Two new temporary ectoparasitic isopods (Cymothoida: Cymothooidea) from Korean waters with a note on geographical distributions of Rocinela Leach, 1818 and Gnathia Leach, 1814

Two new species of temporary ectoparasitic isopods, Rocinela excavata sp. nov. and Gnathia obtusispina sp. nov., are reported from the southern Islands of the Korean Peninsula. Rocinela excavata sp. nov. is distinguishable from its related species by the following characteristics: (1) laterally stepped rostrum; (2) separated eyes; (3) propodal blade having eight robust setae; and (4) merus having four or five blunt robust setae in pereopods 1–3. Gnathia obtusispina sp. nov. differs from its congeners by the combination of the following characteristics: (1) body covered with numerous tubercles and setae, (2) cephalon having tooth-like paraocular ornamentations; and (3) frontal border having two inferior frontolateral processes. These two new species are the 13th Rocinela species and 19th Gnathia species in the temperate Northern Pacific region, respectively. Discovery of these new species represents high species diversity of the genera Rocinela Leach, 1818 and Gnathia Leach, 1814 worldwide as well as in the Northern Pacific region. In addition, faunal diversity analysis on the members of both genera revealed that Rocinela species show high-latitude diversity whereas Gnathia species have low-latitude diversity.


MATERIALS AND METHODS
All materials were collected at the bottom of sublittoral zones using a Smith-McIntyre grab and SCUBA diving. Rocinela specimens were sampled from sandy-mud flats by using the Smith-McIntyre grab. Gnathia specimens were collected from the bryozoans and seaweeds on bedrock. SCUBA diving was used to survey the bedrock of sublittoral zones. These collected materials were immediately fixed in 95% ethyl alcohol and then transferred to the laboratory. Isopods were sorted from the transferred materials and then observed and dissected under a dissecting microscope (Olympus SZH-ILLD, Japan). Measurements and drawings of specimens were conducted with the aid of a drawing tube on a compound microscope (Olympus,BX50,Shinjuku,Tokyo,Japan) or the dissecting microscope. Pencil drawings were digitally scanned, inked, and arranged using a tablet and Adobe Illustrator CS6 as mentioned in Coleman (2003Coleman ( , 2009. All examined type series and additional material were moved into each small glass vial filled with 95% ethanol and deposited at the National Institute of Biological Resource (NIBR), South Korea.
The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: (urn:lsid:zoobank.org: pub:7A53937A-F2EB-49C7-B8DA-F0AA36241310). The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.
Remarks. Synonymy and diagnosis have been well recognized by Brusca & France (1992) and Bruce (2009) and we followed them in this study. Among the members of this family, the genus Rocinela Leach, 1818 is distinguishable from other genera by having pleonite 1 not abruptly narrowing than pereonite 7 and a three-articled maxillipedal palp (Bruce, 2009). Although Rocinela species show a quite uniform appearances to each other, the shape of the frontal margin of the cephalon and the pereopodal armature are most helpful in identifying species (Brusca & France, 1992;Bruce, 2009 posterior margin slightly tri-sinuated, but not distinct; rostrum truncated anteriorly, stepped laterally; eyes large, separate. Pereonite 1 slightly longer than other pereonites; pereonite 3 widest; pereonite 7 narrower than preceding pereonites, tapering posteriorly. Coxal plates visible on dorsal side, acute posteriorly; coxal furrows present in coxal plates 4-7. Pleonite 1 hidden by pereonite 