On the genus Mesopontonia Bruce, 1967 (Crustacea: Decapoda: Palaemonidae) in Korea, with the description of a new species

Mesopontonia verrucimanus and Mesopontonia kimwoni sp. nov. are recorded from high-latitude temperate waters in Munseom Islet, Jejudo Island, Republic of Korea, with both species collected on gorgonians and sponges by trimix diving between 50 and 75 m depth. Mesopontonia kimwoni sp. nov. is morphologically allied to M. brevicarpus, but can be distinguished by the cutting edges of the fingers of the first chela being entire. Significant morphological variation in the rostrum as well as the second pereiopods is documented in M. verrucimanus, although cytochrome c oxidase subunit I (COI) barcode analysis proves this to be infra-specific variation. A key to species of the genus Mesopontonia is provided.


INTRODUCTION
The deep-sea palaemonid shrimp fauna of the Indo-West Pacific is relatively well documented, with to date 23 genera and about 84 species recorded from depths of more than 100 m by trawling and dredging (Bruce, 1991;De Grave & Fransen, 2011;Kou, Li & Bruce, 2016;Li, Mitsuhashi & Chan, 2008;Marin & Chan, 2014;Mitsuhashi & Chan, 2006;Okuno, 2017;Wang, Chan & Sha, 2015). Among them is the rarely recorded genus, Mesopontonia Bruce, 1967 which can be distinguished from related genera by the combination of the absence of both supraorbital and antennal teeth on the carapace, as well as the absence of an exopod on the third maxilliped (Bruce, 1967;Bruce, 1995;Chace & Bruce, 1993).
The most recent classification of carideans by De Grave & Fransen (2011) listed six species in the genus, namely M. gorgoniophila Bruce, 1967 (type species), M. gracilicarpus , M. brucei Burukovsky, 1991, M. monodactylus Bruce, 1991, M. verrucimanus Bruce, 1996and M. brevicarpus Li & Bruce, 2006. The type species was originally described from the northern part of the South China Sea (Bruce, 1967), in association with two species of the gorgonian genus Melithaea Milne-Edwards, 1857 (Cnidaria: Octocorallia: Melithaeidae) in depths of 117-183 m (Fig. 1A). Since then, it has been sparingly reported upon from New Caledonia, the Philippines, and eastern Australia within depths of 130 to 270 m (Bruce, 1979;Bruce, 1984;Bruce, 1985;Bruce, 1991). Mesopontonia gracilicarpus has been reported from New Caledonia and the Chesterfield Islands in depths of 226-600 m Bruce, 1991;Li & Bruce, 2006). The southwestern Indian Ocean species, M. brucei was recorded from depths of 415-460 m from about 850 km south of Madagascar . Mesopontonia monodactylus was described from the Loyalty Islands (Bruce, 1991), associated with species of the hexactinellid sponge genus Pheronema Leidy, 1868 (Porifera: Hexactinellida: Pheronematidae) from 460 m ; M. verrucimanus was reported from the Tanimbar Islands, Indonesia in depths of 184-186 m . Finally, another western Indian Ocean species, M. brevicarpus was recorded from off La Réunion from 270 m (Li & Bruce, 2006). Jejudo Island, the largest island in Korea, is located about 80 km off the southwestern coast of the mainland (Fig. 1B). In summer, the Tsushima current mixes with low-salinity, high-turbidity waters from the Yangtze River to influence the environment around Jejudo Island. In winter, the Yangtze River discharge reduces, resulting in a higher local salinity (Rebstock & Kang, 2003;Lim et al., 2019). Munseom Islet (Fig. 1C) is located off the south coast of the main island, and consists of volcanic rocks covered with rich invertebrate communities (Cho et al., 2014;Lutaenko, Noseworthy & Choi, 2019;Lee et al., 2019), with a maximum depth of less than 75 m. Thus far, only five symbiotic palaemonid shrimps have been reported from the Jejudo Island area (Koo & Kim, 2003;Lee & Ko, 2011;Lee & Ko, 2014;Park, De Grave & Kim, 2019a;Park, De Grave & Kim, 2019b), although many more remain unrecorded and will be covered in future contributions.
