Two New Species of Nipponbathynella Schminke, 1973 from South Korea, with a Brief Discussion on Korean Parabathynellid Genera (Crustacea, Bathynellacea)

: Parabathynellidae is a family well known for its diversity and is endemic to the interstitial hyporheic zone of South Korea. Two new parabathynellid species, Nipponbathynella yeongchunen-sis sp. nov. and N. gapcheonensis sp. nov., were found through groundwater sampling on riverbanks in South Korea. Nipponbathynella yeongchunensis sp. nov. can be distinguished from its congeners by the base socket of the two dorsal setae on the antennule’s ﬁrst segment. Nipponbathynella gapcheo-nensis sp. nov. can be distinguished by the protopod of thoracopod II–IV with hairs. We provide morphological descriptions and remarks about the two new species with molecular information based on 18S rRNA and partial COI gene sequences. In addition, phylogenetic analysis, including Nipponbathynella Schminke, 1973, which had not been previously molecularly evaluated, was inferred. The results of the 18S rRNA gene phylogeny suggest a close relationship between the genera Nip-ponbathynella and Arisubathynella , as well as Europe–North American genera. We have also included a brief discussion and an identiﬁcation key for the ﬁve known parabathynellid genera found in South Korea.


Introduction
The crustacean family Parabathynellidae is a member of freshwater subterranean habitats [1].This group, found on all continents except Antarctica, is included in "stygobionts", which exclusively inhabit underground environments compared to stygophilic organisms found in both surface and subsurface habitats [2,3].Because of this, their evolution and lineage are more closely associated with geological events than any other freshwater organisms [4,5].
Nipponbathynella Schminke, 1973 [6] is one of the five genera of the family Parabathynellidae Noodt, 1965, recognized in Korea: Allobathynella Morimoto and Miura, 1957 [7], Arisubathynella Park and Eun, 2012 [8], Eobathynella Birstein and Ljovuschkin, 1964 [9], Hangangbathynella Park and Cho, 2013 [10], and Nipponbathynella.They have been reported from both the Korean Peninsula and the Japanese Islands, along with Allobathynella and Eobathynella, unlike Arisubathynella and Hangangbathynella, which are recognized exclusively in Korea [11].These distribution characteristics of Parabathynellidae, which exhibit endemism in a specific range, emphasize the need to explore the global distribution, diversity, and evolutionary relationships of subterranean crustaceans.
In this study, we describe two new Nipponbathynella species from the hyporheic zone of South Korea, N. yeongchunensis sp.nov.and N. gapcheonensis sp.nov., based on their morphology and molecular sequences.We obtained mitochondrial cytochrome c oxidase subunit I (COI) and the 18S rRNA gene from the two new species and inferred the phylogenetic relationships of the species of the family Parabathynellidae based on the 18S rRNA gene sequence.A brief discussion based on the 18S-pairwise distance for 14 species of five genera from South Korea and an identification key for the five known parabathynellid genera from South Korea are also presented.

Sampling and Morphological Observation
Samples were collected from the interstitial hyporheic zones of the Namhangang River (37°5′0.52"N, 128°28′57.11"E, Danyang-gun) and the Geumgang River (36°15′39.0"N, 127°20′25.8"E, Daejeon-si), South Korea (Figure 1A).Sampling was performed as described by Lee and Park [17].The specimens were dissected in glycerol under a The genus Nipponbathynella is considered to be more similar to Arisubathynella than to other Asian genera in terms of morphology, sharing similarities in antenna segmentation, male thoracopod VIII, and uropods [8,11,16], suggesting a close relationship between the two genera.However, because of the absence of molecular data for Nipponbathynella, it is not known whether their morphological similarities are a result of simple morphological convergence or indicate a close phylogenetic relationship.
In this study, we describe two new Nipponbathynella species from the hyporheic zone of South Korea, N. yeongchunensis sp.nov.and N. gapcheonensis sp.nov., based on their morphology and molecular sequences.We obtained mitochondrial cytochrome c oxidase subunit I (COI) and the 18S rRNA gene from the two new species and inferred the phylogenetic relationships of the species of the family Parabathynellidae based on the 18S rRNA gene sequence.A brief discussion based on the 18S-pairwise distance for 14 species of five genera from South Korea and an identification key for the five known parabathynellid genera from South Korea are also presented.