7, slightly visible on both lateral sides; pleonites 2-4 with subacute apex, but pleonite 5 with blunt apex. Pleotelson (Fig. 1E) semicircle or shield-shaped, tapering posteriorly, with numerous plumose setae and robust setae distally; lateral margins concave proximally; dorsal surface with one pair of depressions proximally and one medial carina. Antennule (Fig. 1F) reaching anterior margin of pereonite 1; peduncular article 1 wider than article 2, with two penicillate setae distally; article 2 subequal to article 1 in length, with three penicillate setae and one simple seta laterally; article 3 elongated oblong, longest, 1.7 times longer than article 2, with one penicillate seta and two short simple setae distally; flagellar article 1 rectangular, 0.3 times as long as peduncular article 3, without setae; articles 2-5 square, with two aesthetascs distally; article 6 min, with two aesthetascs, one penicillate seta, and three simple setae. Antenna (Fig. 1G) exceeding beyond posterior margin of pereonite 1; peduncular article 1 globular; article 2 short, with three simple setae distally; article 3 4.7 times longer than article 2, with one simple seta distally; article 4 oblong, 1.5 times longer than article 3, with one simple seta; article 5 elongated, longest, 1.3 times longer than article 4, with three penicillate setae and three simple setae distally; flagellum consisting of 16 articles; each article with short simple setae distally except for first article without setae.
Remarks. The material of R. excavata sp. nov. can easily be characterized as new to science by the following combinations of characters: (1) the rostrum is truncated anteriorly and stepped laterally; (2) eyes are separated from each other; (3) pereopods 1-3 have eight robust setae on the propodal blade and four or five blunt robust setae on each merus; (4) ischium to carpus in pereopods 4-7 have tubercles along the posterior margins; and (5) one pair of depressions is located at the proximal region of the pleotelson.
Among the known 41 species of the genus Rocinela, only three species have separated eyes and more than seven robust setae on the propodal blade in pereopods 1-3: R. niponia Richardson, 1909, R. garricki Hurley, 1957, and R. pakari Bruce, 2009(Richardson, 1909Bruce, 2009). Among them, Rocinela excavata sp. nov. most resembles R. garricki by sharing characteristics of the rostrum and propodal blade of pereopods 1-3. However, the former can be rapidly distinguished from the latter in terms of the distal end of the rostrum (truncated in the former vs. rounded in the latter) and the shape of the robust setae on the merus in pereopods 1-3 (blunt in the former vs. subacute in the latter). Rocinela excavata sp. nov. differs from the R. niponia and R. pakari in terms of the laterally stepped rostrum (vs. not stepped rostrum in the latter two species) and pereopods 4-7 having tubercles along the posterior margins (vs. smooth in the latter two species) (Bruce, 2009;Kim & Yoon, 2020).
Among seven species reported from the Far East, Rocinela excavata sp. nov. is most similar to R. japonica in the structure of rostrum and setal armature of pereopods 1-3's merus, while the latter exhibits a distinct difference in the number of setae on the propodal blade in pereopods 1-3 (eight robust setae in the new species vs. three or four robust setae in R. japonica) (Richardson, 1898(Richardson, , 1904(Richardson, , 1909Kussakin, 1974;Vasina, 1993). Rocinela excavata sp. nov. can be distinguishable from other six species by having separated eyes (vs. fused eyes in R. affinis), pereopod 1 bearing eight robust setae on the propodal blade (vs. less than eight in the latter six other species) (Richardson, 1904(Richardson, , 1909Kussakin, 1979;Brusca & France, 1992).
