First records of Gnathia Leach, 1814 and Tachaea Schioedte & Meinert, 1879 from South Korea, with descriptions of two new species (Isopoda, Cymothoida, Cymothooidea)

Abstract Two new species of cymothoid isopods, Gnathiakoreanasp. n. and Tachaeakoreaensissp. n., are described from South Korea. The genera Gnathia Leach, 1814 and Tachaea Schioedte & Meinert, 1879 are recorded for the first time in South Korea. Gnathiakoreanasp. n. is distinguished from its congeners by having the smooth dorsal surface of the pereon, the strongly ridged unornamented paraocular ornamentation, the strong bifid mediofrontal process, and the serrated superior frontolateral process. Tachaeakoreaensissp. n. is distinguished from its congeners by having the expanded propodus with serrated inferior margins in pereopods 1–3, the propodus with serrated inferodistal margins in pereopods 4–7, one seta on the apical lobe of the maxilla, and ten robust setae on the posterior margin of the pleotelson.


Introduction
The isopod crustacean family Gnathiidae Leach, 1814 is one of the nine families belonging to the superfamily Cymothooidea Leach, 1814. This family is unusual among isopods as its members exhibit peculiar morphological differences between juveniles (praniza stage) and adults (Cohen and Poore 1994, Ota 2012, Tanaka 2004, Tanaka and Nishi 2011. In addition, they have biphasic life cycles with ectoparasitic larva (praniza stage) and free-living adults (Hadfield et al. 2009, Smit et al. 2003. Pranizas are regarded to be parasites of fishes, whereas adults are non-feeding and have a degenerated intestine (Golovan 2006;Tanaka and Nishi 2011). The genus Gnathia Leach, 1814 is the largest group in this family and is currently composed of 126 species distributed worldwide Cohen and Poore 1994).
The family Corallanidae Hansen, 1890 also belongs to the superfamily Cymothooidea. The genus Tachaea Schioedte & Meinert, 1879 is the smallest group in this family and is currently composed of seven species . The type species, Tachaea crassipes Schioedte & Meinert, 1879 is the only marine species found in the coral reefs of Singapore, whereas other species are found in freshwater habitats as ectoparasites of fish and various shrimp species (Delaney 1989).
Herein, we report two new species collected from South Korea, Gnathia koreana sp. n. and Tachaea koreaensis sp. n. The genera Gnathia and Tachaea were first found in the United Kingdom and Singapore, respectively, but the present study represents the first record of these genera in South Korea.

Sampling
Specimens of G. koreana sp. n. were collected using light traps from Geomun-do Island (approximately 10 m depth) in South Korea. The sediment at the sampling site was characterized as organic-rich muddy sand. Specimens of T. koreaensis sp. n. were collected as ectoparasites on the freshwater shrimps Macrobrachium nipponense (De Haan, 1849) and Palaemon paucidens De Haan, 1844 collected from reservoirs in South Korea. All specimens were preserved immediately after collection in 95% ethyl alcohol. The type specimens of the two new species have been deposited in the National Institute of Biological Resources (NIBR), Incheon, South Korea.

Morphological analysis
The specimens were transferred to glycerine for dissection, and then examined and dissected under a dissection microscope (Olympus, model SZX-7  (Leica, model no. 11513106) and an ocular micrometre. The photograph of the whole body of G. koreana sp. n. was taken using a digital camera (eXcope, model K6) mounted on a stereomicroscope, and those of the cephalon was taken using a scanning electron microscope (Hitachi, model S-4200). Pre-treatments were performed based on the methods described by Song and Min (2016).
Morphological terminology and the orientation of each appendage largely follows Bruce (2009); some morphological terms were taken from Cohen and Poore (1994) to retain descriptive consistency for the cephalic appendages of G. koreana sp. n. Setal terminology largely follows Watling (1989). Unless otherwise specified, the setae are simple.

