Metarhachotropis parva, Ariyama & Kohtsuka, 2022

Ariyama, Hiroyuki & Kohtsuka, Hisanori, 2022, Metarhachotropis parva, a new genus and species of Eusiridae (Crustacea: Amphipoda) from Sagami Bay, central Japan, Zootaxa 5188 (1), pp. 95-100 : 96-99

Metarhachotropis parva sp. nov.

[Japanese name: Kawari-ryūgūyokoebi, new]

( Figs 1–4 )

Type material. Holotype: male, 2.1 mm (OMNH-Ar-12515), west of Jōgashima Island in Miura City , Kanagawa Prefecture, 35°08'25"N 139°33'42"E –35°08'19"N 139°33'41"E, 95–96 m depth, gravel and sand bottom, using dredge, 5 November 2015, coll. H. Kohtsuka. Paratype: male, 2.0 mm (OMNH-Ar-12516), same data as holotype.

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

Etymology. From the Latin parvus, - a, - um (= small), referring to the body size.

Description of male. Based on holotype (OMNH-Ar-12515, 2.1 mm) and paratype (OMNH-Ar-12516, 2.0 mm) only for mouthparts.

Head ( Figs 1 , 2 ) large, rostrum developed. Eyes situated subventrally, ca. 1/3 length of head, composed of many dispersed ommatidia. Antenna 1 short, about 0.3 times of BL, weakly setose; peduncular articles 1–3 with length ratio of 1:1.05:0.45, article 1 stout, ventral margin lacking robust setae; accessory flagellum short, 1-articulate, tip with 2 setae; primary flagellum with 4 articles, articles 2, 3 each with aesthetasc. Antenna 2 also short, weakly setose; peduncular articles 3–5 with length ratio of 1:2.1:2.25; flagellum with 4 articles. Upper lip with ventral margin rounded, bare. Mandibles, incisors with 2 cusps, laciniae mobilis 2-toothed, wide in left and narrow in right, 3 accessory blades present, minute; molar columnar, poorly triturative; palp stout, article length ratio 1:4.25:5.0, articles 1, 2 lacking setae, article 3 with 2–3 short setae on tip. Lower lip wide, bare; mandibular processes large, inner lobes short. Maxilla 1 with inner plate lacking seta; outer plate with 4 robust setae apically; tip of palp article 2 with 4 short setae. Maxilla 2, inner plate wider and shorter than outer plate, both plates with several short setae distally. Maxilliped stout; inner and outer plates bearing 5 and 2 short setae, respectively; palp developed, medial margin of article 2 and distal margin of article 3 with 2 and 5 short setae, respectively.

Pereon ( Fig. 3 ). Gnathopod 1 large, poorly setose; coxa strongly projected anteroventrally; basis widened distally; merus short; carpus narrow, lobate posteriorly; propodus stout, palm oblique, smooth; dactylus curved posteriorly. Gnathopod 2 also large, poorly setose; coxa pentagonal, basis–dactylus similar to gnathopod 1. Pereopods 3, 4 slender, weakly setose; coxae subtriangular, gill on pereopod 4 wider than that on pereopod 3; dactyli almost linear. Pereopod 5 about 1.2 times as long as pereopod 4, weakly setose; coxa bilobed, posterior lobe deeper; basis relatively wide, subrectagular; propodus long; dactylus almost straight. Pereopod 6 about 1.05 times as long as pereopod 5, weakly setose; coxa bilobed, posterior lobe deeper; basis wide, subrectangular; propodus long; dactylus linear. Pereopod 7 ca. 1.15 times length of pereopod 6, weakly setose; coxa oval; basis expanded, posterodistal corner roundly projected; propodus long; dactylus straight.

Pleon ( Fig. 4 ). Epimeral plates, posteroventral corners of plates 1, 2 angular, that of plate 3 rounded, ventral margins without setae. Pleopods, pleopod 3 shortest; peduncles projected posterodistally, without setae; both rami short, outer rami longer than inner rami, outer and inner rami with 4 and 3 articles, respectively. Uropods relatively slender, outer rami shorter than inner rami, tips of both rami with minute embedded seta; length ratio of uropods 1–3 1:0.9:0.65. Telson elongate, cleft 38% of telson length; lateral margins each with long sensory seta and short seta.

Female. Unknown.

Coloration in preservative (10 months after fixation). Eyes light brown. Body and appendages generally white.

Remarks. The uropods 1 and 2 with rami each bearing only minute embedded seta on the tip are also distinctive in the family Eusuridae sensu stricto. But discussion on the relationship with the senticaudate character is beyond the scope of this paper.

Habitat. Gravel and sand bottom, 95–96 m depth.

Distribution. Japan: Sagami Bay (present study).

Figures

FIGURE 1. Metarhachotropis parva sp. nov. Holotype, male, 2.1 mm (OMNH-Ar-12515). Habitus.

FIGURE 2. Metarhachotropis parva sp. nov. Holotype, male, 2.1 mm (OMNH-Ar-12515) [antennae and maxilla 2], and paratype, male, 2.0 mm (OMNH-Ar-12516) [M, the remaining]. Scales: 0.05 mm.

FIGURE 3. Metarhachotropis parva sp. nov. Holotype, male, 2.1 mm (OMNH-Ar-12515). Scales: 0.1 mm.

FIGURE 4. Metarhachotropis parva sp. nov. Holotype, male, 2.1 mm (OMNH-Ar-12515). Inner ramus of pleopod 2 lost. Scales: 0.1 mm.