During a faunal survey for deep-water invertebrate species from previously unexplored habitats around Jejudo Island in 2015-2020, numerous specimens of Mesopontonia were collected from gorgonians and sponges by trimix SCUBA diving between 50-75 m depth. Detailed examination of their morphology as well as a phylogenetic analysis including related genera indicated that these belong to M. verrucimanus (new record for Korea) and an undescribed species in the same genus, constituting the most northerly record for the genus, as well as the first for temperate waters in the Western Pacific.
In this study, we thus describe both Mesopontonia species and present an identification key for the genus Mesopontonia. To support systematic studies for deep-sea palaemonid shrimps, molecular analyses and ecological information through direct observations are provided. Micro Kit (QIAGEN, Hilden, Germany), following the manufacturer's instructions. Partial sequences of the COI (∼658 bp) and 16S (∼538 bp) markers were amplified via polymerase chain reaction (PCR) with the primers jgHCO2198/jgLCO1490 (Geller et al., 2013) and 16S-ar/16S-1472 (Crandall & Fitzpatrick Jr, 1996;Palumbi et al., 2002), respectively. PCR reactions and sequence data analysis were performed following Park, De Grave & Kim (2019a). Zoobank registration. 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:3CB43670-472F-49AE-80F2-EAE9597E12BD. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.

Taxonomy
Family Palaemonidae Rafinesque, 1815Genus Mesopontonia Bruce, 1967 Mesopontonia verrucimanus Bruce, 1996 Figs. 2-7 Mesopontonia verrucimanus Bruce, 1996: 198, 216-218, figs   slender, about 3.6 times longer than proximal depth, distally curved with acute tip, proximally with two acute teeth at 0.3 and 0.4, distally entire without dorsolateral flange; fixed finger with acute tip, proximally with two small teeth at 0.2 and 0.3, distally entire; palm subcylindrical, about 4.0 times longer than distal width, covered with minutely blunt tubercles and simple setae; carpus about 0.45 of palm length, about 2 times longer than distal width; merus about 2.0 times as long as carpus, as long as palm length, about 6.2 times longer than distal width; ischium as long as carpus length, about 6.0 times longer than distal width; basis and coxa without special features.
Minor second pereiopod (Figs. 5F, 5G) slightly overreaching distal end of scaphocerite; chela about 0.4 of pocl, 0.7 of major chela length, with group of terminal setae; fingers about 0.7 of palm length, distally curved with acute tips, cutting edge entire; palm subcylindrical, about 3 times longer than distal width, smooth slightly tapering proximally; carpus about 1.4 of palm length, about 0.9 of chela length, about 5.6 times longer than distal width; merus about 1.1 times as long as carpus length, about 7.8 times longer than distal width; ischium about 1.1 times as long as merus length, about 7.8 times longer than distal width; basis and coxa without special features.
Ambulatory pereiopods (Figs. 2 and 6)   Variation.  described Mesopontonia verrucimanus based on a single specimen (holotype), with nine dorsal teeth, unarmed ventrally and markedly unequal, dissimilar second pereiopods. The Korean specimens exhibit morphological variation in rostral dentition and the major second pereiopod. The number of dorsal and ventral rostral teeth (Figs. 2 and 3A-3D) varies from 6-9 and 0-2 respectively. A single specimen (Fig.  3D) harbours two epigastric teeth on the carapace. Several specimens (Fig. 2B) bear two symmetrical second pereiopods, which are very similar to the minor second pereiopods in the original description and in other Korean specimens ( Fig. 2A).

Color in life.
