Gnorimosphaeroma rivulare Tomikawa, Yoshii & Nunomura, 2023

Tomikawa, Ko, Yoshii, Junpei, Noda, Akari, Lee, Chi-Woo, Sasaki, Tetsuro, Kimura, Naoya & Nunomura, Noboru, 2023, A new freshwater species of Gnorimosphaeroma (Crustacea, Isopoda, Sphaeromatidae) from Chichi-jima Island, Ogasawara Islands, Japan, Zoosystematics and Evolution 99 (2), pp. 545-556 : 545

Gnorimosphaeroma rivulare Tomikawa, Yoshii & Nunomura sp. nov.

Figs 2 , 3 , 4 , 5 , 6 , 7 New Japanese name: Chichijima-kotsubumushi

Type materials.

Holotype: male 4.9 mm (NSMT-Cr 31485), upper stream of Nagatani River (27°04.051'N, 142°12.938'E), alt. 160 m, Chichi-jima Island, Ogasawara Islands, Tokyo, Japan, collected by Tetsuro Sasaki on 2 February 2021. Paratypes: 3 males, 5.3 mm (NSMT-Cr 31486; G1892), 4.7 mm (NSMT-Cr 31487; G1893), 5.2 mm (NSMT-Cr 31507) 1 female 3.2 mm (NSMT-Cr 31488), data same as for holotype; 2 males, 4.1 mm (NSMT-Cr 31489), 4.0 mm (NSMT-Cr 31490; G1826), locality same as for holotype, collected by Noboru Nunomura on 16 December 2019.

Type locality.

Japan, Tokyo: Ogasawara Islands, Chichi-jima Island, upper stream of Nagatani River (Figs 1 , 2A ).

Diagnosis.

Pleonites incompletely fused; anterior suture line longer than posterior one. Pleotelson posterior margin rounded. Maxillula medial lobe with 4 plumose setae and a short single seta; lateral lobe with 10 robust setae. Maxilla medial lobe with 17 setae; middle lobe with 12 setae; lateral lobe with 13 setae. Pereopod 1 basis with a single seta on posterodistal corner; merus with 4 setae on anterodistal corner. Pereopod 2 propodus subrectangular, not swollen. Pereopod 3 merus, carpus, and propodus sparsely setulose. Uropod exopod length 0.7 times as long as endopod.

Description.

Male [NSMT-Cr 31485, holotype, 4.9 mm]. Body (Figs 2B , 3A ) ovate, vaulted, ca. 1.5 times longer than wide, dorsal surface unornamented, without marginal setae; eyes ovate, simple. Coxal plates (Figs 3A , 7A ) without visible distinct articulation to pereonites, partly overlapped, distal coxal margins rounded without setae; coxa 3 tapered anteriorly and posteriorly such that the animal can conglobulate. Pleon (Figs 3A , 7B ) incompletely fused; pleonite 1 almost invisible, hidden by pereonite 7; anterior suture line between pleonites 2 and 3 longer than posterior one between pleonites 3 and 4; pleonite 5 fused with pleonite 4, barely recognizable. Pleotelson (Figs 3A , 7B ) slightly narrower than pleon, length 0.5 times longer than wide, posterior margin entire, rounded, with minute setae.

Antennule (Fig. 3B ) exceeding posterior margin of head; length ratio of peduncular articles 1-3 as 1.0: 0.4: 0.5; peduncular article 1 ovate, length 1.6 times as long as wide, anterior margin with a few setae, posterior margin lined with setae; peduncular article 2 subquadrate, length 1.2 times as long as wide, anterior and posterior margins lined with setae; peduncular article 3 rectangular, slender, length 3.0 times as long as wide, with setae on anterior and posterior margins; flagellum 6-articulate, articles 2 and 3 each with 2 aesthetaescs and articles 4 and 5 each with an aesthetasc. Antenna (Fig. 3C ) reaching distal margin of pereonite 2; length ratio of peduncular articles 1-5 as 1.0: 0.9: 0.9: 1.5: 1.5; peduncular articles 1-3 subquadrate with fine setae on anterior margin; peduncular articles 4 and 5 subrectangular, each length 1.9 times longer than wide, anterior margin of article 4 with fine setae, article 5 with a few setae; flagellum 12-articulate (only the first five articles drawn in Fig. 3C ).

Labrum (Fig. 3D ) anterior margin rounded with fine setae. Mandibles (Fig. 3E-I ) left and right incisors with 4 and 3 cusps, respectively; lacinia mobilis of left mandible with 3 cusps; setal row of left and right mandibles comprised 7 and 9 serrate setae; palp 3-articulate, length ratio of articles 1-3 as 1.0: 1.2: 0.8, article 1 with fine setae, article 2 with 10 setae on lateral margin and fine setae on medial margin, article 3 with 11 setae on lateral margin. Paragnaths (Fig. 3J ) with rounded shoulders, bearing setae. Maxillula (Fig. 3K ) medial lobe narrow, slightly shorter than lateral lobe, with 4 plumose setae and a short simple seta; lateral lobe with 10 robust setae, 4 of which serrate. Maxilla (Fig. 3L ) medial lobe with 17 setae, some of which plumose; middle lobe with 12 setae; lateral lobe with 13 setae. Maxilliped (Fig. 3M ) endite reaching distal margin of palp article 2, distal margin with 8 plumose setae and 5 simple setae, medial margin with a coupling hook and 4 plumose setae; palp 5-articulate, articles 2-4 with 3, 3 and 6 setae on lateral margin, respectively, produced mediodistally with setae, article 5 narrow with marginal setae.