Etymology. The specific name, excavata, originates from the combination of Latin prefix ex-meaning "out of" and Latin word cavatus meaning "hollow out". It refers to the shape of the rostrum laterally excavated; gender feminine.
Diagnosis. Cephalon with generally straight frontal margin bearing frontal processes, while not deeply concaved; mandibles not elongated, with mandibular incisor and dentate blade; paraocular ornamentation and/or a dorsal sulcus present; pylopod distinct, 2 or 3-articled.
Pereopod 2 (Fig. 5A) with tubercles on ischium to propodus inferiorly; basis with three penicillate setae superiorly, numerous simple setae superiorly and inferiorly; ischium 0.8 times as long as basis, with one serrate seta and six simple setae inferiorly, and one penicillate seta and four simple setae superiorly; merus 0.3 times as long as ischium, with one serrate seta and three simple setae inferiorly, and three simple setae superiorly; carpus similar to merus in length, with one simple seta and two serrate setae inferiorly; propodus oblong, 1.8 times longer than carpus, with two robust simple setae, one simple seta and several short simple setae on inferior margin, and one penicillate seta and one short simple seta at superodistal angle; dactylus rectangular, with four simple setae and one unguis distally. Pereopods 3-6 (Figs. 5B-5E) almost similar to pereopod 2; basis with tubercles superiorly except for pereopod 5.
In the East Asia where the new species were collected, there are nine species characterized by the presence of tubercles on the cephalon and pereonites among 25 Gnathia species reported: G. tuberculata Richardson, 1909 from the Nanao, Japan; G. derzhavini Gurjanova, 1933 from the Askold Island, Russia; G. schmidti Gurjanova, 1933 from the Bay of Vladimir, Russia; G. teruyukiae Ota, 2011 from the Ishigaki Island, Japan; G. rufescens Ota, 2015 from the Okinawa Island Japan; G. albipalpebrata Ota, 2014 from the Okinawa-jima Island, Japan; G. parvirostata Ota, 2014 from the Ishigaki Island, Japan; G. nubila Ota & Hirose, 2009 from the Okinawa Island, Japan; and G. dejimagi Ota, 2014 from the Okinawa-jima Island, Japan (Boyko et al., 2008;Song & Min, 2018;Shodipo et al., 2021). Although G. obtusispina sp. nov. also represents this character state, this new species is easily distinguishable from the latter species by the combination of the following character states: (1) the body is covered with long setae; (2) the cephalon has a pair of remarkable tooth-like blunt paraocular ornamentations; (3) the frontal border of the cephalon is medially concave; (4) two inferior frontolateral processes are present ventrally; (5) the supraocular lobe is prominent and projecting upwards; (6) the dentate blade of the mandible is present and irregular; (7) pereonite 1 is not fused with cephalon dorsally and conspicuous; and (8) the apex of the pleotelson is rounded (Richardson, 1909;Gurjanova, 1933;Ota, 2011Ota, , 2015Ota & Hirose, 2009).
Among the above-mentioned species, G. obtusispina sp. nov. is most similar to G. tuberculata by having inferior frontolateral processes and prominent supraocular lobes on cephalon, and mandible as long as half-length of the cephalon. However, the former differs from the latter in terms of the medially concave frontal border of the cephalon (vs. produced in the latter), presence of a tooth-like paraocular ornamentations (vs. absent in the latter), number of inferior frontolateral processes (two in the former vs. four in the latter), and rounded apex of the pleotelson (vs. acute in the latter) (Richardson, 1909).

