A new species of the genus Opisa Boeck, 1876 (Crustacea, Amphipoda, Opisidae) and a new record for Opisatakafuminakanoi from the East Sea, South Korea

Abstract Two species of the opisid genus Opisa have been collected from the East Sea of South Korea, one of them described as Opisaparvimanasp. nov.. The new species, O.parvimanasp. nov. is similar to O.odontochela; however, it can be clearly distinguished from this species because it has 12 blunt robust setae in the palm of gnathopod 1. The other collected species, Opisatakafuminakanoi Narahara-Nakano, Kakui & Tomikawa, 2016 is previously known from Japanese waters (southeast of Akkeshi Bay, Hokkaido). Both species are illustrated and compared to related species. A key to Opisa species is also provided.


Introduction
The family Opisidae was first established by Lowry and Stoddart (1995) for a small group of lysianassoid amphipods that currently includes 19 species in four genera (Narahara-Nakano et al. 2016;Horton et al. 2020). They are mainly known from the North Pacific Ocean, North Atlantic Ocean, and Mediterranean Sea (Stoddart and Lowry 2010). Most opisids species are considered ectoparasitic in fish (Vader and Romppainen 1985;Bousfield 1987;Stoddart and Lowry 2010), attracted to the smell of the fish, to which they attach instead of scavenging, because they do not have the mouthpart structure of a scavenger (Stoddart and Lowry 2010). Parasitic amphipods are typically found on slow-moving, slow-growing benthic sharks and bony fishes in cold or deep waters; as a group, rockfish, sculpins, goosefishes, and flatfishes may be described as ambush predators (Vader and Romppainen 1985;Bousfield 1987).
The genus Opisa Boeck, 1876 includes a total of four species: O. eschrichtii (Krøyer, 1842), O. odontochela Bousfield, 1987, O. tridentata Hurley, 1963, and O. takafuminakanoi Narahara-Nakano, Kakui & Tomikawa, 2016, with the last one reported in Japan (Narahara-Nakano et al. 2016). In this study, we report an additional new species and a newly recorded species of Opisa from South Korea through illustrations and text descriptions. This study also provides a key to Opisa species around the world.

Materials and methods
The material examined was collected with a fishing net from subtidal waters of the Namae Port, East Sea, South Korea. Specimens were fixed in 70-80% ethanol and dissected in glycerol on Cobb's aluminum hole slides. Examinations were performed using a stereoscope (Olympus SZX 10) and a compound microscope (Olympus BX 51), and the drawings and measurements were made with the aid of a drawing tube. 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 term 'seta' follows Watling (1989), Garm and Watling (2013). Terminology of the setae of the mandibular palp follows G. Karaman (1969Karaman ( , 1971 and Lowry and Stoddart (1993). Type specimens are deposited at the National Institute of Biological Resources (NIBR), Incheon, South Korea and the Marine Amphipoda Resources Bank of Korea (MARBK), Cheonan, South Korea. teeth; palp biarticulate, proximal article short, distal article expanded, with 2 slender setae and 6 blunt robust setae apically.
Maxilliped (Fig. 2I), inner plate rectangular, with 3 pectinate setae medially, apical margin with 2 unequal simple setae and 2 blunt robust setae; outer plate moderately expanded, not reaching distal end of article 3 of palp, with 8 blunt robust setae on inner margin and 7 short simple setae medially; palp 4-articulate, article 1 slightly shorter than article 2, with 1 simple seta on inner margin; article 2 with 7 simple setae on inner margin; article 3 slightly shorter than article 2, with simple setae on inner and distal margins; article 4 falcate, 0.47× shorter than article 3.
Uropod 2 (Fig. 3H), peduncle subequal in length to both rami, with 4 dorsolateral and 3 medial robust setae; each ramus with distinct notch with inserted robust setae; outer ramus subequal in length to inner one, both rami each with 1 dorsolateral and 1 dorsomedial robust setae.
Etymology. The species name is derived from the Latin parvus (=small) and manus (=hand) with reference to the relatively small propodus of the gnathopod 1.   Walker, 1906 in having deep coxal plates, a bilobate telson, small modification or reduction of mandible and maxilliped palps, and distinctly biarticulate outer ramus of uropod 3. However, the genus Opisa is easily distinguished from these genera by the following features: 1) enlarged gnathopod 1, strongly subchelate or cheliform; 2) mandibular molar very reduced or even missing; and 3) maxilliped, broadened outer plate and reduced palp (Bousfield 1987).
Adult female: body (Figs 5B, 7H) about 8.7 mm long. Head similar to that of male except more rounded lateral cephalic lobe.