Antenna 2 (Fig. 3J) 0.7 × length of antenna 1; article 4 of peduncular 1.1 × article 5; peduncular articles 4 and 5 with posterior margins each bearing 5 setal clusters; flagellum comprising 12 articles, calceoli absent.
Upper lip (Fig. 3K) with fine seta on rounded distal margin. Mandibles (Fig. 3L–N), left and right incisors comprising 5 and 4 teeth, respectively, left lacinia mobilis comprising 4 teeth, right lacinia mobilis bifid with many denticles; molar process triturative with plumose seta; left and right mandibles with 7 and 5 blade-like setae on accessory setal rows, respectively; palp comprising 3 articles with length ratio of 1.0 : 3.3 : 2.8; article 1 of palp without setae; article 2 with 25 setae; article 3 bearing pair of setae on inner surface, 3 clusters of setae and single seta on outer surface. Lower lip (Fig. 3O), outer lobes broad, inner lobes indistinct. Maxilla 1 (Fig. 3P) with medial margin of inner plate bearing 20 plumose setae; 11 serrate robust setae on outer plate apically (Fig. 3Q); palp comprising 2 articles, article 1 marginally bare, apical margin of article 2 with 5 robust setae and 2 slender seta. Maxilla 2 (Fig. 4A) with inner plate bearing oblique inner row of 17 plumose setae. Maxilliped (Fig. 4B) with inner plate bearing 3 apical and 2 inner marginal robust setae; outer plate, apical margin with plumose setae and inner margin with robust setae; palp comprising 4 articles, inner margin and submargin of article 2 with rows of setae, article 3 bearing facial setae, slightly curved article 4 with slender nail.
Gnathopod 1 (Fig. 4C, D) with coxa bearing marginal setae ventrally; basis with long setae on anterior and posterior margins; length of carpus 1.4 × width, with seta on anterior margin; length of propodus 1.3 × carpus and 1.4 × width, bearing 2 clusters of setae on anterior margin, propodus with oblique and weakly convex palmar margin bearing 6 medial and 10 lateral peg-like robust setae; dactylus weakly curved, as long as palmar margin. Gnathopod 2 (Fig. 4E, F) with coxa bearing marginal setae ventrally; masis with anterior and posterior margins bearing long setae; length of carpus 1.8 × width, bearing setae on anterior margin; length of propodus 1.1 × carpus and 1.6 × width, respectively, with 2 clusters of setae on anterior margin, propodus with oblique and weakly convex anterior margin bearing 8 medial and 5 lateral peg-like robust setae; dactylus weakly curved, as long as palmar margin.
Pereopods 3 and 4 (Fig. 3A) similar, coxa of pereopod 3 subrectangular with ventral setae; coxa of pereopod 4 expanded with posterior concavity, anterodistal corner and ventral margin with setae. Pereopod 5 (Fig. 3A, 4G) with bilobed coxa bearing apical seta on anterior lobe, 2 robust setae on ventral margin of posterior lobe, posterodistal corner of posterior lobe rounded with robust seta; basis with weakly expanded posterior margin bearing setae, posterodistal corner not lobate. Pereopod 6 (Fig. 3A, 4H) with bilobed coxa bearing anteroproximal setae and apical seta on anterior lobe, 2 robust setae on ventral margin of posterior lobe, posterodistal corner of posterior lobe quadrate with robust seta; basis with weakly expanded posterior margin bearing setae, posterodistal corner not lobate. Pereopod 7 (Fig. 3A, 4I) with weakly concave coxa in ventral margin bearing setae; basis with weakly expanded posterior margin bearing setae, posterodistal corner not lobate with robust and slender setae.
Coxal gills (Figs. 4J–O, 5H, I) with 2 accessory lobes on gills 2–5, posterior lobes longer than or equal to anterior ones, 1 accessory lobe on gills 6 and 7.
Pleopods 1–3 (Fig. 4P) with peduncle bearing paired retinacula (Fig. 4Q) on inner margin; inner ramus with inner basal margin bearing bifid plumose setae.