Sampling and Morphological Observation
Samples were collected from the interstitial hyporheic zones of the Namhangang River (37 • 5 0.52 N, 128 • 28 57.11E, Danyang-gun) and the Geumgang River (36 • 15 39.0 N, 127 • 20 25.8 E, Daejeon-si), South Korea (Figure 1A).Sampling was performed as described by Lee and Park [17].The specimens were dissected in glycerol under a stereomicroscope (SZX12, Olympus Co., Ltd., Tokyo, Japan) and mounted using Eukitt ® quick-hardening mounting medium (Sigma-Aldrich, St. Louis, MO, USA) to obtain permanent slides.Observations and drawings were made using an optical microscope (DM2500, Leica, Germany).To carry out scanning electron microscopy (SEM) imaging, the samples were dehydrated with a graded ethanol series (80%, 95%, and 100% ethanol; 10 min each) and hexamethyldisilazane (Sigma, St. Louis, MO, USA) for one hour.The dried samples were sputtered with platinum and then observed and recorded using SEM (Model Hitachi S-4300; Japan).The type materials of the new species examined in this study have been deposited in the collection at the National Institute of Biological Resources, Korea (NIBR) (Table 1).

DNA Extraction and Amplification
All specimens were immediately preserved in 95% ethanol for molecular analysis and picked using a 200 µL pipet under the stereomicroscope (SZX12, Olympus Co., Ltd., Tokyo, Japan).The genomic DNA was extracted from the tissue of the abdomen part using the LaboPass™ Tissue Genomic DNA Isolation Kit Mini (Cosmo GENETECH, Seoul, South Korea) according to the manufacturer's instructions.Gene sequences were obtained from two loci of COI and 18S rRNA, and the amplification by polymerase chain reaction (PCR) was performed as described by Ji and Min [18].Bidirectional DNA sequencing was performed at Macrogen Corp. (Seoul, South Korea).We combined forward and reverse sequences of raw data of COI and 18S rRNA genes using Geneious v. 8.1.9for the final sequences.

Molecular Analysis
Phylogenetic analyses were performed using maximum likelihood (ML) and Bayesian inference (BI) analyses based on the 18S rRNA genes.The final alignments of the 18S dataset using Geneious v.8.1.9(Biomatters, Auckland, New Zealand) consisted of 1790 bp for 45 sequences, including sequences of two new Nipponbathynella species.The remaining 43 sequences were taken from GenBank (https://www.ncbi.nlm.nih.gov/gene;accessed on 28 July 2023).These alignment data were exported in fasta and nexus format for ML and BI analyses, respectively, and each sequence's missing end was processed as Ns (Adenine/Guanine/Cytosine/Thymine).The ML analyses were performed using ultrafast bootstrap analysis in IQ-TREE v1.6.8, based on the TNe + R2 model.The bootstrap values were calculated with 1000 replications.Prior to the BI analysis, jModelTest 2.1.7software [19] was used to determine the appropriate DNA substitution model, and the GTR + I + G model was found to be optimal.BI assessment was performed using MrBayes v. 3.2.6 [20] for 1,000,000 generations; the first 30% of the generations were discarded as burn-ins.The final trees were displayed in FigTree v1.4.4 and edited using Adobe Illustrator.For calculating the uncorrected p-distance between Korean genera, the alignment data using Geneious v.8.1.9 of 14 sequences were exported in fasta format, each using Geneious v.8.1.9(Biomatters, Auckland, New Zealand), and was determined using MEGA-X v.10.1.8(Table 2) [21].

Diagnosis
Antennule six-segmented with three plumose setae on the first segment; antenna twosegmented with three simple setae; labrum with 10 teeth; mandible incisor process with three teeth; maxilla four-segmented with setal formula 2-4-9-7; protopods of thoracopods I-VII without hairs; thoracopods II-VII each with an epipod; exopod of male thoracopod VIII with a smooth end; epipod of male thoracopod VIII conical-shaped with a narrow base; female thoracopod VIII radicle-shaped with a smooth end; uropod sympod with 10 spines with the distal-most prominent spine and proximal-most smallest spine; furcal ramus with six spines; anal operculum protruded.

