Grandidierella nana, Ariyama, 2020

Ariyama, Hiroyuki, 2020, Six species of Grandidierella collected from the Ryukyu Archipelago in Japan with descriptions of four new species (Crustacea: Amphipoda: Aoridae), Zootaxa 4810 (1), pp. 1-44 : 29-34

Grandidierella nana sp. nov.

[Japanese name: Chibi-dorosokoebi, new]

( Figs 25–29 )

Type material. Holotype: male, 2.6 mm (OMNH-Ar-11546), Funaura, Iriomote Island , Okinawa Prefecture, 24°23'49"N, 123°49'31"E ( Fig. 1G ), middle intertidal, fine sand bottom, 18 July 2015, coll. H. Ariyama. Paratypes: 3 males, 2.3, 2.0, 1.8 mm (OMNH-Ar-11547–11549) and 2 ovigerous females, 3.2, 2.7 mm (OMNH-Ar-11550, 11551), same data as holotype; male, 2.3 mm and ovigerous female, 3.4 mm (OMNH-Ar-11552, 11553), mouth of Nakama River, Iriomote Island , Okinawa Prefecture, 24°16'50"N, 123°52'53"E ( Fig. 1I ), upper–middle intertidal, sandy mud bottom with gravel, 15 July 2015, coll. H. Ariyama.

Type locality. Funaura in Iriomote Island , Okinawa Prefecture, Japan .

Etymology. From the Greek nanus, -a, -num (= very small), referring to the size of the species.

Diagnosis. Male pereon, ventral process absent. Male gnathopod 1, posterodistal corners of ischium and merus without projection; carpus wide, with 2 teeth posterodistally; propodus curved posteriorly, posterodistal corner swollen. Male gnathopod 2, coxa without posteromedial projection; basis curved anteriorly; carpus longer than propodus; propodus with a few robust setae on posterodistal corner and posterior margin. Uropod 1 peduncle with inter-ramal process.

Description of male. Based on holotype, male, 2.6 mm (OMNH-Ar-11546).

Head ( Figs 25 , 26 ) depressed dorsoventrally. Eyes medium-sized, about quarter length of head. Antenna 1 slender, about 0.6 times as long as body; length ratio of peduncular articles 1–3 1:1.15:0.4, article 1 with 4 ventral robust setae; accessory flagellum vestigial, uniarticulate; primary flagellum with 11 articles, terminal article minute, posterodistal corners of articles 5–10 each bearing aesthetasc. Antenna 2 stout, length about 0.45 times as long as body; length ratio of peduncular articles 3–5 1:2.9:2.8, article 3 with 2 medial robust setae; flagellum short, consisting of 3 normal and 1 minute articles, articles 2, 3 with 3 and 2 robust setae, respectively. Upper lip slightly hollowed ventrally, bearing many thin setae. Mandibles with length ratio of palp articles 1–3 1:1.4:1.2, articles 1, 2 each with 2 setae, article 3 clavate, setose distally; incisor with 4 and 5 cusps in left and right, respectively, lacinia mobilis with 3 cusps, 5 accessory setae present. Lower lip, distal margin of outer lobe slightly projected in middle, apical parts of outer and inner lobes covered with thin setae. Maxilla 1, outer plate with 12 distal robust setae, palp article 2 bearing 5 robust and several slender setae apically. Maxilla 2 with inner plate setose on distal and medial margins, lacking mediofacial row of setae, outer plate setose on distal margin. Maxilliped, distal part of inner plate setose, with 3 robust setae; outer plate bearing 7 long-to-short robust setae mediodistally; palp article 2 long, articles 2, 3 with several setae.

Pereon ( Fig. 27 ). Ventral surface lacking process. Gnathopod 1, coxa roundish trapezoidal; basis wide, lateral surface with ridge, posterior margin a little convex; ischium short, without posterodistal projection; merus posterodistal margin bearing a few setae, lacking projection; carpus ovoid, length about 1.2 times width, distal margin with small blunt tooth, posterodistal corner with large sharp tooth, posterior margin setose moderately; propodus curved posteriorly, posterodistal corner swollen roundly, setose; dactylus short, slightly curved posteriorly, posterior margin without denticle. Gnathopod 2, coxa trapezoidal; basis slender, curved anteriorly, with ridge anterolaterally; merus posterodistal margin setose; carpus, distal half of posterior margin densely setose; propodus 0.8 times as long as carpus, posterodistal corner and posterior margin with 2 and 1 short robust setae, respectively; dactylus short, curved posteriorly, posterior margin with 2 denticles. Pereopods 3 and 4, pereopod 3 slightly longer than pereopod 4; coxae trapezoidal; bases long; meri wide, a little dilated distally; carpi shorter than meri; propodi and dactyli narrow. Pereopod 5 shorter than pereopod 4; coxa bilobed; basis rectangular, posteroproximal corner angular, anterior margin with several setae, posterior margin bearing a few short setae and long plumose seta; merus with long thick seta each on anterodistal and posterodiatal corners; carpus bearing anterodistal thick seta and posterodistal robust seta; propodus with 2 lateral and 2 posterodistal robust setae; dactylus short, strongly curved. Pereopod 6 about 1.75 times as long as pereopod 5; coxa short, slightly bilobed; basis rectangular, posteroproximal corner angular, anterior margin with several short setae, posterior margin with many plumose setae; merus anterodistal corner with robust seta, posterior margin and posterodistal corner each bearing thick seta; carpus with lateral robust seta, posterodistal corner bearing 1 robust and 1 thick setae; propodus with 3 anterior robust setae, posterodistal corner setose; dactylus curved. Pereopod 7 about 1.2 times as long as pereopod 6; coxa short, slightly bilobed; basis rectangular, posteroproximal corner angular, anterior margin with several short setae and plumose seta, posterior margin bearing many plumose setae and several short setae; merus anterodistal corner with robust seta, posterior margin bearing 3 thick setae; carpus with lateral robust seta, posterodistal corner bearing 1 robust and 1 thick setae; propodus with 2 anterior robust setae and posterior thick seta, posterodistal corner setose; dactylus curved.

