Leptosphaeroma gyoja Noda, Ariyama and Tomikawa, 2025

Noda, Akari, Ariyama, Hiroyuki & Tomikawa, Ko, 2025, A new species of the genus Leptosphaeroma (Crustacea: Isopoda: Sphaeromatidae) from Japan, Journal of Natural History 59 (1 - 4), pp. 227-240 : 229-239

Leptosphaeroma gyoja Noda, Ariyama and Tomikawa sp. nov.

(New Japanese name: Gyōja-hirataumisemi)

( Figures 2–5 )

Material examined

Holotype. Male, 3.3 mm (NSMT-Cr 32348), JAPAN: Wakayama, Tomogashima Island, Kosukematsu (34.2793°N, 135.0020°E), collected by H. Ariyama on 1 August 2023.

Paratypes. Seven males, 3.3 mm (NSMT-Cr 32349; G2053), 3.1 mm (NSMT-Cr 32350; Figures 2 and 3 (a)), 3.2 mm (NSMT-Cr 32351), 2.9 mm (NSMT-Cr 32352), 3.1 mm (NSMT-Cr 32353), 2.7 mm (OMNH-Ar 13044; G2025), 3.0 mm (OMNH-Ar 13045; G2026); and two females, 2.6 mm (NSMT-Cr 32354), 2.0 mm (OMNH-Ar 13046); data same as for the holotype.

Referred material of Leptosphaeroma gottschei . Male 5.1 mm (NSMT-Cr 32355; G 2095), female 2.6 mm (NSMT-Cr 32356; G 2096), Mogi , Nagasaki, Japan (32.7090° N 129.9166° E), collected by A. Noda, C. Yamanaka and H. Yoshimura on 12 April 2024. Three males, 5.4 mm (OMNH-Ar 13047; G 2027), 5.5 mm (OMNH-Ar 13048; G 2028), 6.6 mm (OMNH-Ar 13049; A 15-1), northern coast, Tomogashima Island , Wakayama, Japan (34.2830° N, 135.0019° E), collected by H. Ariyama on 1 August 2023. Two males, 5.9 mm (OMNH-Ar 13050; G 2005), 6.1 mm (OMNH-Ar 13051; A 6-1), one female, 3.5 mm (OMNH-Ar 13052; G 2006), Toyokunizaki , Osaka, Japan (34.3223° N, 135.1160° E), collected by H. Ariyama on 17 June 2023.

Etymology

The specific name ‘gyoja’ refers to a Japanese ascetic and mystic ‘En no Gyōja (634–701)’ who developed Tomogashima Island as one of the ascetic practices. Thus, the name is not a Latin or Latinised word.

Diagnosis

Body sparsely setose all along margin. Antennule peduncular article 1 length 2.3–2.5 times of width. Maxilliped endite with 3 robust setae on distal margin. Pereopod 3 with 1–2 plumose setae at dorsodistal corner of carpus. Pleopod 1 endopod length 2.8–3.9 times of width.

Description

Male [holotype, NSMT-Cr 32348]. Body ( Figures 2 (a) and 3(a)) ovate, exceedingly flattened, 1.5 times longer than width, only slightly convex dorsally in central region, sparsely setose all along margin, dorsal surface without teeth and granules, but with many minute pits; cephalon entire, not pointed, eyes ovate, simple. Coxal plates ( Figures 2 (a) and 3(a)) without visible distinct articulation to pereonites, distal coxal margins of pereonites 2–7 well developed, quadrangular in outline. Lateral parts of anterior segments of pleonite also developed. Pleon ( Figures 2 (a, b) and 3(a)) incompletely fused; pleonite 1 almost invisible, hidden by pereonite 7; anterior suture line between pleonites 2 and 3 shorter than posterior pleonites 3 and 4. Pleotelson ( Figures 2 (a) and 3(a)) completely surrounded by very long endopods of uropods, length 1.4 times of width, apex of posterior margin turned somewhat upwards, vaulted, with a longitudinal groove.