Description of Adult Male and Female
Body: (Figure 2A) length 1.6 mm, more than 10 times as long as wide; head as long as wide, shorter than the three anterior thoracic segments combined.
Antennule: (Figure 3A) six-segmented; first segment with one small seta on the inner distal margin, two simple dorsal setae of different sizes with sockets on the bases, and three plumose setae; second segment with one simple seta ventrally and dorsally, respectively, one group of four plumose setae on the outer side; third segment with two simple setae of different sizes on the inner margin, with two simple lateral setae of different sizes; inner flagellum of the third segment with three simple setae of different sizes; fourth segment with one stub seta and one plumose seta on the dorsal margin, two stub setae and two plumose setae on the outer distal apophysis being slightly protruded; on the median of the fifth segment with one dorsal simple seta and one aesthetasc dorsally and with two simple setae laterally; fifth segment distally with three setae, two dorsal aesthetascs, one dorsal seta, and one lateral aesthetasc; sixth segment as large as one-half the fifth segment, with three aesthetascs and four simple setae.
two-segmented with three simple setae; labrum with 10 teeth; mandible incisor process with three teeth; maxilla four-segmented with setal formula 2-4-9-7; protopods of thoracopods I-VII without hairs; thoracopods II-VII each with an epipod; exopod of male thoracopod VIII with a smooth end; epipod of male thoracopod VIII conical-shaped with a narrow base; female thoracopod VIII radicle-shaped with a smooth end; uropod sympod with 10 spines with the distal-most prominent spine and proximal-most smallest spine; furcal ramus with six spines; anal operculum protruded.

Description of Adult Male and Female
Body: (Figure 2A) length 1.6 mm, more than 10 times as long as wide; head as long as wide, shorter than the three anterior thoracic segments combined.Labrum: (Figure 3C) with eight median teeth flanked by one tooth on the lateral sides; ventral surface with two nipple-like protrusions (red arrows) on both sides, numerous ctenidia (blue arrow), and one projection in the middle.
Mandible: (Figure 3D) with incisor process of three teeth; tooth of the ventral edge triangular; spine row consisting of four spines; palp one-segmented with one apical seta.Maxillule: (Figure 3E) two-segmented; proximal segment with three setae on the distal margin; distal segment with two terminal setae, the distalmost one with seven teeth and the other one smooth; four dentated spines on the inner edge with one tiny spine.
Thoracopod VIII of male: (Figure 6C) bell-shaped, 1.3 times longer than wide; protopod massive with penial region of three lobes; frontal lobe with 10-11 spines; dentate lobe with seven teeth; inner lobe bur-like; epipod conical-shaped with a narrow base; basipod with one simple seta near the base of the endopod; exopod half the size of the basipod, Thoracopod VIII of female: (Figure 6B) in the shape of a pair of radicles with smooth round ends.
Thoracopod VIII of male: (Figure 6C) bell-shaped, 1.3 times longer than wide; protopod massive with penial region of three lobes; frontal lobe with 10-11 spines; dentate lobe with seven teeth; inner lobe bur-like; epipod conical-shaped with a narrow base; basipod with one simple seta near the base of the endopod; exopod half the size of the basipod, one-lobed and with two small teeth on one side only; endopod with one plumose seta and one simple shorter seta.
First pleopod: (Figure 5B) absent.Uropod: (Figure 7A) with load-shaped sympod bearing 10 spines on the inner margin; most distal spine significantly thicker and most proximal one smallest; size of spines decreasing toward the distal end; endopod drawn into a spur, with two setae on the outer base of the spur; exopod longer than endopod with one outer seta, two terminal setae, and one inner medial seta; inner seta strongly barbed and thicker than the other exopodal seta.
Pleotelson: (Figure 7B) without seta; anal operculum protruded.Furcal rami: (Figure 7B) 1.2 times as long as wide, with two large distal spines,six smaller spines on the inner margin, and two dorsal plumose setae.Uropod: (Figure 7A) with load-shaped sympod bearing 10 spines on the inner margin; most distal spine significantly thicker and most proximal one smallest; size of spines decreasing toward the distal end; endopod drawn into a spur, with two setae on the outer base of the spur; exopod longer than endopod with one outer seta, two terminal setae, and one inner medial seta; inner seta strongly barbed and thicker than the other exopodal seta.
Pleotelson: (Figure 7B) without seta; anal operculum protruded.Furcal rami: (Figure 7B) 1.2 times as long as wide, with two large distal spines, six smaller spines on the inner margin, and two dorsal plumose setae.

