A New Record of the Genus Orchomenella (Crustacea, Amphipoda, Tryphosidae) from Korean Waters

Abstract

The species Orchomenella littoralis Nagata, 1965 belong to the family Tryphosidae Lowry & Stoddart, 1997. Previously this species was only known from a single record identified in Japanese waters, and newly recorded from Baengnyeongdo-Island, on the west coast of Korea. The species is similar to Orchomene breviceps Hirayama, 1986 in carrying posteroventral teeth on epimeron 3. However, O. littoralis is characterized by the presence of 4-7 posteroventral teeth of epimeron 3, a round posterodistal lobe of coxa 4, and elongated antenna 2 in male. The species is described and fully illustrated in the present study. Also, a key to Korean Orchomenella species is provided.

INTRODUCTION

The superfamily Lysianassoidea Dana, 1849 is a very abundant and diverse taxonomic group of amphipod crustaceans found in almost every marine benthic habitat (Lowry and Stoddart, 2012b). Currently, this superfamily is actively discussed and studied taxonomically (Lowry and Stoddart, 2012a; Lowry and Kilgallen, 2014; Lowry and Myers, 2017). Recently, the subfamily Tryphosinae was removed from the family Lysianassidae and elevated to the family level within the Lysianassoidea (Lowry and Myers, 2017). Family Tryphosidae Lowry & Stoddart, 1997 is a large group of 376 species including 43 genera (Horton et al., 2019) and generally exhibit the following characteristics: (1) mandible, molar triturative; (2) maxilliped, apical margin of outer plate with robust setae; (3) gnathopod 1 subchelate; and (4) telson cleft (Lowry and Stodaart, 1997). However, not all of these features are shared, and a few taxa create confusion. The generic/subgeneric concepts of the Orchomene complex were revised by De Broyer (1984, 1985), and include the following genera: Abyssorchomene De Broyer, 1984; Allogaussia Schellenberg, 1926; Falklandia De Broyer, 1985; Orchomene Boeck, 1871; Orchomenella Sars, 1890; Orchomenopsis Sars, 1891; Orchomenyx De Broyer, 1984, and Tryphosa Boeck, 1871. Among these genera, Orchomenella comprises 34 species (Horton et al., 2019) and is similar to Orchomene. Oleröd (1975) and De Broyer (1984, 1985) defined these two genera based on mandible morphology. However, this definition is not entirely satisfactory because of several overlapping features between Orchomenella and other genera included in the Orchomene complex (Barnard and Karaman, 1991a, 1991b; Lowry and Stoddart, 1994). Recently, Jung et al. (2017) have described two new species of the genus Orchomenella in Korean waters, and their characteristics are mentioned in detail. In Korea, four Orchomenella species have been reported to date:(1) O. obtusa (G. O. Sars, 1891), (2) O. japonica Gurjanova, 1962, (3) O. paucisetigera Jung, Yi, Coleman & Yoon, 2017, and (4) O. rugosa Jung, Yi, Coleman & Yoon, 2017. Here, we add another species, Orchomenella littoralis Nagata, 1965 to the Korean lysianassid gammaridean fauna. Nomenclature of the terms ‘spine’ and ‘seta’ follows Barnard and Karaman (1991a, 1991b) and terminology of the setae of the mandibular palp follows Lowry and Stoddart(1993).

Specimens were collected using a light trap from the subtidal waters of the Baengnyeongdo Island, Korea. They were deposited at the National Institute of Biological Resources (NIBR), Incheon, Korea and the Department of Biological Science, Dankook University (DKU), Cheonan, Korea.

SYSTEMATIC ACCOUNTS

Order Amphipoda Latreille, 1816

Superfamily Lysianassoidea Dana, 1849

Family Tryphosidae Lowry & Stoddart, 1997

Genus Orchomenella Sars, 1890

Orchomenella littoralis Nagata, 1965 (Figs. 1-4)

Orchomenella sp. Nagata, 1960: 167, Pl. 13, figs. 7-8.

Orchomenella littoralis Nagata, 1965: 143.

Orchomene naikaiensis Hirayama, 1986: 1.