Pereopod 1 (Fig. 4A, B ) basis with a ventrodistal simple seta, ventral margin with fine setae; merus lobate dorsodistally, dorsodistal corner with 4 setae, ventral margin with robust setae and fine setae; carpus short with 2 robust setae and fine setae on ventral margin; propodus oval, swollen, length 2.5 times as long as wide, with 2 robust setae on ventral margin and 5 setulate setae close to ventral margin; dactylus length 0.6 times as long as propodus. Pereopod 2 (Fig. 4C, D ) basis with a ventrodistal simple seta; merus weakly lobate dorsodistally, with 5 setae on dorsodistal corner, ventral margin with fine setae; carpus with 6 robust and 3 slender setae on distal surface, ventral margin with fine setae; propodus subrectangular, not swollen, length 2.8 times as long as wide, with 3 robust setae on ventral margin; dactylus length 0.5 times as long as propodus. Pereopod 3 (Fig. 4E, F ) basis with a ventrodistal simple seta; merus lobate dorsodistally, dorsodistal corner with 6 setae, ventral margin with sparse fine setae; carpus with 5 robust and some slender setae on distal margin, ventral margin with sparse fine setae; propodus length 2.8 times as long as wide, with 2 robust setae and a slender single seta on ventral margin; dactylus length 0.5 times as long as propodus. Pereopod 4 (Fig. 4G ) basis with a simple seta on ventrodistal corner and some broom setae on dorsal margin; ischium with fine setae on ventral margin; merus lobate dorsodistally with robust setae, dorsal and ventral margins with fine setae; carpus with sparse fine setae on ventral margin; propodus with 3 robust setae on ventral margin, dorsal and ventral margins with sparse fine setae. Pereopod 5 (Fig. 4H ) basis with a simple seta on dorsodistal corner; merus and carpus with robust setae on distal margins; propodus with 3 robust setae on dorsal margin. Pereopods 6 and 7 (Fig. 5A, B ) basis with a simple seta on dorsodistal corner; merus and carpus with robust setae on distal margins; propodus with 2 robust setae on dorsal margin.

Penial process (Fig. 5C ) simple, length 3.5-4.2 times as long as basal width, close set.

Pleopod 1 (Fig. 5D ) peduncle length 0.5 times width with 4 setae on mediodistal corner; exopod oval, length 1.9 times width, 1.2 times length of endopod, with plumose setae marginally; endopod subtriangular, length 1.4 times width, with distal plumose setae. Pleopod 2 (Fig. 5E ) peduncle length 0.4 times width, mediodistal and laterodistal corners with 2 and 1 setae, respectively; exopod oval, length 1.8 times width, with marginal plumose setae; endopod subtriangular, length 1.6 times width, with plumose setae on distal margin; appendix masculina slender, length 6.0 times width, 1.2 times as long as endopod. Pleopod 3 (Fig. 5F ) peduncle length 0.4 times width, mediodistal and laterodistal corners with 2 and 1 setae, respectively; exopod oval, length 1.9 times width, with transverse suture, bearing plumose setae marginally; endopod slightly shorter than exopod with plumose setae on distal margin. Pleopod 4 (Fig. 5G ) peduncle small, length 0.5 times width, laterodistal corner with a single seta; exopod length 1.8 times width with transverse suture, bearing 4 plumose setae and simple setae marginally; endopod oval, length 1.4 times width, marginally bare. Pleopod 5 (Fig. 5H, I ) peduncle with 2 setae on mediodistal corner; exopod length 1.6 times width transverse suture, bearing lateral and distal scale patches, lateral margin with simple setae; endopod oval, length 0.7 times as long as exopod, marginally bare.

Uropod (Figs 3A , 5J , 7B ) not quite extending to posterior margin of pleotelson; peduncle rectangular, length 0.6 times width; exopod length 3.5 times width, 0.7 times as long as endopod, with fine sparse setae; endopod length 2.5 times width with fine setae.

Female [NSMT-Cr 31488, paratype, 3.2 mm]. Body shape similar to that of male. Body length 1.7 times width.

Pereopod 1 (Fig. 6A ) propodus subrectangular, length 2.6 times as long as wide. Pereopod 2 (Fig. 6B ) propodus subrectangular, length 3.1 times as long as wide, with 2 robust setae on posterior margin.