Antennule ( Figure 3 (b)), length ratio of peduncular articles 1–3 as 1.0: 0.8: 0.3; peduncle much deformed, proximal articles 1–2 thickened and strongly expanded, forming a rounded broad rim in front of cephalon; peduncular article 1 length 2.5 times of width, anterior margin sparsely setose, posterior margin without setae, left and right inner margins touching each other in medial line; peduncular article 2 length 1.9 times of width, anterior margin sparsely setose, posterior margin without setae, posterodistal corner with a seta; peduncular article 3 rectangular, slender, length 3.1 times of width, anterior and posterior margins without setae; flagellum 4-articulate, articles 2 and 3 with 2 and 1 aesthetascs, respectively. Antenna ( Figure 3 (c)), peduncular article 1 subquadrate, without setae; peduncular article 2 subquadrate, with 2 setae at anterodistal corner; peduncular article 3 weakly lobate, with 3 setae at anterodistal corner; peduncular articles 4 and 5 subrectangular, with marginal setae, article 5 1.3 times as long as article 4; flagellum 10-articulate, articles 1–9 with simple setae on anterodistal margin, article 10 with setae on distal margin.

Epistome ( Figure 3 (d)) triangular, anterior margin with setae, posterior part widened and embracing basal half of labrum. Labrum ( Figure 3 (d)) circular, posterior margin with fine setae. Mandible ( Figure 3 (e–g)), left and right incisors 3-cuspid; lacinia mobilis absent; setal rows comprising 2 and 3 plumose setae in left and right, respectively; molar oblique and expanded; palp 3-articulate, length ratio of articles 1–3 as 1.0: 0.9: 0.2, article 1 with a slender seta and fine setae, articles 2 and 3 each with 6 plumose setae on ventral margin. Paragnath ( Figure 3 (h)) with rounded shoulders, bearing setae. Maxillula ( Figure 3 (i)), medial lobe narrow, with 3 plumose setae and a short simple seta; lateral lobe with 6 teeth and 2 robust setae. Maxilla ( Figure 3 (j)), medial lobe with 3 plumose setae and 4 robust setae; middle lobe with 4 serrate setae; lateral lobe with 3 serrate setae. Maxilliped ( Figure 4 (a)), endite reaching distal margin of palp article 3, distal margin with 2 plumose, 3 robust and 1 simple setae, medial margin with a coupling hook and fine setae; palp 5-articulate, article 1 with a seta on lateral margin, articles 2 and 3 with 12 and 13 setae on medial margin, article 4 with 7 and 1 setae on medial and lateral margins, respectively, article 5 narrow, with 9 distal setae and a seta on lateral margin.

Pereopod 1 ( Figure 4 (b)), basis with a ventrodistal simple seta, dorsal margin bearing robust setae; ischium with a ventrodistal simple seta; merus lobate dorsodistally, dorsodistal corner with 2 simple and 1 plumose robust setae, ventrodistal corner bearing 2 simple setae; carpus short, with 2 simple setae on ventral margin; propodus with 4 plumose and 1 simple robust setae on ventral margin; dactylus length 0.6 times as long as propodus. Pereopod 2 ( Figure 4 (c)), basis with a ventrodistal simple seta; ischium with a ventrodistal simple seta; merus weakly lobate dorsodistally, with 1 simple and 1 plumose setae at dorsodistal corner, ventrodistal corner bearing 2 simple setae; carpus with a few setae on distal surface, dorsodistal corner bearing 2 plumose robust setae and a simple seta; propodus with 1 simple and 1 plumose setae at dorsodistal corner, ventrodistal corner bearing 3 simple setae; dactylus length 0.7 times as long as propodus. Pereopod 3 ( Figure 4 (d)), basis with a ventrodistal simple seta; ischium with a ventrodistal simple seta; merus weakly lobate dorsodistally, with 1 simple and 1 plumose setae at dorsodistal corner, ventrodistal corner bearing a simple seta; carpus swollen, length 2.3 times of width, with 2 plumose setae at dorsodistal corner, ventrodistal corner bearing a simple seta; propodus with 1 simple and 1 plumose setae at dorsodistal corner, ventrodistal corner bearing 3 simple setae; dactylus length 0.2 times as long as propodus. Pereopod 4 ( Figure 5 (a)), basis with a ventrodistal simple seta; ischium with a ventrodistal simple seta; merus weakly lobate dorsodistally, with 1 simple and 2 plumose setae at dorsodistal corner, ventrodistal corner bearing 2 simple setae; carpus slender, with 7 setae distally; propodus with 2 setae at dorsodistal corner, ventrodistal corner with 2 simple setae; dactylus length 0.4 times as long as propodus. Pereopod 5 ( Figure 5 (b)), basis with a ventrodistal simple seta; ischium with a ventrodistal simple seta; merus lobate dorsodistally, with 1 simple and 2 plumose setae at dorsodistal corner, ventrodistal corner bearing 2 simple setae; carpus slender, with 7 setae distally; propodus with 2 setae at dorsodistal corner, ventrodistal corner bearing 2 simple setae; dactylus length 0.4 times as long as propodus. Pereopod 6 ( Figure 5 (c)), basis with a ventrodistal simple seta; ischium with a ventrodistal simple seta; merus lobate dorsodistally, with 1 simple and 2 plumose setae at dorsodistal corner, ventrodistal corner bearing 2 simple setae; carpus slender, with 7 setae distally; propodus with 2 simple setae at dorsodistal corner, ventrodistal corner bearing 3 simple setae; dactylus length 0.4 times as long as propodus. Pereopod 7 ( Figure 5 (d)), basis with a ventrodistal simple seta; ischium with a ventrodistal simple seta; merus lobate dorsodistally, with 2 plumose setae at dorsodistal corner, ventrodistal corner bearing 2 simple setae; carpus slender, with 8 setae distally; propodus with 2 simple setae at dorsodistal corner, ventrodistal corner with 3 simple setae; dactylus length 0.3 times as long as propodus.