Remarks
N. yeongchunensis sp.nov. is morphologically the most similar to N. leesookyungae Park and Cho, 2015 [11] in having (1) three setae on the maxillule proximal segment, (2) nine setae on the third segment of the maxilla, and (3) six spines on the furcal ramus.However, the new species can be clearly distinguished from N. leesookyungae by the following characteristics (features of N. leesookyungae in parentheses): (1) two dorsal setae with base sockets on the first segment of the antennule (without sockets on the bases); (2) a maxillule distal segment having six (five) spines and one tiny additional spine; (3) a maxillule distal spine with seven dentils (without dentils); and (3) male thoracopod VIII of the exopod with a smooth end (with three spinules and two teeth).

Etymology
The species name is derived from Yeongchun-myeon, where the material was collected.

Diagnosis
Antennule six-segmented with three plumose setae on the first segment; antenna twosegmented with three simple setae; labrum with 12 teeth; mandible incisor process with three teeth; maxilla four-segmented with setal formula 2-4-8-7; protopods of thoracopods II-IV with hairs; thoracopods II-VII each with an epipod; exopod of male thoracopod VIII with three spinules; epipod of male thoracopod VIII conical-shaped with a wide base; female thoracopod VIII radicle-shaped with slight bifurcation; uropod sympod with 10 spines, with the distal-most spine being most prominent; furcal ramus with eight spines; anal operculum protruded.

Description of Adult Male and Female
Body: (Figure 2B) length 1.5 mm, more than 10 times as long as wide; head as long as wide, shorter than the three anterior thoracic segments combined.
Antennule: (Figure 8A) six-segmented; first segment with one small seta on the inner distal margin, two simple dorsal setae of different sizes, and three plumose setae; second segment with one simple seta ventrally and dorsally, respectively, and one group of four plumose setae on the outer side; third segment with two simple setae of different sizes on the inner margin, with one simple lateral seta; inner flagellum of the third segment with three simple setae of different sizes; fourth segment with one stub seta and one plumose seta on the dorsal margin, two stub setae and two plumose setae on the outer distal apophysis being slightly protruded; on the median of the fifth segment with one dorsal simple seta and one aesthetasc dorsally and two simple setae laterally; fifth segment distally with three setae, two dorsal aesthetascs, one dorsal seta, and one lateral aesthetasc; sixth segment as large as one half of the fifth segment, with three aesthetascs and four simple setae.
Antenna: (Figure 8B) two-segmented; proximal segment without seta; distal segment with two simple terminal setae and one subterminal seta.
Labrum: (Figure 8C) with seven median teeth flanked by three (left) or two (right) teeth on the lateral sides; ventral surface with two nipple-like protrusions (red arrows) on both sides, numerous ctenidia (blue arrow), and three projections in the middle.
Mandible: (Figure 8D) with incisor process of three teeth; tooth of the ventral edge triangular; spine row consisting of four spines; palp one-segmented with two apical setae of different sizes.
Maxillule: (Figure 8E) two-segmented; proximal segment with three setae on the distal margin; distal segment with two terminal setae; distal most seta with four teeth and the other smooth; four dentated spines on the inner edge.Thoracopod VIII of female: (Figure 11C) in the shape of a pair of radicles slightly bifurcated with rounded ends.
Thoracopod VIII of male: (Figure 12C) bell-shaped, 1.4 times longer than wide; protopod massive with penial region of three lobes; frontal lobe with 10-11 spines; dentate lobe with 10 teeth; inner lobe bur-like; epipod conical-shaped with a wide base; basipod with one seta near the base of the endopod, on one side simple and the other side plumose; exopod half the size of the basipod with two spinules distally; endopod with two simple seta of similar sizes.
First pleopod: (Figure 2B) absent.Uropod: (Figure 12A) with a load-shaped sympod bearing 10 spines on the inner margin; distal-most spine significantly thicker than the others; the size of the spines decreasing toward the distal end; endopod: drawn into a spur, with two setae on the outer base of the spur; exopod: one outer seta, two terminal setae, and one inner medial seta; inner seta: strong-barbed and thicker than other exopodal setae.Pleotelson: (Figure 12B) without seta; anal operculum protruded.Furcal rami: (Figure 12B) 1.3 times as long as wide, with two large distal spines and six smaller spines on the inner margin, and two dorsal plumose setae.