Whole body and appendages almost transparent with scattered emerald green chromatophores (Fig. 7); longitudinal pale red band along the ventral surface of the body from the carapace to the fifth abdominal somite.
Habitat and host. The present specimens were obtained from gorgonian and sponge colonies below 50 m (Figs. 14A, 14B), with the deepest samples from 75 m depth. The present specimens demonstrate a lack of host specificity and the species cannot be considered as restricted to gorgonians, as previously postulated. Most specimens were collected on the orange colored sea whip, Ellisella cf. limbaughi (Bayer & Deichmann, 1960), with further specimens obtained on the white colored gorgonian, Cirrhipathes cf. anguina, as well as the orange colored sponge, Raspailia (Raspaxilla) hirsuta Thiele, 1898. A single specimen was collected on the white colored antipatharian, Myriopathes lata. Remarks. Mesopontonia verrucimanus can be immediately separated from most other species in the genus which have a biunguiculate dactylus of ambulatory pereiopods, except M. monodactylus with which it shares a non-biunguiculate dactylus. M. monodactylus differs from M. verrucimanus primarily by having a distinct dorsolateral flange on the chela of the major second pereiopod (Bruce, 1991;.  (Fig. 8) smooth, glabrous, with epigastric tooth at anterior 0.3 of pocl; without supraorbital and antennal teeth; inferior orbital angle produced; hepatic tooth large, acute, extending to anterior margin of carapace; pterygostomial angle bluntly rounded.
First pereiopod (Figs. 8 and 9A, 9B) overreaching distal end of scaphocerite; fingers about 0.6 of palm length, tips hooked, cutting edge entire, with transverse row of setae and group of terminal setae; palm ventrolaterally with transverse row of serrulate setae; carpus 1.1 times length of chela with row of serrulate setae along distomesial margin; merus 1.1 times length of carpus; ischium about 0.5 times length of merus; basis and coxa without special features.
Second pereiopods (Figs. 8 and 10C-10G) well developed, similar in shape, unequal in size. Major second pereiopod (Figs. 10C-10E) overreaching distal end of rostrum by middle of propodus; chela about 1.3 times as long as pocl, with group of terminal setae; fingers about 0.4 of palm length; dactylus slender, about 3.8 times longer than proximal depth, distally curved with acute tip, proximally with two blunt teeth at proximal 0.3 and 0.4, distally entire without dorsolateral flange; fixed finger with distally curved with acute tip, proximally with single acute tooth at 0.4, distally entire; palm subcylindrical, about 4.2 times longer than distal width, covered with minutely blunt tubercles and short simple setae; carpus about 0.4 of palm length, about 2.8 times longer than distal width; merus about 2.1 times as long as carpus, about 0.8 of palm length, 7.0 times longer than distal width; ischium subequal to carpus length, about 7.0 times longer than distal width; basis and coxa without special features.
Minor second pereiopod (Figs. 10F, 10G) overreaching distal end of rostrum by end of carpus; chela about 0.7 of pocl, 0.7 of major chela length, with group of terminal setae; fingers about 0.7 of palm length, with distally curved with acute tips, cutting edge entire; palm subcylindrical, about 3.75 times longer than distal width, smooth, slightly tapering proximally; carpus about 1.3 of palm length, about 0.75 of chela length, about 7.2 times longer than distal width; merus about 1.1 times as long as carpus length, about 9 times longer than distal width; ischium about 0.9 of merus length, about 10 times longer than distal width; basis and coxa without special features.
