A new species of Socarnes Boeck, 1871 (Crustacea, Amphipoda, Lysianassidae) from Korean waters

Abstract A new species of lysianassid amphipod belonging to the genus Socarnes Boeck was collected from Korean coastal waters. This is the first record of the genus Socarnes from Korea. The new species is fully illustrated and extensively compared with related species. A key to Socarnes speciesis provided.


Introduction
The subfamily Lysianassinae Dana, 1849 is a large group of the family Lysianassidae, with 29 genera. Among these genera, Socarnes has close affinity to the Concarnes, Socarnoides, Socarnella, Socarnopsis, and Socarnella by having a simple gnathopod 1 and cleft telson. However, the Socarnes is further characterized by the protruded upper lip, apically serrated palp of maxilla 1, and biarticulated outer ramus of uropod 3.
Species of Socarnes are widely distributed and are found in shallow to deep water, from the Arctic to the tropical South Pacific including temperate regions (Mediterranean and East Asia). Ecologically, they are well known scavengers (Lowry and Stoddart 1994;Hall-Spencer and Bamber 2007). The genus is relatively small, being comprised of 10 species. Hitherto, only two species of the subfamily Tryphosinae belonging to the family Lysianassidae, Orchomenella obtusa (Sars, 1895) and O. japonica Gurjanova, 1962 have been recorded in Korea (KSSZ 1997, Jung andKim 2008), even though the family Lysianassidae is one of the most speciose. This is the first record of the genus Socarnes from Korea and brings the total recorded number of lysianassid species to three from Korea. Many more are to be expected with increased sampling in this area, considering the high diversity of the group. A key to the world Socarnes species is also provided.

Material and methods
Specimens were collected with a hand-net from various algae, seagrass and coral rubble from shallow waters of Gyeongpo, Tongyeong-si, Korea. The specimens were fixed with 80% ethyl alcohol and dissected in glycerol on Cobb's aluminum hollow slides. Permanent mounts were made using polyvinyl lactophenol with lignin pink added. Drawings and measurements were performed with the aid of a drawing tube, mounted on an Olympus SZX 12 stereomicroscope and Olympus BX 51 interference contrast compound microscope. The body length was measured from the tip of rostrum to the end of the telson, along the dorsal parabolic line of the body. Nomenclature of the terms 'tooth', 'spine' and 'seta' follows Barnard and Karaman (1991) and terminology of the setae of the mandibular palp follows Lowry and Stoddart (1993). Type specimens are deposited at the National Institute of Biological Resources (NIBR), Incheon, Korea, Department of Biological Science, Dankook University (DKU), Cheonan, Korea and the Canadian Museum of Nature (CMN), Ottawa, Canada.
Lower lip ( Fig. 2E) with broad outer lobe covered with patch of pubescence medially, mandibular process thin and blunt apically.
Maxilla 2 (Fig. 2H) inner plate with dense pubescence medially, apical margin with 2 rows of simple or pectinate setae; outer plate subequal in width to inner and slightly longer, with simple and pectinate setae apically.
Male. Unknown. Remarks. The new species is morphologically similar to Socarnes vahlii (Krøyer, 1838) in size, in the short dactylus of gnathopod 2 and constricted inner ramus of uropod 2. However, the new species is obviously distinguished from S. vahlii (different characters of S. vahlii in brackets) by the combination of the following features: 1) gnathopod 2, propodus subequal to half the length of carpus and narrowing anterodistally (vs less than half the length of carpus and broadened distally); 2) epimeron 3 with rounded posterior corner (vs obtusely truncated posterior corner); 3) telson broad, length 1.3 × width, moderately cleft, about half of its length (vs narrow, length 1.6 × width, cleft > the middle of its length). The redescription of S. vahlii (Krøyer, 1838) by Nagata (1965), which is the only species of the genus recorded in Japan, was brief and lacked illustrations. He mentioned that his specimens were in agreement with Sars' figures (1895, Pl.16, fig. 2) except for the shape of the posteroventral corner of epimeron 3, which is similar to S. erythrophthalmus Robertson, 1892 and S. tongyeongensis sp. n. Our specimens show morphological similarities to the redescription of Nagata (1965). However, as we have not had the opportunity to examine the material of Nagata (1965), we cannot confidently determine whether Nagata's specimens are conspecific with ours, despite the geographic proximity. Therefore at this time we conservatively maintain Nagata's specimens as S. vahlii until a detailed, comparative study can be made on the type material. Given the great geographic distance however, we would expect Nagata's specimens to differ from S. vahlii, as that species was recorded from Greenland, North Western Atlantic and Arctic Oceans.
Etymology. The species name is derived from the type locality, Tongyeong-si located on the south coast of Korea.