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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Mar. Sci.</journal-id>
<journal-title>Frontiers in Marine Science</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Mar. Sci.</abbrev-journal-title>
<issn pub-type="epub">2296-7745</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.3389/fmars.2024.1359503</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Marine Science</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>The host range and distribution pattern of rhizocephalan parasitic barnacles in Korean coasts and their relationship with geographical factors</article-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Jung</surname><given-names>Jibom</given-names>
</name>
<xref ref-type="author-notes" rid="fn001"><sup>*</sup></xref>
<uri xlink:href="https://loop.frontiersin.org/people/2611002"/>
<role content-type="https://credit.niso.org/contributor-roles/data-curation/"/>
<role content-type="https://credit.niso.org/contributor-roles/formal-analysis/"/>
<role content-type="https://credit.niso.org/contributor-roles/funding-acquisition/"/>
<role content-type="https://credit.niso.org/contributor-roles/investigation/"/>
<role content-type="https://credit.niso.org/contributor-roles/methodology/"/>
<role content-type="https://credit.niso.org/contributor-roles/project-administration/"/>
<role content-type="https://credit.niso.org/contributor-roles/resources/"/>
<role content-type="https://credit.niso.org/contributor-roles/software/"/>
<role content-type="https://credit.niso.org/contributor-roles/visualization/"/>
<role content-type="https://credit.niso.org/contributor-roles/writing-original-draft/"/>
<role content-type="https://credit.niso.org/contributor-roles/writing-review-editing/"/>
</contrib>
</contrib-group>
<aff id="aff1"><institution>Population Genomics Laboratory, School of Biological Sciences, College of Natural Sciences, Seoul National University</institution>, <addr-line>Seoul</addr-line>, <country>Republic of Korea</country></aff>
<author-notes>
<fn fn-type="edited-by">
<p>Edited by: Jin Sun, Ocean University of China, China</p>
</fn>
<fn fn-type="edited-by">
<p>Reviewed by: Wong Yue Him, Shenzhen University, China</p>
<p>Olga Korn, National Scientific Center of Marine Biology, Far East Branch (RAS), Russia</p>
</fn>
<fn fn-type="corresp" id="fn001">
<p>*Correspondence: Jibom Jung, <email xlink:href="mailto:apociv@naver.com">apociv@naver.com</email>
</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>22</day>
<month>02</month>
<year>2024</year>
</pub-date>
<pub-date pub-type="collection">
<year>2024</year>
</pub-date>
<volume>11</volume>
<elocation-id>1359503</elocation-id>
<history>
<date date-type="received">
<day>21</day>
<month>12</month>
<year>2023</year>
</date>
<date date-type="accepted">
<day>02</day>
<month>02</month>
<year>2024</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2024 Jung</copyright-statement>
<copyright-year>2024</copyright-year>
<copyright-holder>Jung</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p>
</license>
</permissions>
<abstract>
<sec>
<title>Introduction</title>
<p>This study undertakes a detailed examination of the host range and distribution patterns of Rhizocephala, a group of specialized parasitic barnacles, within Korean marine environments. It aims to expand the understanding of their biodiversity and ecological impact.</p>
</sec>
<sec>
<title>Methods</title>
<p>Employing both morphological observation and mitochondrial DNA sequencing, the study identifies the variety of rhizocephalan species infecting decapod hosts in Korean waters, aiming to document their diversity and distribution.</p>
</sec>
<sec>
<title>Results</title>