Remarks
Nipponbathynella gapcheonensis sp.nov. is morphologically most similar to N. donggangensis Park and Cho, 2015 [11] in having (1) six setae on the maxillule distal segment, (2) thoracopod VIII of the conical-shaped epipod on thoracopod VIII of males with a wide base, and (3) eight spines on the furcal ramus.However, the new species can be clearly distinguished from N. donggangensis based on the following characteristics (characteristics Thoracopod VIII of male: (Figure 12C) bell-shaped, 1.4 times longer than wide; protopod massive with penial region of three lobes; frontal lobe with 10-11 spines; dentate lobe with 10 teeth; inner lobe bur-like; epipod conical-shaped with a wide base; basipod with one seta near the base of the endopod, on one side simple and the other side plumose; exopod half the size of the basipod with two spinules distally; endopod with two simple seta of similar sizes.
First pleopod: (Figure 2B) absent.Uropod: (Figure 12A) with a load-shaped sympod bearing 10 spines on the inner margin; distal-most spine significantly thicker than the others; the size of the spines decreasing toward the distal end; endopod: drawn into a spur, with two setae on the outer

Etymology
The species name is derived from the Gapcheon River, where the material was collected.base of the spur; exopod: one outer seta, two terminal setae, and one inner medial seta; inner seta: strong-barbed and thicker than other exopodal setae.
Pleotelson: (Figure 12B) without seta; anal operculum protruded.Furcal rami: (Figure 12B) 1.3 times as long as wide, with two large distal spines and six smaller spines on the inner margin, and two dorsal plumose setae.

Molecular Analysis
In this study, a total of 11 partial COI sequences (789 bp) and 18S rRNA gene sequences (1682 bp) of the two new Nipponbathynella species were obtained (Table 2).The interspecies genetic distances between the two new species were 20.8-20.9% for COI sequences and 0.7% for 18S rRNA gene sequences.For Korean parabathynellid genera from South Korea, including 14 species of Allobathynella, Arisubathynella, Eobathynella, Hangangbathynella, and Nipponbathynella, we analyzed the pairwise p-distance for 1377 bp of the 18S rRNA gene sequences (Table 2).Within the genus, the interspecies genetic distances were 0.0-0.7% for Arisubathynella, Allobathynella, Hangangbathynella, and Nipponbathynella.For the genus Eobathynella, we were able to analyze two sequences of Korean Eobathynella: E. cf.yeojuensis Shin, 2014 [22] and E. gwangjuensis Ji and Min, 2022 [23], which exhibited a relatively large p-distance of 4.1%.The intergenetic p-distances for the five Korean genera were all ≥1.8%.Regarding the relationships between the genus Nipponbathynella and the four other Korean genera, the distance between Nipponbathynella and Arisubathynella was closest at 1.8-2.0%.Compared with the other Korean genera excluding Arisubathynella, the genus Nipponbathynella showed a genetic distance of 2.5-5.3%; the minimum distance was obtained for Allobathynella hongcheonensis Park and Cho, 2016 [24], and the maximum distance was obtained for E. gwangjuensis.

