Aora biarticulata, Ariyama & Kohtsuka, 2022

Ariyama, Hiroyuki & Kohtsuka, Hisanori, 2022, Three new species of the family Aoridae collected from Sagami Bay, central Japan (Crustacea: Amphipoda), Zootaxa 5159 (3), pp. 393-413 : 395-400

Aora biarticulata sp. nov.

[Japanese name: Nisetsu-himeyumbo-sokoebi, new]

( Figs 1–4 )

Type material. Holotype: male, 6.3 mm (OMNH-Ar-12429), west of Jōgashima Island in Miura City , Kanagawa Prefecture, 35°07′10″N 139°34′11″E –35°06′46″N 139°34′10″E, 238–282 m depth, 25 June 2015 . Paratypes: 3 males, 5.1, 4.7, 3.6 mm (OMNH-Ar-12430–12432), female with juveniles, 6.2 mm (OMNH-Ar-12433) and female, 5.2 mm (OMNH-Ar-12434), west of Jōgashima Island in Miura City , Kanagawa Prefecture, 35°06′57″N 139°33′44″E –35°06′43″N 139°34′03E″, 230–398 m depth, 10 December 2015 .

Type locality. Sagami Bay , west of Jōgashima Island in Miura City, Kanagawa Prefecture, Japan .

Etymology. The Latin biarticulatus, - a, - um (= 2-articulated) refers to the number of the accessory flagellum articles.

Description of male. Based on holotype (OMNH-Ar-12429, 6.3 mm).

Head ( Figs 1 , 2 ). Eyes medium (diameter: ca. 0.3 times of head length). Antenna 1 long, ca. 0.7 times of BL, slender; peduncular articles 1–3 with length ratio of 1:1.5:0.35, article 1 stout, ventral margin lacking robust setae; accessory flagellum short, 2-articulate, second article minute, with 4 setae on tip; primary flagellum with 22 articles, last article minute. Antenna 2 far shorter than antenna 1 (about 0.55 times), slender, weakly setose; peduncular articles 3–5 with length ratio of 1:3.1:3.55; flagellum with 6 articles, last article minute, articles 3–5 each with 2 robust setae distally. Upper lip with ventral margin rounded, bearing many short thin setae. Mandibles, left one with incisor bearing 5 cusps, 4-toothed lacinia mobilis, 4 accessory blades and 2 plumose setae, right one with incisor bearing 7 cusps, 6-toothed lacinia mobilis, 3 accessory blades and 1 plumose seta; palp stout, article 3 semi-falciform, article length ratio of right one 1:3.2:4.4, article 1 bare, article 2 with 5–6 setae, article 3 heavily setose. Lower lip with acute-tipped mandibular process, distal margin of outer lobe with several thick setae, dorsal surfaces of inner and outer lobes and ventral surface of outer lobe covered with short thin setae. Maxilla 1 with small inner plate bearing single long seta; outer plate with 10 robust setae apically; tip of palp article 2 with 7 distal robust setae. Maxilla 2, outer plate setose mediodistally, inner plate with many distal setae and mediofacial row of setae. Maxilliped with inner plate bearing 3 short robust setae and several plumose setae; outer plate broad, not reaching apex of palp article 2, distomedial margin with 11 robust setae; palp weakly setose, dorsodistal end of article 3 projected triangularly.

Pereon ( Fig. 3 ). Sternal process absent. Gnathopod 1 very large; coxa trapezoidal, anteroventral corner rounded, ventral margin bearing 3 short setae; basis stout, anterior margin and lateral surface bare, posterodistal corner with 2 short setae; ischium bearing 2 short setae on posterodistal corner; merus long, lanceolate, weakly setose; carpus roundish rectangular, anterior margin without setae, posterior margin and medial surface weakly setose; propodus almost straight, notched posterodistally, ca. 0.65 times as long as carpus, anterior margin with a few setae, posterior margin and medial surface weakly setose; dactylus slightly curved, posterior margin with 5 small denticles. Gnathopod 2 small, carpus–dactylus strongly twisted (counter clockwise in left, Fig. 1 ); coxa roundish rectangular, ventral margin with 5 short setae; basis slightly curved anteriorly, anterior margin with several short setae, posterior margin bearing single long and several short setae; meus short, setose posterodistally; carpus longish triangular, posterior margin heavily setose; propodus ca. 0.9 times as long as carpus, palm oblique, defined by robust seta, anteromedial surface and posterior margin heavily setose; dactylus short, curved, posterior margin with 5 denticles. Pereopods 3, 4 slender, slightly longer than gnathopod 2; coxae roundish rectangular; bases narrow, anterior margins with a few setae, posterior margins bearing 1 long and 4–6 short setae; meri and carpi without robust setae on posterior margins; propodi and dactyli narrow. Pereopod 5 almost same length as pereopod 4; coxa bilobed; basis wide, posteroproximal corner rounded, anterior margin with a few short robust setae, posterodistal corner bearing thick seta; carpus with 1 lateral and 3 distal robust setae; propodus anterior margin bearing 4 robust setae, posterodistal corner setose. Pereopod 6 about 1.5 times as long as pereopod 5; coxa bilobed; basis wide, posteroproximal corner rounded, anterior margin with a few short robust setae, posterodistal corner bearing thick seta; merus with 3 long thick setae on posterior margin; carpus bearing 1 lateral and 2 posterodistal robust setae; propodus anterior margin with several robust setae, posterodistal corner weakly setose. Pereopod 7 ca. 1.3 times length of pereopod 6; coxa oval; basis wide, posteroproximal corner roundly projected, anterior margin with a few short robust setae, posterodistal corner bearing thick seta; merus with 5 thick setae on posterior margin; carpus bearing a few thick setae each on anterior and posterior margins; propodus elongate, anterodistal corner with 2 short robust setae, posterodistal corner weakly setose.