<p>The investigation identifies 26 rhizocephalan species parasitizing 28 decapod hosts, including the discovery of three new parasite-host relationships not previously reported. It was observed that while a few rhizocephalan species infect multiple hosts, most have a narrow host range. Additionally, the study maps out regional differences in Rhizocephala diversity across three Korean marine ecoregions, noting the highest diversity in the warmer East China Sea ecoregion and lower diversity in the colder Yellow Sea and East Sea ecoregions.</p>
</sec>
<sec>
<title>Discussion</title>
<p>The findings underscore the necessity for continued taxonomic research on Rhizocephala and their host species to better understand and manage these parasites, which hold significant ecological and economic importance. The study suggests that geographical variations in host-parasite relationships are influenced by regional climatic conditions, particularly winter water temperatures.</p>
</sec>
</abstract>
<kwd-group>
<kwd>Rhizocephala</kwd>
<kwd>Decapoda</kwd>
<kwd>parasite</kwd>
<kwd>infestation rate</kwd>
<kwd>host resources</kwd>
<kwd>biogeography</kwd>
<kwd>ecoregion</kwd>
<kwd>water temperature</kwd>
</kwd-group>
<contract-num rid="cn001">2021R1A6A3A01086374, 2021R1A6A1A10039823, No. RS-2023-00301976</contract-num>
<contract-sponsor id="cn001">National Research Foundation of Korea<named-content content-type="fundref-id">10.13039/501100003725</named-content>
</contract-sponsor>
<counts>
<fig-count count="3"/>
<table-count count="4"/>
<equation-count count="0"/>
<ref-count count="36"/>
<page-count count="12"/>
<word-count count="5180"/>
</counts>
<custom-meta-wrap>
<custom-meta>
<meta-name>section-in-acceptance</meta-name>
<meta-value>Marine Evolutionary Biology, Biogeography and Species Diversity</meta-value>
</custom-meta>
</custom-meta-wrap>
</article-meta>
</front>
<body>
<sec id="s1" sec-type="intro">
<label>1</label>
<title>Introduction</title>
<p>Rhizocephala is a taxon of specialized parasitic barnacles that infect a variety of crustacean hosts. They are mainly parasites on decapods, and some rhizocephalans infect commercial decapod species such as king crabs and swimming crabs (<xref ref-type="bibr" rid="B23">Noever et&#xa0;al., 2016</xref>; <xref ref-type="bibr" rid="B35">Yang et&#xa0;al., 2018</xref>). Rhizocephala has the potential to act as an invasive species and harm the ecological function of native decapod hosts. One of the rhizocephalans, <italic>Loxothylacus panopaei</italic> (Gissler, 1884), native to the Gulf of Mexico and Florida, has already invaded Cheshire Peak Bay and New York and is harming the ecological function of its host, <italic>Eurypanopeus depressus</italic> (Smith, 1869), which exerts top-down control in oyster reefs (<xref ref-type="bibr" rid="B7">Hines et&#xa0;al., 1997</xref>; <xref ref-type="bibr" rid="B24">O'Shaughnessy et&#xa0;al., 2014</xref>). Many previous studies have reported biological characteristics of Rhizocephala such as their complex life cycle, the body size correlation between the parasite and the host, and biological effects on their hosts (feminization, growth and survival rate, etc.) (<xref ref-type="bibr" rid="B27">Ritchie &amp; H&#xf8;eg, 1981</xref>; <xref ref-type="bibr" rid="B1">Alvarez et&#xa0;al., 1995</xref>; <xref ref-type="bibr" rid="B19">Kristensen et&#xa0;al., 2012</xref>; <xref ref-type="bibr" rid="B22">Nagler et&#xa0;al., 2017</xref>). A comprehensive understanding of the host range and distribution of Rhizocephala is fundamental to effective control of their prevalence on commercial decapod species and invaded areas and the biological control of invasive and harmful decapods through their parasitic barnacles (<xref ref-type="bibr" rid="B4">Boyko &amp; Williams, 2016</xref>).</p>