Pleon ( Fig. 28 ). Epimeral plates 1–3 rounded posteroventrally, without setae, ventral margins bare ( Fig. 25 ). Pleopods, each peduncle bearing several plumose setae and 2 coupling hooks; inner ramus longer than outer, both rami each with 6–7 articles. Uropod 1, dorsal surface of peduncle bearing 2 lateral and 3 medial robust setae, ventrodistal end with inter-ramal process; both rami shorter than peduncle, outer ramus subequal to inner ramus, former with 4 terminal robust setae, latter bearing 1 dorsomedial and 3 terminal robust setae. Uropod 2 about half length of uropod 1; peduncle dorsal surface with 1 laterodistal and 1 mediodistal robust setae; both rami shorter than peduncle, outer ramus shorter than inner ramus, former with 2 terminal robust setae, latter bearing 3 terminal robust setae. Uropod 3 about 0.55 times as long as uropod 2; peduncle slightly wider than long, swollen medially, lateral margin with seta; single ramus almost same length as peduncle, with tiny second article and 1 lateral and 4–5 terminal setae. Telson wider than long, laterodistal corners each with 2 setae.

Description of female. Based on paratype, ovigerous female, 3.2 mm (OMNH-Ar-11550). Gnathopod 1 ( Fig. 28 F-G1), coxa trapezoidal; basis almost straight; ischium and merus short, both lacking projection; carpus wide, posterior margin without projection, setose; propodus about 0.9 times as long as carpus, posterior margin with 2 robust setae; dactylus slightly curved posteriorly, posterior margin with 3 denticles. Gnathopod 2 ( Fig. 28 F-G2) almost same as that of holotype, male, 2.6 mm (OMNH-Ar-11546) except for oostegite, but coxa longer, basis almost straight, carpus shorter.

Variation. Paratype, small male, 1.8 mm (OMNH-Ar-11549). Gnathopod 1 ( Fig. 28 SM-G1) almost same as that of holotype, male, 2.6 mm (OMNH-Ar-11546), but propodus wider, posterior margin of dactylus with 2 denticles.

Coloration in recently fixed specimen ( Fig. 29 ). Eyes dark brown; dorsal surfaces of head, pereonites and pleonites yellowish light brown. Antenna 1 peduncles and proximal part of antenna 2 peduncular article 4 also yellowish light brown. Other parts white.

Remarks. Grandidierella nana sp. nov. is a small species (maximum BL: 2.6 mm in male and 3.4 mm in female). The wide and short carpus (length:width=1:1.2) of the male gnathopod 1 in adults is unique in the genus. Grandidierella propodentata Moore, 1986 and G. rhizophorae Myers, 2009 both from Australia have a relatively wide and short carpus (length:width=1:1.4) of the male gnathopod 1; however, G. propodentata possesses a posteroproximal tooth on the male gnathopod 1 propodus and G. rhizophorae has a single posterodistal tooth on the male gnathopod 1 carpus.

Habitat. Fine sand bottom in the middle intertidal zone, marine; sandy mud bottom with gravel in the up-per–middle intertidal zone, brackish.

Distribution. Japan: Iriomote Island (present study).

Figures

FIGURE 1. Map showing collecting sites of the examined specimens in the Ryukyu Archipelago.

FIGURE 25. Grandidierella nana sp. nov. Holotype, male, 2.6 mm (OMNH-Ar-11546). Habitus.

FIGURE 26. Grandidierella nana sp. nov. Holotype, male, 2.6 mm (OMNH-Ar-11546). Scales: 0.05 mm.

FIGURE 27. Grandidierella nana sp. nov. Holotype, male, 2.6 mm (OMNH-Ar-11546). Scales: 0.05 mm.

FIGURE 28. Grandidierella nana sp. nov. All except for F and SM: holotype, male, 2.6 mm (OMNH-Ar-11546); F, paratype, ovigerous female, 3.2 mm (OMNH-Ar-11550); SM, paratype, male, 1.8 mm (OMNH-Ar-11549). Scales: 0.05 mm.

FIGURE 29. Grandidierella nana sp. nov. Photograph of fixed specimen (male) collected together with the type specimens (6.5 hours after fixation).