Uropod 1 (Fig. 4R) with peduncle bearing basofacial robust seta, 2 robust setae on inner and outer margins, 1 and 2 robust setae on inner and outer distal corners, respectively; length of inner ramus 0.8 × that of peduncle, inner margin of inner ramus with 2 robust setae; length of outer ramus 0.9 × that of inner ramus, inner margin of outer ramus with robust seta. Uropod 2 (Fig. 4S) with peduncle bearing 2 robust setae on inner and outer margins, respectively, and robust seta on inner and outer distal corners; length of inner ramus 0.9 × that of peduncle, inner margin of inner ramus with 2 robust seta; length of outer ramus 0.8 × that of inner ramus, inner margin of outer ramus with robust seta. Uropod 3 (Fig. 4T, U) with peduncle length 0.3 × that of outer ramus; length of inner ramus 0.3 × that of outer ramus, inner ramus with slender setae on inner margin and setae apically; outer ramus comprising 2 articles, proximal article with 2 clusters of setae on inner and outer margins, some of which robust, lacking plumose setae, length of terminal article 0.2 × that of proximal article, apical part of terminal article with simple setae.
Telson (Fig. 4V) 0.8 times as long as wide, cleft for 67% of length, with robust seta and slender setae on each lobe.
Female [7.3 mm, NSMT-Cr 29004]. Antenna 1 (Fig. 5A), length ratio of peduncular articles 1–3 1.0 : 0.8 : 0.6; peduncular article 1 with pair of setae and single seta on posterior margin; peduncular article 2 with 5 clusters of setae on posterior margin; accessory flagellum comprising 3 articles; primary flagellum comprising 17 articles. Antenna 2 (Fig. 5B) with peduncular article 4 bearing 6 clusters or single setae on posterior margin; peduncular article 5 with 5 clusters or single setae on posterior margin; flagellum comprising 11 articles, lacking calceoli.
Gnathopod 1 (Fig. 5C, D) with carpus bearing cluster of setae on anterior margin; length of propodus 1.2 × that of carpus and 1.5 × width; propodus with 8 medial and 2 lateral robust setae on palmar margin. Gnathopod 2 (Fig. 5E, F) with carpus bearing cluster of setae on anterior margin; propodus and carpus about the same length, propodus with 3 medial and 2 lateral robust setae and 1 medial and 1 lateral pectinate robust setae on palmar margin.
Pereopods 5–7 with more expanded posterior margin of bases than those of male (Fig. 5G–I).
Brood plates (= oostegites) (Fig. 5J) wide, with numerous setae on its margins.
Uropod 3 (Fig. 5K), length of peduncle 0.4 × that of outer ramus; length of inner ramus 0.2 × that of outer ramus.
Variation. Although almost all specimens have a pleonite 1 with a pair of setae on the dorsal margin, a few specimens have three setae. Some specimens have a urosomite 1 with a pair of lateral robust setae and a pair of clusters of robust setae on its dorsal margin. The numbers of setal clusters on the posterior margins of the peduncular articles 1–3 ranged from two to four, six to seven, and two to four, respectively. The number of setal clusters on the posterior margins of the peduncular articles 4 and 5 ranged from five to six and four to five, respectively. Some specimens have robust setae on the outer margin of the outer ramus of uropod 1 and lack robust setae on the inner margin of the outer ramus of uropod 2. Some specimens have a telson with two robust setae on each lobe. The number of eggs number is up to nine.
Etymology. The new specific name derived from the absence of calceolus.
Remarks. Jesogammarus acalceolus (J.) sp. nov. differs from its congeners by lacking a calceolus on the flagellum of antenna 2 in male. This new species is similar to J. (J.) bousfieldi Tomikawa, Hanzawa and Nakano, 2017 and J. (J.) paucisetulosus Morino, 1984 in having the following features: eyes are small; antenna 1 lacks robust setae on the posterodistal corner of the peduncular article 1; antennae 1 and 2 have many long setae on the posterior margins of the peduncular articles; the maxilla 1 lacks setae on the outer margin of the palp article 2; and gnathopods 1 and 2 have few setae on the ventral margins of the coxae in female. In addition to the absence of a calceolus, J. (J.) acalceolus sp. nov. is distinguished from J. (J.) bousfieldi by the pleonites 1–3 each with less than three setae on the dorsal margins (vs. more than 4 setae in J. (J.) bousfieldi).