Fourth pereiopod (Figs. 11C, 11D) with dactylus about 0.2 times length of propodus, about 4 times longer than proximal width, about 0.65 of corpus length, biunguiculate; propodus with four distolateral spiniform setae including single distoventral one, with long simple setae distally; carpus about 0.45 times length of propodus, unarmed; merus subequal to propodus length, unarmed; ischium about 0.46 length of propodus; basis and coxa without special feature. Fifth pereiopod (Fig. 11E) similar to fourth pereiopod. Uropod (Fig. 11D) overreaching distal end of telson; exopod with distolateral tooth and movable acute tooth.   M. kimwoni sp. nov., appears most closely related to the west Indian species, M. brevicarpus, sharing a similar rostral formulation, a tuberculate major second pereiopod chela, as well as the ratio of the ambulatory pereiopods. Both species can be most easily distinguished on the basis of the combination of the following characters, (1) fingers of first chela with entire cutting edge (vs. fine pectinated serrations subapically on both fingers in M. brevicarpus); (2) hepatic tooth reaching to the anterior margin of the carapace (vs. reaching or extending to anterior margin of carapace in M. brevicarpus) and (3) the finger of minor chela being about 0.7 of the palm length (vs. fingers subequal to palm in M. brevicarpus).  suggested two further new species may be present in the genus, one collected from Indonesia , as well as the juvenile specimen assigned to M. gorgoniophila in Bruce (1985), both however were left unnamed. Given their incomplete or juvenile status these taxa are not considered herein, but are unlikely to be the same species as M. kimwoni, as the details of the first and second pereiopods are different.

Molecular data analyses
Fragments of 658 and 462 bp were obtained for the COI and 16S markers, respectively. The multiple sequence alignment revealed that the K2P distance between the five specimens of M. verrucimanus which showed minor morphological variations in the dentition of the rostrum and proportions of the second pereiopods fall within an intraspecific range, being 0-0.5% (Table 2). The intraspecific divergence between both specimens of M. kimwoni sp. nov. was higher at 1.1% (Table 2). To eludicate the phylogenetic position of the genus, an analysis was performed on 22 specimens of 16 species of 12 genera ( Table 1). The ML and BI analyses showed the same topology (Fig. 15), and the combined phylogenetic tree clearly demonstrated the monophyly of Mesopontonia with high support values (BP = 100, PP = 100). Furthermore, their distant relationship was supported by the K2P distance, which was 13.6% (Table 2).  From the concatenated tree in the present analysis,the genera Mesopontonia and Paraclimenes are postulated to be sister taxa with high support values (BP = 100, PP = 92), indicating that they are more genetically related to each other than the remaining analysed genera, supported by morphological similarities.

DISCUSSION
The present study explored the commensal palaemonid fauna of Jejudo Island, recording Mesopontonia kimwoni sp. nov. and M. verrucimanus at higher latitude temperate waters than the genus was previously known from. While the six species previously known in the genus had been reported from between 117 and 600 m depth by trawling and dredging (Bruce, 1967;Bruce, 1979;Bruce, 1984;Bruce, 1985;Bruce, 1991;Li & Bruce, 2006), the present specimens were collected from shallower depths of less than 75 m. As they were directly collected with technical SCUBA diving equipment, more details are available on their habitat and ecology, whilst color patterns are recorded for the first time for the genus as a whole.
Mesopontonia kimwoni sp. nov. can be distinguished from all other Mesopontonia species by the combination of the biunguiculate dactylus of the ambulatory pereiopods, the lack of a dorsolateral dactylar flange on the major second chela, the relatively long carpus of the minor second pereiopod and the entire cutting edge of fingers of first chela. Specimens of M. verrucimanus exhibited minor morphological variation in rostral dentition and proportions of the major second pereiopod, but all specimens are clearly conspecific.
Due to its rarity, Mesopontonia had not been previously included in family level phylogenies (e.g., Kou et al., 2013;Gan et al., 2015;Horká et al., 2016), but is herein shown to be phylogenetically close to Paraclimenes. This is also supported by a relatively similar morphology with both antennal and supraorbital teeth being absent; and the epigastric and hepatic teeth being present. Nevertheless, Paraclimenes can be readily distinguished from Mesopontonia by the presence of a well-developed exopod on the third maxilliped (Bruce, 1995). Li & Bruce, 2006)