Key to the species of Gnathia from Japan, Korea, and Russian Far East
This key is based on males. Etymology. The specific name 'koreana' is derived from the name of the nation from which the specimens were collected.
Antennula ( Figure 2C) peduncle article 2 0.6 times as long as article 1; article 3 1.7 times as long as article 2; flagellum with five articles. Antenna ( Figure 2D) peduncle article 4 3.0 times as long as wide, 1.2 times as long as article 3, with two penicillate setae; flagellum with seven articles.  Mandible ( Figures 1C, D, 2E) 0.5 times as long as width of cephalon, curved upward; mandibular seta present; carina present, unornamented; incisor elevated, distally rounded; dentate blade with five weak processes; pseudoblade absent; internal lobe absent; dorsal lobe absent; basal neck short; erisma absent; lamina dentate absent. Maxilliped ( Figure 2F) article 1 lateral margin with continuous marginal setae; article 2 lateral margin with three plumose setae; article 3 lateral margin with seven plumose setae; article 4 lateral margin with four plumose setae; article 5 with seven plumose setae; endite extending to distal margin of article 2; without coupling setae. Pylopod ( Figure 2G) with three articles; article 1 1.4 times as long as wide, without distolateral lobe, with three areolae; posterior and lateral margins forming rounded curve; lateral margin with 30 plumose setae; mesial margin with continuous setae, distal margin with 5-6 setae; article 2 1.3 times as long as wide; article 3 minute.
Pereopods 2-6 ( Figure 3A-E) without long plumose setae; basis superior margin with 2-3 penicillate setae; dactylus superodistal margin with one penicillate seta. Pereopod 2 basis 2.4 times as long as greatest width, superior margin with two setae, inferior margin with three setae; ischium 0.7 times as long as basis, 2.4 times as long as wide, superior margin with three setae, inferior margin with four setae and four tubercles; merus 0.6 times as long as ischium, 1.8 times as long as wide, superodistal margin with one seta, inferior margin with one seta and four tubercles; carpus as long as merus, 2.1 times as long as wide, superodistal margin with one seta, inferior margin with three setae and three tubercles; propodus 0.8 times as long as ischium, 3.5 times as long as wide, superior margin with three setae, inferior margin with two robust setae; dactylus 0.3 times as long as propodus. Pereopods 3-6 similar, but basis superior margin with 3-6 tubercles.
Habitat. This species was collected at approximately 10 m depth corresponding to a sedimentary bottom of muddy sand.
Remarks. Gnathia koreana sp. n. is distinguished from other known species of Gnathia by the following characters: 1) the dorsal surface of the pereon without tubercles, 2) the paraocular ornamentation is strongly developed, forming a ridge, without tubercles, 3) the mediofrontal process is strong and bifid, 4) the superior frontolateral process is shorter than the mediofrontal process, rounded, and serrated, and 5) the mandible without internal lobe.
Gnathia koreana sp. n. is most similar to G. excavata from Japan in terms of the following characters: the body is smooth, the mediofrontal process is bifid, and the mandible without internal lobe. However, the new species is distinguished from G. excavata by the shape of the paraocular ornamentation and superior frontolateral process. In G. excavata, the paraocular ornamentation with three indistinct tubercles and the superior frontolateral process is acute and not serrated. In comparison, in the new species, the paraocular ornamentation with distinct unornamented ridge and the superior frontolateral process is rounded and serrated.
Distribution. Only known from the type locality.

Key to the species of Tachaea
This key is based on females. Therefore, we excluded T. crassipes that is designated the holotype based on the male specimen. Etymology. The specific name 'koreaensis' is derived from the name of the nation from which the specimens were collected.
Frontal lamina ( Figure 5B) elongate, narrow; clypeus short and broad. Antennula ( Figure 5C) peduncle article 1 triangular, 1.1 times as long as wide, with four setae and two penicillate setae; article 2 0.7 times as long as article 1, with two setae and one penicillate seta; article 3 minute, 0.2 times as long as article 2, with three setae distally; flagellum with seven articles, articles 3-6 with 2 aesthetascs, articles 6 and 7 minute. Antenna ( Figure 5D) peduncle article 1 0.8 times as long as wide, with one seta; article 2 shortest, with two setae; article 3 2.0 times as long as article 2, with two setae; article 4 with two setae; article 5 with six setae and two penicillate setae; articles 4 and 5 similar length; flagellum with 12 articles, each articles with six setae.   Mandible ( Figure 5E) palp article 2 with four serrate setae and two setae; article 3 with seven short serrate setae, three serrate setae. Lacinia mobilis and molar process absent; incisor monocuspid. Maxillula ( Figure 5G) lateral lobe forming single large curved spine. Maxilla ( Figure 5H) short, apical lobe with one seta. Maxilliped (Figure 5F) without endite; palp with four articles, narrow, 4.4 times as long as wide.
Pereopods 1-3 ( Figure 6A-C) similar, propodus inferior margins expanded with serrations. Pereopod 1 basis 2.7 times as long as greatest width, superior margin with three setae, inferodistal margin with one seta; ischium 0.6 times as long as basis, 1.8 times as long as wide, superior margin with two setae, inferior margin with two setae; merus 0.6 times as long as ischium, 0.8 times as long as wide, superodistal margin with two setae and one robust setae, inferodistal margin with three robust setae, inferior margin with one seta; carpus shortest, 0.4 times as long as merus, 0.3 times as long as wide, superodistal margin without setae, inferodistal margin with five robust setae; propodus 1.2 times as long as ischium, 1.7 times as long as wide, superior margin with one seta and one penicillate seta, inferior margin with three robust setae, one comb seta and one seta; dactylus 0.5 times as long as propodus. Pereopods 4-7 ( Figure 6D-G) similar, propodus inferodistal margins with serrations. Pereopods 4 and 5 carpi with three comb setae; pereopod 6 carpus with four comb setae, propodus with three comb setae; pereopod 7 carpus with seven comb setae, propodus with five comb setae.
Remarks. Tachaea koreaensis sp. n. is distinguished from other known species of Tachaea by the following combination of characters: 1) the inferior margins of the propodus of pereopods 1-3 is expanded with serrations, 2) the inferodistal margins of the propodus of pereopods 4-7 with serrations, 3) the apical lobe of the maxilla with one seta, and 4) the posterior margin of the pleotelson with ten robust setae.
Tachaea koreaensis sp. n. is most similar to T. spongillicola from India, but it can be distinguished from the latter by the following characters: the ratio of pereonite 1 to pereonite 2, the number of cusps on the mandible, the presence or absence of setae on the apical lobe of the maxilla, and the number of robust setae on the posterior margin of the pleotelson. In T. spongillicola, the pereonite 1 is as long as the pereonite 2, the incisor of the mandible with two cusps, the apical lobe of the maxilla without seta, and the posterior margin of the pleotelson with eight robust setae. In comparison, in the new species, the pereonite 1 is 1.7 times as long as the pereonite 2, the incisor of the mandible with one cusp, the apical lobe of the maxilla with one seta, and the posterior margin of the pleotelson with ten robust setae.