Fig. 1. Orchomenella littoralis Nagata, 1965. A, Adult male, 7.2 mm, habitus; B, Adult female, 4.5 mm, habitus. Scale bars: A, B=1.0 mm.

Fig. 2. Orchomenella littoralis Nagata, 1965, adult male, 7.3 mm. A, Habitus; B, Antenna 1; C, Antenna 2; D, Right mandible, E, Maxilla 1; F, Maxilla 2; G, Maxilliped; H, Gnathopod 1. Scale bars: A=1.0 mm, B, C, H=0.2 mm, D-G=0.1 mm.​​​​​​​

Fig. 3. Orchomenella littoralis Nagata, 1965, adult male, 7.3 mm. A, Gnathopod 2; B, Pereopod 3; C, Pereopod 4; D, Pereopod 5; E, Pereopod 6; F, Pereopod 7; G, Epimeral plates. Scale bars: A-G=0.4 mm.​​​​​​​

Fig. 4. Orchomenella littoralis Nagata, 1965, adult male, 7.3 mm. A, Uropod 1; B, Uropod 2; C, Uropod 3; D, Telson. Adult female, 6.3 mm: E, Antenna 1; F, Antenna 2; G, Lower lip; H, Pleon; I, Urosome; J, Telson and uropod 3, dorsal. Scale bars: A-C, E, F=0.2 mm, D, G, J=0.1 mm, H, I=0.4 mm.​​​​​​​

Material examined. 1♂(cat No. NIBRIV0000843928), 1♀(cat No. NIBRIV0000843927), Korea: Incheon, Baengnyeong-myeon, Baengnyeongdo Island, Yonggipo port, 37°57ʹ26ʺN, 124°44ʹ23ʺE, 12 Jul 2006, depth 5-10 m. The remaining paratypes 14♀♀(DKUAMP201901), in the collection of the corresponding author

Description. Male (cat No. NIBRIV0000843928): body 7.3 mm long.

Body (Figs. 1A, 2A) dorsally smooth; head, lateral cephalic lobe produced, apically rounded; eye not apparent in alcohol.

Epimera 1-2 (Fig. 3G) weakly and irregularly serrated posteroventrally; epimeron 3 (Fig. 3G) posteroventral corner with 4 teeth.

Antenna 1 (Fig. 2B) short, 0.31 times as long as antenna 2; peduncular article 1 swollen with a row of penicillate setae on ventral margin; peduncular articles 2-3 strongly telescoping, short; length ratio of peduncular articles 1-3=1.00 : 0.20 : 0.10; flagellum long, 2.2 times as long as peduncle, 12-articulate; article 1 with strong callynophores, calceoli absent; accessory flagellum 6-articulate, article 1 rather elongate.

Antenna 2 (Fig. 2C) greatly elongate, about the same length as body; peduncular article 4 widening distally, 1.57 times as long as article 5, with 2 penicillate setae near ventral margin and simple setae ventrodistally, distal half of dorsal margin with 11 simple setae; flagellum elongate, 52- articulate, calceoli absent.

Right mandible (Fig. 2D), incisor smooth, with 1 small tooth on cutting edge; lacinia mobilis absent; accessory setal row with 3 spines; molar process well developed, subcylindrical, truncate and triturative distally; palp attached slightly below molar level, 3-articulate; article 1 short, unarmed; article 2 longest, with 15 A2-setae; article 3 slender, 0.71 times as long as article 2, with 11 D3-setae, and 3 E3-setae.

Maxilla 1 (Fig. 2E), inner plate subtriangular, with 2 pectinate setae apically; outer plate with 11 dentate spine-teeth; palp biarticulate, proximal article short, distal article curved with 8 blunt spines and 1 long pectinate seta apically.

Maxilla 2 (Fig. 2F), inner plate slender, with rows of simple setae mediodistally, 1 pappose seta medially; outer plate 1.06 times longer than inner one, with 7 denticulate and 9 simple setae distally; inner and outer plates generally covered with pubescence.