Discussion
In the present study, two new species, Nipponbathynella yeongchunensis sp.nov.and N. gapcheonensis sp.nov., from two different sites in South Korea are described based on morphological and molecular data.They not only exhibit significant morphological differences from each other but also display distinct differences from their closest congeners (see Remarks and Table 3).In addition, no molecular data were previously available for comparison of Nipponbathynella species.However, the mitochondrial and nuclear gene sequences obtained from the two new species show a clear separation between them.
We analyzed the phylogenetic relationships among 20 parabathynellid genera, including the five genera from South Korea, Allobathynella, Arisubathynella, Eobathynella, Hangangbathynella, and Nipponbathynella, based on the 18S rRNA gene (Figure 13).Our analysis supports the three geographical clades, representing Gondwana, the Laurasia continent, and the cosmopolitan genus, of the family Parabathynellidae supported by previous studies [18,25].The phylogenetic reconstruction based on ML and BI analyses shows that the Korean Nipponbathynella belongs to Clade A, which includes some Korean genera such as Arisubathynella and Allobathynella together with the Europe-North American genera (ML, bootstrap support = 95 and BI, posterior probabilities = 1.0).The morphological affinity among the genera Nipponbathynella, Arisubathynella, and Parabathynella has been reported in previous studies [8,11,16].In addition, previous morphological and molecular phylogenetic analyses have revealed a close relationship between Arisubathynella and the Europe-North American genera [18].Thus, our phylogenetic results, including those for the genus Nipponbathynella, are consistent with both the morphological and molecular evidence.
Additionally, we provided a pairwise p-distance using the 18S rRNA data, including all available and morphologically verified sequences for five parabathynellid genera that occur on the Korean peninsula (Table 2).Our analysis based on molecular data shows the validity of the five genera that were previously classified as a 'morpho-genus' relying primarily on morphological characteristics (see Molecular Analysis in the result).These results provide important data for the genetic standard for genus delimitation of the family Parabathynellidae, not only in the Korean peninsula but also in other regions.Here, we discuss some of the implications of this analysis.
The relationship between Nipponbathynella and Arisubathynella shown in the results of the two analyses is consistent with the fact that Nipponbathynella is morphologically more similar to Arisubathynella than to the other Korean genera (Figure 13 and Table 2).Thus, the morphology of the antennae segmentation, thoracopod VIII of males, and uropods of Parabathynellidae may be significant characteristics for investigating phylogenetic relationships.Table 3. Type localities of each species and morphological comparison among species of Nipponbathynella, excluding N. miurai (cited and modified from Park and Cho [11]).Categories within the horizontally separated sections refer to characters within the major appendage indicated in bold.There is no information about items marked with a question mark.We propose a new phylogenetic position for the genera Allobathynella and Hangangbathynella from South Korea.In the previous and current studies [18,25], parabathynellid genera from the Laurasia continent split into two clades.In our analysis, Allobathynella and Hangangbathynella from South Korea were included in the two separate clades A, related Europe-North American genera, and B, related East Asian genera, respectively.The molecular phylogenetic position of Allobathynella is relatively unambiguous, supported by both ML/BI analysis (ML, bootstrap support = 98 and BI, posterior probabilities = 1.0) through analysis using a significantly longer sequence (1790 bp) than previous research (716 bp) [14].However, it is too early to conclude about the Allobathynella phylogeny at the moment, and it is necessary to further confirm the phylogenetic position of Allobathynella by including the 18S rRNA gene sequences of more species found in Europe and North America or using multiple molecular sequence data.Additionally, we provided a pairwise p-distance using the 18S rRNA data, including all available and morphologically verified sequences for five parabathynellid genera that occur on the Korean peninsula (Table 2).Our analysis based on molecular data shows the validity of the five genera that were previously classified as a 'morpho-genus' relying primarily on morphological characteristics (see Molecular Analysis in the result).These results provide important data for the genetic standard for genus delimitation of the family Parabathynellidae, not only in the Korean peninsula but also in other regions.Here, we discuss some of the implications of this analysis.
The relationship between Nipponbathynella and Arisubathynella shown in the results of the two analyses is consistent with the fact that Nipponbathynella is morphologically more similar to Arisubathynella than to the other Korean genera (Figure 13 and Table 2).Thus, the morphology of the antennae segmentation, thoracopod VIII of males, and uropods of Parabathynellidae may be significant characteristics for investigating phylogenetic relationships.
We propose a new phylogenetic position for the genera Allobathynella and Hangangbathynella from South Korea.In the previous and current studies [18,25], parabathynellid genera from the Laurasia continent split into two clades.In our analysis, Allobathynella and Hangangbathynella from South Korea were included in the two separate clades A, related Europe-North American genera, and B, related East Asian genera, respectively.The molecular phylogenetic position of Allobathynella is relatively unambiguous, supported by both ML/BI analysis (ML, bootstrap support = 98 and BI, posterior probabilities = 1.0) through analysis using a significantly longer sequence (1790 bp) than previous research We gained important insights into the phylogenetic relationships of the family Parabathynellidae based on their morphological characteristics.However, considering their highly convergent morphological evolution as subterranean crustaceans, acquiring molecular data using multiple genetic markers could provide a more comprehensive understanding of the diversity within Parabathynellidae.Furthermore, expanding sampling efforts in South Korea and neighboring countries, particularly Japan and China, could contribute to a better understanding of their distribution patterns and diversity.
Key to five parabathynellid genera from South Korea.

