Jesogammarus (J.) acalceolus, Tomikawa & Kimura, 2021

Tomikawa, Ko & Kimura, Naoya, 2021, On the brink of extinction: a new freshwater amphipod Jesogammarus acalceolus (Anisogammaridae) from Japan, ZooKeys 1065, pp. 81-100 : 81

Jesogammarus (J.) acalceolus sp. nov.

[New Japanese name: Shitsuko-yokoebi] Figures 1B , 3 , 4 , 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 N. Kimura on 12 December 2020 .

Diagnosis.

Dorsal surface of pereonites smooth. Pleonites 1-3 each with fewer than three dorsal setae. Antenna 1 without robust seta on posterodistal corner of peduncular article 1. Male antenna 2 without Sternophysinx calceoli . Mandible with palp article 1 lacking setae. Uropod 3 without plumose setae on outer ramus.

Description.

Male [7.4 mm, NSMT-Cr 29003].

Body. 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 three, two, and two 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 three setae, seta on weakly produced posteroventral corner, two and one robust setae on ventral margin and submargin, respectively (Fig. 3A ); epimeral plate 3 with posterior margin almost straight bearing three setae, seta on quadrate posteroventral corner, three 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.

Antennae. Antenna 1 (Fig. 3H ) 0.6 ×length of body; length ratio of peduncular articles 1-3 in 1.0: 0.9: 0.6; peduncular article 1 with posterodistal corner lacking robust seta, posterior margin with three pairs of setae and single seta; peduncular article 2 with posterior margin bearing six clusters of setae; peduncular article 3 with posterior margin bearing four clusters of setae; accessory flagellum comprising four 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 five setal clusters; flagellum comprising 12 articles, Sternophysinx calceoli absent.

Mouth parts. Upper lip (Fig. 3K ) with fine seta on rounded distal margin. Mandibles (Fig. 3L-N ), left and right incisors comprising five and four teeth, respectively, left lacinia mobilis comprising four teeth, right lacinia mobilis bifid with many denticles; molar process triturative with plumose seta; left and right mandibles with seven and five 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, three 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; eleven serrate robust setae on outer plate apically (Fig. 3Q ); palp comprising 2 articles, article 1 marginally bare, apical margin of article 2 with five robust setae and two slender seta. Maxilla 2 (Fig. 4A ) with inner plate bearing oblique inner row of 17 plumose setae. Maxilliped (Fig. 4B ) with inner plate bearing three apical and two inner marginal robust setae; outer plate, apical margin with plumose setae and inner margin with robust setae; palp comprising four articles, inner margin and submargin of article 2 with rows of setae, article 3 bearing facial setae, slightly curved article 4 with slender nail.

Gnathopods . 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 two clusters of setae on anterior margin, propodus with oblique and weakly convex palmar margin bearing six medial and ten lateral peg-like robust setae; dactylus weakly curved, as long as palmar margin. Gnathopod 2 (Fig. 4E, F ) with coxa bearing marginal setae ventrally; basis 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 two clusters of setae on anterior margin, propodus with oblique and weakly convex anterior margin bearing eight medial and five lateral peg-like robust setae; dactylus weakly curved, as long as palmar margin.

Pereopods. 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 (Figs 3A , 4G ) with bilobed coxa bearing apical seta on anterior lobe, two 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 (Figs 3A , 4H ) with bilobed coxa bearing anteroproximal setae and apical seta on anterior lobe, two 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 (Figs 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 (Fig. 4J-O ) with two accessory lobes on gills 2-5, posterior lobes longer than or equal to anterior ones, one 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.

Uropods. Uropod 1 (Fig. 4R ) with peduncle bearing basofacial robust seta, two robust setae on inner and outer margins, one and two robust setae on inner and outer distal corners, respectively; length of inner ramus 0.8 × that of peduncle, inner margin of inner ramus with two 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 two 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 two 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 two articles, proximal article with two 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].

Antennae. Antenna 1 (Fig. 5A ), length ratio of peduncular articles 1-3 in 1.0: 0.8: 0.6; peduncular article 1 with pair of setae and single seta on posterior margin; peduncular article 2 with five clusters of setae on posterior margin; accessory flagellum comprising three articles; primary flagellum comprising 17 articles. Antenna 2 (Fig. 5B ) with peduncular article 4 bearing six clusters or single setae on posterior margin; peduncular article 5 with five clusters or single setae on posterior margin; flagellum comprising eleven articles, lacking Sternophysinx calceoli .

Gnathopods . 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 eight medial and two lateral robust setae on palmar margin. Gnathopod 2 (Fig. 5E, F ) with carpus bearing cluster of setae on anterior margin; propodus and carpus approximately the same length, propodus with three medial and two lateral robust setae and one medial and one 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.

Variations.

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 of antenna 1 ranged from two to four, six or 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 or six and four or 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 2 robust setae on each lobe. The number of eggs is up to 9.