Distribution. South Korea (the Yellow Sea)
Host. Unknown.
Etymology. The specific name, obtusispina, originates from the combination of Latin words obtusus, meaning "blunt" and spina, meaning "thorn". This name refers to tooth-like paraocular ornamentation; gender feminine.

DISCUSSION
Rocinela is distributed worldwide. It particularly shows high-latitude diversity (Bruce, 2009). Indeed, based on marine ecoregions of the world by Spalding et al. (2007), 29 of 41 known Rocinela species have been reported from a temperate region (Table 1). Among the temperate species, 21 known species are recorded from the Pacific, with 12 species from the temperate Northern Pacific region, including seven species from the Far East. This means that the majority of Rocinela species have been described from the temperate Northern Pacific, so the region could be considered as diversity hotspot for the genus Rocinela. However, given that Bruce (2009) has mentioned that a significant number of undescribed species from the tropical western Pacific region is held at the Muséum national d'Histoire naturelle in Paris, the lack of attention on the Rocinela species was likely to negatively affect our knowledge of the Rocinela species diversity in trophic region. So, undescribed species can be discovered through further study in this region. While among 29 species are known from the temperate region, only two species, R. angustata and R. belliceps, show a broad distribution ranging from the Northwest to Northeast Pacific despite most Rocinela species having endemic distribution ranges (Richardson, 1904(Richardson, , 1905(Richardson, , 1909Kussakin, 1979;Brusca & France, 1992). Considering that host-association times is correlated with the distribution range and that Rocinela species can attach to the host temporally, these endemic distribution ranges of Rocinela species might be due to their feeding strategy with temporary ectoparasites attaching to fishes in their particular life history (Bruce, 2009;Smit, Bruce & Hadfield, 2019). Although hosts of R. angustata and R. belliceps remain unknown, broad distribution ranges of these two species could be related to their host's distribution patterns (Smit, Bruce & Hadfield, 2019).
Fifty-six and 76 species of 133 known Gnathia species have been reported from a temperate region and tropical region, respectively (Table 2; Song & Min, 2018;Shodipo et al., 2021). Only two species, G. fragilis Schultz, 1977 andG. tuberculosa (Beddard, 1886), are from the Southern Ocean, Antarctic (Monod, 1926;Schultz, 1977). According to the marine ecoregions of the world, the Central Indo-Pacific (with 47 species) is thought to be the most diverse hotspot of Gnathia (Shodipo et al., 2021). After the Central Indo-Pacific, the second-most rich species of 18 species have been reported from the temperate Northern Pacific that includes the study area of the present study. Consequently, the temperate Northern Pacific is considered to be the second most diverse hotspot following the Central Indo-Pacific. Within the temperate Northern Pacific, the Far East, from which 11 Gnathia species are recorded, could be regarded as a representative hotspot. While looking for substrate types from which Gnathia species are collected, most temperate species have been collected from soft substrates such as mud, silt, and sandy flats in contrast to tropical Gnathia species reported from coral-reef habitats (Cohen & Poore, 1994;Svavarsson & Bruce, 2012). This result is a mismatch to the general knowledge that gnathiid species prefer coral reef-associated habits (Cohen & Poore, 1994;Santos & Sikkel, 2019;Smit, Bruce & Hadfield, 2019;Svavarsson & Bruce, 2012. Furthermore, the feature of the substratum strongly affects the distribution of gnathiids, and each species has a different habitat depending on its life stages (Smit, Bruce & Hadfield, 2019). Taken all together, the life history of Gnathia species is likely to differ depending on whether they live in a temperate or a tropic region (Santos & Sikkel, 2019). However, further study about the substratum preference between temperate and tropic Gnathia species is needed because most ecological studies of these species have been conducted from coral reef-associated habitats (Grutter, Morgan & Adlard, 2000, Grutter et al., 2018Santos & Sikkel, 2019;Smit, Bruce & Hadfield, 2019;Shodipo et al., 2021). Additionally, although most Gnathia species are known as endemic, two species, Gnathia calmani Monod, 1926 andGnathia nasuta Nunomura, 1992, have wide distributions ranging from the tropic to Table 2 Summary of Gnathia species from the temperate region.

CONCLUSION
The present study of Korean ectoparasitic isopods revealed high species diversity of Rocinela and Gnathia species in the temperate Northern Pacific region by the discovery of two new species, Rocinela excavata sp. nov. and Gnathia obtusispina sp. nov. The two new species are the species records for the 13 th Rocinela species and the 19 th Gnathia species in this region, respectively. Our investigation on the geographical distributions of known Rocinela and Gnathia species indicated that the temperate Northern Pacific has the most Rocinela species and the second most Gnathia species in the regional species richness of each genus. It also showed that even if both genera indicate great diversity in the western Pacific, Rocinela species reveal high-latitude diversity while Gnathia species represent low-latitude diversity, particularly in the Central Indo-Pacific region.

ADDITIONAL INFORMATION AND DECLARATIONS Funding
This study was supported by research funds from Chosun University (2022) and a grant (201902204) of the National Institute of Biological Resources (NIBR) funded by the Ministry of Environment (MOE), the Republic of Korea. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Field Study Permissions
The following information was supplied relating to field study approvals (i.e., approving body and any reference numbers): Field experiments were approved by Ministry of Environment (MOE) of the Republic of Korea (project number: NIBR201902204; NIBR202102204).

Data Availability
The following information was supplied regarding data availability: The raw data are line drawings of two new species and the morphological character.