Figure 1 .
Figure 1.Map showing the distribution of the genus Nipponbathynella.(A) The type localities of two new Nipponbathynella (1.N. yeongchunensis sp.nov.; 2. N. gapcheonensis sp.nov.); (B) the type localities of all previously reported species of Nipponbathynella.

Figure 1 .
Figure 1.Map showing the distribution of the genus Nipponbathynella.(A) The type localities of two new Nipponbathynella (1.N. yeongchunensis sp.nov.; 2. N. gapcheonensis sp.nov.); (B) the type localities of all previously reported species of Nipponbathynella.

Figure 2 .
Figure 2. Habitus of two new species.(A) N. yeongchunensis; (B) N. gapcheonensis.Scale bar: 0.5 mm.Figure 2. Habitus of two new species.(A) N. yeongchunensis; (B) N. gapcheonensis.Scale bar: 0.5 mm.Antenna: (Figure 3B) two-segmented; proximal segment without seta; distal segment with two simple terminal setae and one simple subterminal seta.Labrum: (Figure3C) with eight median teeth flanked by one tooth on the lateral sides; ventral surface with two nipple-like protrusions (red arrows) on both sides, numerous ctenidia (blue arrow), and one projection in the middle.Mandible: (Figure3D) with incisor process of three teeth; tooth of the ventral edge triangular; spine row consisting of four spines; palp one-segmented with one apical seta.

Figure 7 .
Figure 7. Nipponbathynella yeongchunensis sp.nov., holotype female.(A) uropod; (B) furcal rami.Scale bars: 0.05 mm.3.2.5.RemarksN.yeongchunensis sp.nov. is morphologically the most similar to N. leesookyungae Park and Cho, 2015[11] in having (1) three setae on the maxillule proximal segment, (2) nine setae on the third segment of the maxilla, and (3) six spines on the furcal ramus.However, the new species can be clearly distinguished from N. leesookyungae by the following characteristics (features of N. leesookyungae in parentheses): (1) two dorsal setae with base sockets on the first segment of the antennule (without sockets on the bases); (2) a maxillule distal segment having six (five) spines and one tiny additional spine; (3) a maxillule distal spine with seven dentils (without dentils); and (3) male thoracopod VIII of the exopod with a smooth end (with three spinules and two teeth).

Diversity 2023 , 23 Figure 13 .
Figure 13.Maximum likelihood and Bayesian inference analyses based on nuclear 18S rRNA gene sequences for 19 parabathynellid genera (majority consensus tree of BI).Numbers on nodes represent ultrafast bootstrap values for maximum likelihood and Bayesian posterior probabilities.Country abbreviations are as follows: AU = Australia, IN = India, ES = Spain, MA = Morocco, KR = South Korea, SI = Slovenia, TH = Thailand, VN = Viet Nam.

Figure 13 .
Figure 13.Maximum likelihood and Bayesian inference analyses based on nuclear 18S rRNA gene sequences for 19 parabathynellid genera (majority consensus tree of BI).Numbers on nodes represent ultrafast bootstrap values for maximum likelihood and Bayesian posterior probabilities.Country abbreviations are as follows: AU = Australia, IN = India, ES = Spain, MA = Morocco, KR = South Korea, SI = Slovenia, TH = Thailand, VN = Viet Nam.

Table 1 .
Voucher numbers corresponding with GenBank accession numbers for specimens in this study.

Table 2 .
Pairwise p-distances of the 18S rRNA gene sequences for the 14 species of five genera.