Etymology.

The new specific name derived from the absence of calceolus.

Remarks.

Jesogammarus (J.) acalceolus 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 & 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; 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 four setae in J. (J.) bousfieldi ).

Assessment of conservation status.

Jesogammarus (J.) acalceolus sp. nov. was found in a spring located 120 m above sea level, on the slope of the volcanic Mt. Iwaki, Aomori Prefecture, Japan. Although we conducted an intensive survey of inland waters at more than 400 sites in the Aomori Prefecture, this new species was present only in this one spring described above and not found in any others (unpublished data). In most of the freshwater habitats that were investigated, J. (J.) jesoensis Schellenberg, 1937, which is distributed in Hokkaido and northern Honshu, was present. Because J. (J.) acalceolus sp. nov. and J. (J.) jesoensis are not closely related (Fig. 2 ), it is expected that the current distributions of both species are a result of different evolutionary processes. As a positive aspect, the type locality of J. (J.) acalceolus sp. nov. is in the precincts of the Iwaki Haguro Shrine, built in AD 807, as a result of which this type locality has been treated with care by locals for more than 1,000 years ( Sasaki 1995). Therefore, the environment of this spring has been preserved in good condition, allowing the present J. (J.) acalceolus sp. nov. population to survive. At present, this spring has an abundance of water (60 m3/day) ( Yamamoto 1994), and its environment is stable. However, amphipods are known to be highly sensitive to chemicals, such as pesticides ( Schulz 2003; Nyman et al 2013). This species inhabits only a few meters of a spring brooklet surrounded by apple plantations. Therefore, the deterioration of its habitat due to an inflow of agricultural chemicals into spring water may lead to its extinction.

Figures

Figure 1. Habitat and live specimens of Jesogammarus (Jesogammarus) acalceolus sp. nov. A the type locality, Haguro Shrine Spring, Hirosaki, Aomori Prefecture, Japan B mate guarding pair, male is upper and female is lower, photographed by Ryu Uchiyama.

Figure 2. Maximum likelihood tree and ancestral state reconstructions for calceolus on male antenna 2. Filled circles at each species represent states of habitat; pie charts at internal nodes present proportional likelihoods of reconstruction. Key nodes are labelled with the proportional likelihood of the presence or absence of the calceolus on male antenna 2, which was reconstructed as most likely to be at that node.

Figure 3. Jesogammarus (Jesogammarus) acalceolus sp. nov., male (7.4 mm), NSMT-Cr 29003 A habitus, lateral view B-D dorsal margins of pleonites 1 - 3, respectively, dorsal views E-G dorsal margins of urosomites 1 - 3, respectively, dorsal views H peduncular articles 1 - 3, accessory flagellum, and flagellar articles 1 - 4 of antenna 1, medial view I aesthetasc and associate setae on the flagellum of antenna 1, medial view J peduncular articles 1 - 5 and flagellar articles 1 - 3 of antenna 2, medial view K upper lip, posterior view L right mandible, medial view M-N incisor and lacinia mobilis of left and right mandibles, medial views O lower lip, ventral view P maxilla 1, medial view Q serrate robust setae on outer plate of maxilla 1, medial view.

Figure 4. Jesogammarus (Jesogammarus) acalceolus sp. nov., male (7.4 mm), NSMT-Cr 29003 A maxilla 2, medial view B maxilliped, dorsal view C gnathopod 1, medial view D palmar margin of propodus and dactylus of gnathopod 1, medial view, some setae omitted E gnathopod 2, medial view F palmar margin of propodus and dactylus of gnathopod 2, medial view, some setae omitted G-I coxa-ischium of pereopods 5 - 7, respectively, lateral views J-O coxal gills on gnathopod 2 - pereopod 7, respectively, lateral views P pleopod 1, lateral view, distal parts of rami omitted Q retinacula on peduncle of pleopod 1, lateral view R-S uropods 1 - 2, respectively, dorsal views T uropod 3, ventral view U distal part of proximal article and terminal article of outer ramus of uropod 3, ventral view V telson, dorsal view.

Figure 5. Jesogammarus (Jesogammarus) acalceolus sp. nov., female (7.3 mm), NSMT-Cr 29004 A peduncular articles 1 - 3, accessory flagellum, and flagellar articles 1 - 4 of antenna 1, medial view B peduncular articles 1 - 5 and flagellar articles 1 - 3 of antenna 2, medial view C ischium-dactylus of gnathopod 1, medial view D palmar margin of propodus and dactylus of gnathopod 1, medial view, some setae omitted E ischium-dactylus of gnathopod 2, medial view F palmar margin of propodus and dactylus of gnathopod 2, medial view, some setae omitted G-I coxa-ischium of pereopods 5 - 7, respectively, lateral views J brood plate on gnathopod 2, lateral view K uropod 3, ventral view.