Pleon ( Figs 1 , 4 ). Epimeral plates 1–3 each with minute projection and short seta on posteroventral corner, ventral margins bare. Pleopods, pleopod 3 shortest; peduncles with 9, 3, 3 plumose setae in pleopods 1–3, respectively; outer rami shorter than inner rami, outer ramus of pleopod 3 with 8 articles, other outer rami and inner rami bearing 9 articles. Uropod 1, peduncle with 1 basofacial, 3 dorsolateral and 2 dorsomedial robust setae, inter-ramal process long, 62% length of peduncle; both rami ca. 1.35 times as long as peduncle, outer ramus with 3 dorsolateral and 4 terminal robust setae, inner ramus bearing 2 ventrolateral, 1 dorsolateral, 6 dorsomedial and 4 terminal robust setae. Uropod 2 about 3/4 length of uropod 1, peduncle with 2 dorsolateral and 1 dorsomedial robust setae, inter-ramal process short, 12% length of peduncle; outer and inner rami ca. 1.2 and 1.4 times of peduncle, respectively, outer ramus with 4 dorsal and 4 terminal robust setae, inner ramus bearing 5 dorsal and 4 terminal robust setae. Uropod 3 about 0.6 times length of uropod 2, peduncle with 2 laterodistal and 1 mediodistal robust setae, inter-ramal process minute; outer and inner rami ca. 1.45 and 1.25 times of peduncle, respectively, outer ramus with marginal robust seta in middle, tip with tiny second article bearing 2 long setae, 2 robust setae and 1 short seta, inner ramus with 3 marginal and 1 terminal robust setae and long apical seta. Telson roundish pentagonal, distal margin with pair of 2 long and 1 short setae, lateral margins each with 2 sensory setae.

Description of female. Based on paratype (OMNH-Ar-12433, 6.2 mm). Generally similar to male holotype except for gnathopods and oostegites. Gnathopod 1 ( Fig. 4 F-G1) subchelate; coxa roundish trapezoidal; basis stout, poorly setose; ischium with posterodistal setae; merus short, posterodistal margin setose; carpus longish triangular, posterior margins setose; propodus about 1.2 times length of carpus, posterior margin and medial surface setose, palm oblique, defined by long robust seta; dactylus weakly curved, posterior margin with 7 denticles. Gnathopod 2 ( Fig. 4 F-G2), carpus–dactylus strongly twisted like male holotype; coxa large, roundish square; basis relatively narrow, anterior margin with several short setae, posterior margin bearing 1 long and a few short setae; meus short, weakly setose posterodistally; carpus longish triangular, posterior margin setose; propodus long, ca. 1.05 times length of carpus, palm oblique, defined by robust seta, posterior margin weakly setose; dactylus short, distal 1/3 narrowed, posterior margin with 2 denticles.

Variation. Paratype, small male (OMNH-Ar-12430, 5.1 mm). Gnathopod 1 ( Fig. 4 SM-G1), carpus wider than that of holotype ( Fig. 3G 1 ); propodus ca. 0.85 times as long as carpus, palm oblique, defined by long robust seta; dactylus posterior margin with 6 denticles. Paratype, juvenile male (OMNH-Ar-12432, 3.6 mm). Gnathopod 1 ( Fig. 4 JM-G1), merus short, with distal tip reaching half-length of carpus; propodus ca. 1.1 times length of carpus, palm oblique, defined by long robust seta; dactylus posterior margin with 4 denticles.

Coloration in life. Unknown.

Remarks. Aora biarticulata sp. nov. is characterized by (1) accessory flagellum with 2 articles, (2) weakly setose male gnathopod 1 with rounded coxa, and (3) male gnathopod 2 with setose carpus and propodus (setose only on posterior margin of carpus and both anterior and posterior margins of propodus). There is no other Aora species possessing all these characters. Especially in the accessory flagellum, the number of articles varies from 3 to 6 in the Aora species except for this new species (unknown in Aora adpressa Myers & Moore, 1983 , A. anomala Schellenberg, 1926 , and A. gibbula K.H. Barnard, 1932 ), whereas A. biarticulata has a 2-articulated accessory flagellum. Although Aora adpressa has the above characters (2) and (3), A. adpressa can be distinguished from A. biarticulata by the male pereon segments 2–4 each bearing strong sternal process, the male gnathopod 1 with shorter propodus, and the male gnathopod 2 with longer carpus.

Habitat. Bottom sediment unknown, 230–398 m depth.

Distribution. Japan: Sagami Bay (present study).

Figures

FIGURE 1. Aora biarticulata sp. nov. Holotype, male, 6.3 mm (OMNH-Ar-12429). Habitus.

FIGURE 2. Aora biarticulata sp. nov. Holotype, male, 6.3 mm (OMNH-Ar-12429). Scales: 0.05 mm.

FIGURE 3. Aora biarticulata sp. nov. Holotype, male, 6.3 mm (OMNH-Ar-12429). Scales: 0.1 mm.

FIGURE 4. Aora biarticulata sp. nov. All except for F, SM and JM: holotype, male, 6.3 mm (OMNH-Ar-12429); F: paratype, female, 6.2 mm (OMNH-Ar-12433); SM: paratype, small male, 5.1 mm (OMNH-Ar-12430), slender setae omitted; JM: paratype, juvenile male, 3.6 mm (OMNH-Ar-12432), slender setae omitted. Scales: 0.1 mm.