Isaea concinnoides, Ariyama & Moritaki, 2023

Ariyama, Hiroyuki & Moritaki, Takeya, 2023, Two New Amphipods Associated with a Hermit Crab from the Kumano-nada, Central Japan (Crustacea: Amphipoda: Isaeidae, Stenothoidae), Records of the Australian Museum (Rec. Aust. Mus.) 75 (4), pp. 357-370 : 363-367

Isaea concinnoides sp. nov.

urn:lsid:zoobank.org:act:576DF834-A761-43C9-B450-9E7BD1F507DA

[Japanese name: Yadokari-yokoebi, new]

Figs 6–11

Holotype: OMNH-Ar-12499, male, 4.8 mm, Kumano-nada, off Kumano City , Mie Prefecture, 33°48'54.0"N 136°11'42.1"E, 190–380 m depth, associated with Propagurus obtusifrons , coll. T. Moritaki, 7 February 2022 . Paratypes (TAMBL-CR 1776): OMNH-Ar-12500, male, ca. 5.6 mm (damaged); OMNH-Ar-12501, male, 3.4 mm; OMNH-Ar-12502, male, 3.0 mm; OMNH-Ar-12503, female, 3.6 mm, off Owase City , Mie Prefecture, 34°01'22.0"N 136°20'57.6"E, 190–350 m depth, associated with Propagurus obtusifrons , coll. T. Moritaki, 13 October 2016 .

Type locality. Kumano-nada, off Kumano City in Mie Prefecture, Japan.

Etymology. From concinna (closely related species) + the Greek oides (= resembling).

Diagnosis. Eyes medium-sized. Antennae relatively stout, with short setae on posterior margins; accessory flagellum with 2 articles. Gnathopod 2 stout, carpus short. Basis of pereopod 5 oval. Posterodistal corner of epimeral plate 3 angular.

Description. Male (based on holotype, 4.8 mm). Body ( Fig. 6 ) subovate, smooth. Coxae 1–4 long, coxae 5–7 short.

Head ( Figs 6 , 7 ). Rostrum small. Ocular lobes subtriangular. Eyes medium-sized (diameter: ca. 0.25 times head length). Antenna 1 relatively short, ca. 0.3 times BL; peduncular articles 1–3 with length ratio of 1.0:1.3:1.0, article 1 stout, posterodistal corner setose (twisted in preparation), anterodistal corner of article 2 setose; accessory flagellum short, 2-articulate, tip with several setae; primary flagellum with 5 articles, poorly setose. Antenna 2 subequal to antenna 1 in length, stout, weakly setose; peduncular articles 3–5 with length ratio of 1.0:2.5:2.2; flagellum short, with 5 articles, terminal article minute, articles 1–4 bearing 6, 3, 3, 2 robust setae, respectively. Upper lip rounded ventrally, ventral margin with many minute setae. Mandibles stout, left and right incisors with 3 and 5 cusps, respectively, left lacinia mobilis 4-toothed and right 3-toothed, accessory blades 4 in left and 3 in right, molar well-developed, fan-shaped structure attached on distal side of left molar; palp strong, article length ratio 1.0:2.8:1.9, article 1 bare, articles 2, 3 weakly and strongly setose, respectively, article 3 clavate. Lower lip with acute-tipped mandibular process, mediodistal margin of outer lobe setose, dorsomedial surface covered with short thin setae; distal margin of inner plate with thin setae. Maxilla 1 small; inner plate reduced, bullet-shaped, with bare tip; outer plate with 9 robust setae apically; tip of palp with 5 robust and 4 slender setae. Maxilla 2 small; inner plate setose mediodistally; outer plate longer, tip setose. Maxilliped with inner plate bearing 4 robust and 4 plumose setae mediodistally; outer plate broad, not reaching apex of palp article 2, distomedial margin with 13 long-to-short robust setae; palp slender, weakly setose, article 4 bearing 2 long robust setae on tip.

Pereon ( Figs 8 , 9 ). Gnathopod 1 small; basis slightly curved anteriorly, bare; posterior margins of merus and carpus heavily setose; propodus ca. 1.3 times as long as carpus, palm rounded, defined by long robust seta; dactylus slightly curved, posterior margin minutely serrated. Gnathopod 2 stout; basis slightly curved anteriorly, anterior margin with 2 short setae, anterodistal corners of basis and ischium produced roundly; merus short and broad, posterodistal corner setose; carpus narrow, triangular, with setose posterior lobe, anterolateral surface with long robust seta; propodus greatly expanded, broad, ca. 5.5 times carpal length, palm strongly oblique, defined by short robust seta, bearing numerous plumose setae, distal part of palmer margin with triangular process; dactylus short, strongly curved posteriorly. Pereopod 3 slender; basis poorly setose; merus, anterodistal and posterodistal corners setose; carpus short, setose posterodistally; propodus long, ca. 1.65 times length of carpus, widened distally, palm oblique, with 7 and 5 robust setae on lateral and medial surfaces, respectively; dactylus strongly curved posteriorly. Pereopod 4 slender, similar to pereopod 3 except for coxa; coxa wide, posterior margin weakly excavated; palm oblique, with 5 and 4 robust setae on lateral and medial surfaces, respectively. Pereopod 5 shorter than pereopod 4; coxa bilobed; basis broad, oval, length ca. 1.1 times width, posterodistal corner expanded, rounded distally, anterodistal corner setose; merus produced posterodistally, anterodistal and posterodistal corners of merus and carpus setose; propodus widened distally, palm slightly oblique, with 2+3+4 and 3 robust setae on lateral and medial surfaces, respectively. Pereopod 6 slightly longer than pereopod 5; coxa bilobed; basis broad, subrectangular, length ca. 1.35 times width, posterodistal corner expanded, rounded distally, lateral surface with vertical ridge in middle, posterior half slightly bending mediodistally; propodus widened distally, palm slightly oblique, with 2+3+5 and 3 robust setae on lateral and medial surfaces, respectively. Pereopod 7 longer than pereopod 6; coxa not lobate; basis subrectangular, length ca. 1.4 times width, posterodistal corner expanded, rounded distally, lateral surface with vertical ridge in middle, posterior half slightly bending mediodistally; propodus widened distally, palm slightly oblique, with 2+3+4 and 3 robust setae on lateral and medial surfaces, respectively.