Pleopod 1 ( Figure 6 (a)), peduncle length 0.4 times of width, with 3 robust setae at mediodistal corner; exopod slender, oval, length 2.8 times of width, 1.3 times as long as endopod, with 17 plumose setae on distal margin; endopod shorter and more slender than exopod, length 3.7 times of width, with 10 distal plumose setae. Pleopod 2 ( Figure 6 (b)), peduncle length 0.3 times of width, mediodistal and laterodistal corners with 3 and 1 robust setae, respectively; exopod oval, length 1.8 time of width, with 25 marginal plumose setae and fine setae; endopod subtriangular, length 1.2 times of width, with 13 plumose setae on distal margin and fine setae marginally; appendix masculina slender, length 6.9 times of width, 1.5 times as long as endopod. Pleopod 3 ( Figure 6 (c)), peduncle length 0.4 times of width, mediodistal and laterodistal corners with 3 and 1 robust setae, respectively; exopod oval, length 2.5 time of width, with transverse suture, margin bearing 21 plumose setae and fine setae; endopod subtriangular, length 1.5 times of width, margin with 14 distal plumose setae and fine setae. Pleopod 4 ( Figure 6 (d)), peduncle deformed, mediodistal and laterodistal corners with 2 and 1 setae, respectively; exopod length 1.5 times of width, with transverse suture, distal margin with 4 plumose setae, medial margin bearing simple setae and fine setae; endopod oval, length 1.9 times of width, with a few minute setae. Pleopod 5 ( Figure 6 (e)), peduncle with a short seta at laterodistal corner; exopod length 2.1 times of width, medial and distal surfaces with patches; endopod length 2.1 times of width, without setae. Uropod ( Figure 6 (f)), peduncle small; exopod short, lanceolate, length 2.4 times of width, 0.7 times as long as endopod, lateral margin sparsely setose, inner margin fitting lateral margin of endopod; endopod large, expanded distally, length 2.1 times of width, laterodistal and mediodistal margins with setae, inner margin completely surrounding pleotelson.

Female [paratype, NSMT-Cr 32354]. Body smaller than that of male, 1.5 times longer than width. Pereopod 1 ( Figure 7 (a)), propodus with 3 plumose and 1 simple robust setae on ventral margin. Pereopod 7 ( Figure 7 (b)), merus with 3 plumose setae at dorsodistal corner. Pleopod 1 ( Figure 7 (c)), exopod length 2.3 times of width; endopod length 2.8 times of width, with 12 plumose setae on distal margin. Pleopod 2 ( Figure 7 (d)), peduncle length 0.4 times of width; exopod with 23 plumose setae distally; endopod broad, length 1.3 times of width, with 15 marginal plumose setae; appendix masculina absent.

Variation

Body length 1.4–1.5 times longer than width in both sexes. Antennule peduncular article 1 length 2.3–2.5 times of width; flagellum 4- or 5-articulate. Left and right mandibles with setal rows comprising 0–3 and 2–4 setae. Pereopod 3 carpus with 1–2 plumose setae at dorsodistal corner. Pereopods 4–7 meri with 3–4, 3–4, 3–4 and 2–3 setae at dorsodistal corners; carpi with 6–7, 7–8, 7–8 and 7–8 setae at dorsodistal corners. Pleopod 1 endopod length 2.8–3.9 times of width. Uropod exopod length 1.9–2.4 times width.

Colouration in life

Body light tan with melanophores. Eyes black.