Pleopod 1 (Fig. 6C ) peduncle length 0.5 times width with 3 setae on mediodistal corner; exopod oval, length 1.6 times width, 1.2 times length of endopod; endopod subtriangular, length 1.5 times width. Pleopod 2 (Fig. 6D ) peduncle length 0.4 times width, mediodistal and laterodistal corners with 2 and 1 setae, respectively; exopod oval, length 1.8 times width; endopod subtriangular, length 1.6 times width.

Uropod (Fig. 6E ) peduncle rectangular, length 0.6 times width; exopod length 3.0 times width, 0.7 times as long as endopod; endopod length 2.6 times width.

Variation.

Mandible left incisor of paratype male (NSMT-Cr 31486) with 4 cusps.

Etymology.

The specific name rivulare is derived from a Latin adjective rivularis, which means brook living, referring to the habitat of the new species.

Distribution and habitat.

This species is known only from the type locality. The specimens were collected from beneath the cobbles in an upper stream of Nagatani River.

Remarks.

Gnorimosphaeroma rivulare sp. nov. is morphologically similar to G. boninense Nunomura, 2006, G. naktongense Kwon & Kim, 1987, and G. saijoense Nunomura, 2013 in having anterior suture line longer than posterior one, maxilla middle and lateral lobes each with more than 10 setae, pereopod 1 basis with a simple seta on ventrodistal corner, pereopod 1 merus with 4 setae on dorsodistal corner, and uropodal exopod twice as long as endopod. However, G. rivulare sp. nov. differs from these three species by the following features (features of the species that are being compared are in parentheses): from G. boninense , pleotelson posterior margin rounded (almost straight), pereopod 3 carpus and propodus sparsely setulose (densely setulose), pereopod 2 propodus subrectangular (subtriangular), and pleopod 3 exopod almost same size as endopod (smaller than endopod); from G. naktongense , antennule peduncular article 3 not elongate, length 1.3 times as long as article 2 (elongate, 1.7 times as long as article 2), pereopod 2 propodus ventral margin straight (weakly expanded), and pleopod 5 endopod length 0.7 (0.9) times as long as exopod; from G. saijoense , maxillula medial lobe with 4 (5) plumose setae, maxilliped palp articles 2 and 3 each with 3 (1) setae on laterodistal corner, pereopod 2 propodus ventral margin straight (weakly expanded), pereopod 3 merus, carpus, and propodus sparsely setulose (densely setulose).

Figures

Figure 1. Map showing the type localities of Gnorimosphaeroma species.

Figure 2. Habitat and live specimen of Gnorimosphaeroma rivulare sp. nov. A. Habitat; B. Living state of the holotype, male 4.9 mm (NSMT-Cr 31485).

Figure 3. Gnorimosphaeroma rivulare sp. nov. A-F, H, J-M. Holotype, male 4.9 mm (NSMT-Cr 31485); G, I. Paratype, male 5.3 mm (NSMT-Cr 31486). A. Habitus, dorsal view; B. Antennula, medial view; C. Antenna, medial view; D. Labrum, dorsal view; E. Right mandible, medial view; F. Incisor and setal row of right mandible, medial view; G. Incisor of right mandible, lateral view; H. Incisor, lacinia mobilis and setal row of left mandible, medial view; I. Incisor and lacinia mobilis of left mandible, medial view; J. Paragnaths, posterior view; K. Maxillula, anterior view; L. Maxilla, anterior view; M. Maxilliped, anterior view.

Figure 4. Gnorimosphaeroma rivulare sp. nov., holotype, male 4.9 mm (NSMT-Cr 31485). A. Right pereopod 1, medial view; B. Merus, carpus, propodus and dactylus of right pereopod 1, medial view; C. Right pereopod 2, medial view; D. Carpus, propodus and dactylus of right pereopod 2, medial view; E. Right pereopod 3, medial view; F. Merus, carpus, propodus and dactylus of right pereopod 3, medial view; G. Right pereopod 4, medial view; H. Right pereopod 5, medial view.

Figure 5. Gnorimosphaeroma rivulare sp. nov., holotype, male 4.9 mm (NSMT-Cr 31485). A. Right pereopod 6, medial view; B. Right pereopod 7, medial view; C. Penes, anterior view; D. Right pleopod 1, anterior view; E. Right pleopod 2, anterior view; F. Right pleopod 3, anterior view; G. Right pleopod 4, anterior view; H. Left pleopod 5, posterior view; I. Detail of distal endopod of pleopod 5, posterior view; J. Left uropod, dorsal view.

Figure 6. Gnorimosphaeroma rivulare sp. nov., paratype, female 3.2 mm (NSMT-Cr 31488). A, B. Merus, carpus, propodus and dactylus of right pereopods 1 and 2, medial views; C, D. Right pleopods 1 and 2, anterior views; E. Right uropod, dorsal view.

Figure 7. SEM photographs of G. rivulare sp. nov., paratype, male 5.2 mm (NSMT-Cr 31507). A. Habitus, lateral view; B. Pleon and pleotelson, dorsal view.