Pleon ( Fig. 10 ). Epimeral plates 1–3, posterodistal corners round, weakly angular and angular, respectively; ventral margins bare. Pleopods slender, pleopod 3 shortest; peduncles with 1, 5, 8 plumose setae in pleopods 1–3, respectively; outer rami subequal to inner rami in length, former with 9, 9, 8 articles, respectively and latter each with 7 articles. Uropod 1, peduncle with inter-ramal process (ca. 0.2 times length of peduncle), bearing 3 dorsolateral and 2 dorsomedial robust setae; outer ramus shorter than peduncle, with 2 dorsolateral, 1 dorsomedial and 4 terminal robust setae; inner ramus as long as peduncle, bearing 1 dorsolateral, 2 dorsomedial and 4 terminal robust setae. Uropod 2 ca. 0.85 times length of uropod 1; peduncle with minute inter-ramal process (ca. 0.08 times length of peduncle), bearing 2 dorsolateral and 1 dorsomedial robust setae; outer ramus ca. 0.9 times length of peduncle, with 2 dorsolateral, 1 dorsomedial and 4 terminal robust setae; inner ramus ca. 1.15 times length of peduncle, bearing 1 dorsolateral, 3 dorsomedial and 4 terminal robust setae. Uropod 3 ca. 0.75 times length of uropod 2; peduncle lacking inter-ramal process, bearing 1 robust and 3 short setae medioproximally, distal part with 3+3+2 robust setae; outer ramus curved laterally, ca. 0.75 times length of peduncle, with 1 dorsomedial robust seta and 1 terminal minute robust seta; inner ramus ca. 0.9 times as long as peduncle, bearing 1 dorsolateral and 1 dorsomedial robust setae and 1 terminal minute robust seta. Telson roundish triangular, length ca. 0.9 times width, with pair of subapical cusps bearing 2 robust setae (1 long and 1 short) and a sensory seta, each lateral margin with 2 sensory setae.

Female (based on paratype, OMNH-Ar-12503, 3.6 mm). Immature (oostegites very small). Gnathopod 2 ( Fig. 10 SF-RG 2 ), distal process on palmer margin smaller than that of holotype .

Variation in male gnathopod 2. Paratype, large male (OMNH-Ar-12500, ca. 5.6 mm; Fig. 10 LM-RG2), posterodistal corner of propodus produced roundly, posterior margin with large distal and small proximal processes and without robust seta. Paratype, small male (OMNH-Ar-12501, 3.4 mm; Fig. 10 SM-G2), distal process on palmar margin small.

Colour in life ( Fig. 11 ). Eyes black (they appear silver in the figure probably due to reflection of light). Body almost translucent, with poorly-defined pale yellow bands, pale brown internal organs visible.

Remarks. Isaea concinnoides sp. nov. is quite different from I. elmhirsti and I. montagui in the shape of the male gnathopod 2. The male gnathopod 2 of the new species closely resembles that of I. concinna ; however, I. concinnoides can be distinguished from I. concinna by the relatively stout antennae with short setae (slender, with long setae in I. concinna ) and the oval basis of pereopod 5 (subrectangular in I. concinna ).

Habitat. Bottom sediment unknown, 190–380 m depth, associated with Propagurus obtusifrons .

Distribution. Japan: Kumano-nada (present study).

Figures

Figure 6. Isaea concinnoides sp. nov., holotype male, 4.8 mm, OMNH-Ar-12499, habitus.

Figure 7. Isaea concinnoides sp. nov., holotype male, 4.8 mm, OMNH-Ar-12499. Scale bars = 0.1 mm.

Figure 8. Isaea concinnoides sp. nov., holotype male, 4.8 mm, OMNH-Ar-12499. Scale bars = 0.1 mm.

Figure 9. Isaea concinnoides sp. nov., holotype male, 4.8 mm, OMNH-Ar-12499, distal parts of pereopods 3–7. Scale bars = 0.1 mm.

Figure 10. Isaea concinnoides sp. nov.: holotype male, 4.8 mm, OMNH-Ar-12499 (except for LM, SM, and SF); LM: paratype male, ca. 5.6 mm, OMNH-Ar-12500; SM: paratype male, 3.4 mm, OMNH-Ar-12501; SF: paratype female, 3.6 mm, OMNH-Ar-12503. Slender setae of gnathopods omitted. Scale bars = 0.1 mm.

Figure 11. Isaea concinnoides sp. nov., paratype male, ca. 5.6 mm, OMNH-Ar-12500, photographed in aquarium, T. Moritaki.