Maxilliped (Fig. 2G), inner plate subrectangular, with row of 6 plumose setae medially, apical margin with trifurcated cutting edge and 2 simple setae; outer plate subovate, reaching distal end of article 2 of palp, with 13 teeth on medial and apical margins, larteral surface with 4 spinules; palp 4-articulate, article 1 slightly shorter than article 2, with 1 simple seta on inner and laterodistal margins, respectively; article 2 with a row of 22 simple setae on inner margin, 2 simple setae laterodistally; article 3 subrectangular, with simple setae on inner margin and distal corner; article 4 falcate, with small unguis, shorter than article 3.

Gnathopod 1 (Fig. 2H) subchelate; coxa large, not widening distally, anterior margin slightly concave; basis subrectangular, with simple setae anteriorly; merus with pubescence posteriorly and simple setae posterodistally; carpus subtriangular, with two simple setae distally and posterodistal portion with pubescence; propodus rectangular, both margins nearly parallel, 1.32 times as long as carpus; palm short, perpendicular, defined by 2 stout spines posterodistally; dactylus falcate, fitting palm, with accessory tooth.

Gnathopod 2 (Fig. 3A) slender; coxa subrectangular, with 8 simple setules anteriorly, 0.36 times as wide as long; basis slender, elongate, with short setae anteriorly; ischium elongate, with 3 short setae posteriorly, 0.90 times as long as carpus; merus 0.64 times as long as ischium, with pubescence posteriorly; carpus 0.53 times as long as basis, posterior margin slightly convex, generally covered with simple setae; propodus short, 0.47 times as long as carpus, obtusely angled distally with unequal simple setae; dactylus small, acute, fitting palm.

Pereopod 3 (Fig. 3B), coxa similar to that of gnathopod 2, width 0.34 times length; basis with 2 simple setae posterodistally; merus and carpus with simple setae posteriorly; propodus 1.22 times as long as carpus, with a row of 5 spines and 3 simple setae posteriorly; dactylus falcate, 0.45 times as long as propodus.

Pereopod 4 (Fig. 3C) similar to pereopod 3 except coxa much broader than that of pereopod 3, posterior margin excavate, posterodistal lobe strongly produced.

Pereopod 5 (Fig. 3D), coxa large, bilobate, rounded quadrate, subequal in width and length, basis subovate, with a row of 11 spines and pubescence along anterior margin; merus slightly expanding posteriorly, anterior margin with 4 simple setae and 2 spines, posterior margin with 3 spines; carpus 0.78 times as long as merus, with 6 spines anteriorly and 1 spine posterodistally; propodus subrectangular, 1.43 times as long as carpus, with 2 duad spines and 2 spines anteriorly; dactylus falcate, 0.50 times as long as propodus.

Pereopod 6 (Fig. 3E), basis moderately broad, width 0.62 times length, with a row spines along anterior margin, posterior margin slightly convex, with a row setules; merus slightly expanding posteriorly, with 4 long simple setae and 2 spines anteriorly, posterior margin with 2 spines; carpus subequal in length to merus, with 4 duad spines anteriorly, posterodistal margin with 1 spine; propodus equal in length to carpus, with 2 duad spines and 1 spine anteriorly; dactylus falcate, 0.42 times as long as propodus.

Pereopod 7 (Fig. 3F) similar to pereopod 6, basis slightly broader than that of pereopod 6, posterior margin convex.

Uropod 1 (Fig. 4A), peduncle subrectangular, 1.15 times as long as outer ramus, with 3 small spines and pubescence dorsolaterally, 2 bifid spines and 1 robust spine dorsomedially; outer ramus slightly longer than inner one, with 2 dorsolateral spines; inner ramus with 2 dorsolateral spines.

Uropod 2 (Fig. 4B), peduncle subequal in length to outer ramus, with 3 dorsolateral spines and 1 apicolateral spine; outer ramus slightly longer than inner one, with 2 spines dorsolaterally; innter ramus unarmed.

Uropod 3 (Fig. 4C), peduncle short, 0.69 times as long as outer ramus; outer ramus biarticulate, 1.08 times as long as inner ramus, proximal article with 8 plumose setae along inner margin, outer margin with 3 spines and pubescence, distal portion with 2 single spines; distal article slender, 0.30 times as long as proximal one; inner ramus 0.69 times as long as peduncle, inner margin with long plumose setae and outer margin with 5 spines and 5 plumose setae.