Habitat

Gravel beach, lower intertidal zone.

Distribution

Known only from the type locality.

DNA sequences

In total, three and six 16S rRNA sequences of L. gyoja sp. nov. (LC830700–LC830702) and L. gottschei (LC830703–LC830708) were determined, respectively. The uncorrected pairwise genetic distance (p -distance; %) based on the mitochondrial 16S rRNA sequence was 0.0 and 0.0–3.8% within Leptosphaeroma gyoja sp. nov. and L. gottschei , respectively. Genetic distances between Leptosphaeroma gyoja sp. nov. and L. gottschei were 15.2–16.3% ( Table 1 ).

Remarks

Leptosphaeroma gyoja sp. nov. is distinguished from L. gottschei by the following features (features of L. gottschei in parentheses): body sparsely setose all along the margin (densely setose), antennule peduncular article 1 length 2.3–2.5 times of width (1.8–1.9 times of width), maxilliped endite with 3 robust setae on distal margin (4 robust setae), and pleopod 1 endopod length 2.8–3.9 times of width (4.3–4.7 times of width). Leptosphaeroma gyoja sp. nov. is distinguished from L. gottschei by the number of plumose setae at the dorsodistal corner of the pereopod 3 carpus: 1–2 in L. gyoja sp. nov. and 4–5 in L. gottschei ( Figure 8 (b, c)). In a redescription of L. gottschei by Nishimura (1976), two setae are illustrated. However, observations of the newly obtained L. gottschei specimens, including those from the type locality, suggest that some setae were omitted or overlooked by Nishimura (1976). In addition, Nishimura (1976) described 3 teeth on the lateral lobe of the maxillula of L. gottschei , whereas specimens from the type locality have 6 ( Figure 8 (a)). The range of variation for this feature needs to be clarified for both species in the future. Genetic distances between 16S rRNA sequences of L. gyoja sp. nov. and L. gottschei of 15.2–16.3% are larger than the genetic distances between species of Sphaeromatidae known from previous studies (approximately 8%; Wetzer et al. 2018), suggesting that L. gyoja sp. nov. and L. gottschei are genetically differentiated to the level of distinct species.

Leptosphaeroma gyoja sp. nov. and L. gottschei were collected from Tomogashima Island, and the former seems to prefer coasts with stronger waves than the latter. It might be suggested that habitat segregation is achieved there. Future studies are needed on the habitat preference of Leptosphaeroma .

Figures

Figure 2. SEM photographs of Leptosphaeroma gyoja sp. nov., paratype, male 3.1 mm (NSMT-Cr 32350). (a) Habitus, dorsal view. (b) Pleon and pleotelson, dorsal view. Arrows indicate suture lines on the pleonites.

Figure 3. Leptosphaeroma gyoja sp. nov. (a) Paratype, male, NSMT-Cr 32350; (b–k) holotype, male, NSMT-Cr 32348. (a) Habitus, dorsal view. (b) Left antennule, ventral view. (c) Right antenna, ventral view. (d) Labrum, dorsal view. (e) Left mandible, medial view. (f, g) Incisors and setal rows of left and right mandibles, respectively, medial view. (h) Paragnath, ventral view. (i) Left maxillula, dorsal view. (j) Left maxilla, dorsal view.

Figure 4. Leptosphaeroma gyoja sp. nov., holotype, male, NSMT-Cr 32348. (a) Left maxilliped, ventral view. (b–d) Right pereopods 1–3, respectively, medial views.

Figure 5. Leptosphaeroma gyoja sp. nov., holotype, male, NSMT-Cr 32348. (a–d) Left pereopods 4–7, respectively, medial views.

Figure 6. Leptosphaeroma gyoja sp. nov., holotype, male, NSMT-Cr 32348. (a) Right pleopod 1, anterior view. (b) Left pleopod 2, posterior view. (c) Left pleopod 3, posterior view. (d) Left pleopod 4, anterior view. (e) Left pleopod 5, posterior view. (f) Left uropod, dorsal view.

Figure 7. Leptosphaeroma gyoja sp. nov., paratype, female, NSMT-Cr 32354. (a) Left pereopod 1, lateral view. (b) Right pereopod 7, medial view. (c) Right pleopod 1, posterior view. (d) Right pleopod 2, anterior view.

Figure 8. Leptosphaeroma gottschei, male, NSMT-Cr 32355. (a) Left maxillula, ventral view. (b) Right pereopod 3, medial view. (c) Distal part of right pereopod 3 carpus, medial view.