Telson (Fig. 4D) longer than broad, cleft 78% of its length, each lobe with a pair of penicillate setae and 1 spine dorsolaterally, and 1 spine apically.

Adult female (cat No. NIBRIV0000843927): body 6.3 mm long.

Epimera 1-2 (Fig. 4H) similar to those of male; epimeron 3 (Fig. 4H), posteroventral corner with 7 teeth.

Antenna 1 (Fig. 4E) similar to that of male; flagellum 8-articulate.

Antenna 2 (Fig. 4F) short; peduncular articles 4-5 slenderer than those of male; flagellum 7-articulate, much shorter than that of male.

Lower lip (Fig. 4G), inner lobe indistinct; outer lobe pubescent medially, and mandibular process elongate, well developed.

Urosomite 1 (Fig. 4I) with dorsal depression and mid-dorsal carina; uropod 1 slightly longer than uropod 2, subequal to apex of uropod 3.

Telson and uropod 3 (Fig. 4J) similar to those of male, except much less setose and spinose than male.

Remarks. The taxonomical status of O. littoralis Nagata, 1965 was changed several times. Nagata (1960) described a species, Orchomenella sp. with brief illustrations which had collected from the Seto Inland Sea of Japan. In 1965, Nagata named this species as O. littoralis with additional description. Hirayama (1986) reestablished the species name after it was synonymized with Orchomene naikaiensis for two reasons: (1) to follow the rearrangement of genus by Barnard (1969) reporting that the genus Orchomenella belongs to the genus Orchomene, and (2) to avoid homonymy with Orchomene littoralis reported by Schellenberg (1926). Due to the rules of priority in zoological nomenclature, the species has been correctly named as Orchomenella littoralis.

Orchomenella littoralis is very similar to O. breviceps Hirayama, 1986 in carrying posteroventral teeth on epimeron 3. However, O. littoralis is distinguished by the following characteristics: (1) males display elongated antenna 2, nearly reaching the apex of the uropod 1 (vs. half of body in O. breviceps); (2) antenna 2, flagellum article 1 short, 0.41 times as long as peduncular article 5 (vs. subequal to peduncular article 5 in O. breviceps); (3) epimeron 3 with 4-7 posteroventral teeth (vs. 3 teeth in O. breviceps); and (4) coxa 4, posterodistal lobe round (vs. angular in O. breviceps). Our specimens are in accordance with the original description provided by Nagata (1965). As mentioned above, Oleröd (1975) and De Broyer (1984, 1985) defined these two genera based on the shape of molars on the mandible. Accordingly, O. breviceps might belong to genus Orchomenella because O. breviceps carries subcylindrical (button-shaped) molar

Distribution. Korea (Baengnyeongdo-island).

Key to the species of Orchomenella in Korean waters

1. Lateral cephalic lobe rounded·········································· 2

Lateral cephalic lobe pointed or subpointed···················· 3

2. Gnathopod 1, propodus not exceed half the length of the basis, margins parallel; palmar margin irregular ··············· ··········· O. paucisetigera Jung, Yi, Coleman & Yoon, 2017 Gnathopod 1, propodus exceed half the length of the basis, ventral margin slightly concave; palmar margin convex·········································O. obtusa (G. O. Sars, 1891)

3. Antenna 2, ventrodistal margin of peduncular article 5 not expansion in male; urosomite 1 without keel hanging over urosomite 2 dorsodistally ················································· 4 Antenna 2, ventrodistal margin of peduncular article 5 expansion in male; urosomite 1 with rounded keel hanging over urosomite 2 dorsodistally ···································· ············································ O. japonica Gurjanova, 1962

4. Posterior margin of epimeral plate 3 not serrated; posterior margin of epimeral plates 1-2 and urosomite 1 serrated ······················ O. rugosa Jung, Yi, Coleman & Yoon, 2017 Posterior margin of epimeral plate 3 serrated; posterior margin of empimeral plates 1-2 and urosomite 1 not serrated ·········································· O. littoralis Nagata, 1965