<p>Nevertheless, the distribution and host relationships of rhizocephalans are largely unknown. Recently, a few studies have contributed to elucidating the distribution and host range of Rhizocephala in Japan and Korea (<xref ref-type="bibr" rid="B36">Yoshida et&#xa0;al., 2014</xref>; <xref ref-type="bibr" rid="B11">Jung et&#xa0;al., 2019</xref>, <xref ref-type="bibr" rid="B12">2021</xref>). However, these studies, while pioneering, were constrained by their limited examination of specific areas or a few of the host taxa. <xref ref-type="bibr" rid="B36">Yoshida et&#xa0;al. (2014)</xref> focused on the limited paguroids (hermit crab) hosts in the limited area of southern Honshu (Japan). While <xref ref-type="bibr" rid="B11">Jung et&#xa0;al. (2019)</xref> expanded this research to the entire Korean coast, however, the host taxon in this study was also limited to Paguroidea. <xref ref-type="bibr" rid="B12">Jung et&#xa0;al. (2021)</xref> investigated the interaction of Korean Rhizocephala with Brachyura (crab) and Gebiidea (mud shrimp) hosts, however, this study has a limitation in that it mostly focused on the southern Korean Peninsula. As the host specificity of Rhizocephala is expected to vary depending on geographic location (<xref ref-type="bibr" rid="B11">Jung et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B12">2021</xref>), a more expansive and inclusive study is required to fully elucidate the relationship between Rhizocephala and their hosts.</p>
<p>To fill the gap in our knowledge, an extensive examination of three host taxa&#x2014;Gebiidea, Anomura, and Brachyura&#x2014;sampled from 28 locations representing three marine ecoregions of (<xref ref-type="bibr" rid="B16">Kim et&#xa0;al., 2020</xref>; <xref ref-type="bibr" rid="B17">2022</xref>) was performed, building upon previous work of <xref ref-type="bibr" rid="B11">Jung et&#xa0;al. (2019</xref>; <xref ref-type="bibr" rid="B12">2021)</xref>
<xref ref-type="fn" rid="fn1"><sup>1</sup></xref>. Due to the highly degenerated morphological features of rhizocephalan species (<xref ref-type="bibr" rid="B8">H&#xf8;eg,&#xa0;1992</xref>; <xref ref-type="bibr" rid="B9">H&#xf8;eg &amp; L&#xfc;tzen, 1995</xref>; <xref ref-type="bibr" rid="B25">&#xd8;ksnebjerg, 2000</xref>) that often make species identification very challenging, the author also determined mtDNA <italic>cox1</italic> sequences as molecular evidence of their correct identification. Afterward, the decapod host and distribution of each identified Rhizocephala were confirmed based on the three Korean coast ecoregions, i.e., Yellow Sea, East China Sea, and East Sea (<xref ref-type="bibr" rid="B16">Kim et&#xa0;al., 2020</xref>; <xref ref-type="bibr" rid="B17">2022</xref>). The author analyzed the relationship between Rhizocephala and its host and the diversity of Rhizocephala in each marine ecoregion to confirm trends and special features of the results. Through this integrative approach, this study concluded that the relationship with its host and the distribution of Rhizocephala might have been influenced by differences in geographical factors such as water temperature.</p>
</sec>
<sec id="s2" sec-type="materials|methods">
<label>2</label>
<title>Materials and methods</title>
<sec id="s2_1">
<label>2.1</label>
<title>Taxon sampling, morphological and molecular identifications</title>
<p>The author examined 173 Korean Rhizocephalans from 18 host decapod species collected from 21 sampling sites in Korea (<xref ref-type="fig" rid="f1"><bold>Figure&#xa0;1A</bold></xref>). Voucher specimens were deposited in the National Institute of Biological Resources (NIBR), the National Marine Biodiversity Institute of Korea (MABIK), and the Honam National Institute of Biological Resources (HNIBR). All rhizocephalan specimens were fixed in 70-95% ethanol and subjected to morphological examination and molecular analysis. For morphological analysis, the externa and mantle were examined using an MZ8 dissection microscope (Leica, Wetzlar, Germany).</p>
<fig id="f1" position="float">
<label>Figure&#xa0;1</label>
<caption>
<p>Map and heatmap showing the collection sites and density of the Korean rhizocephalan species. <bold>(A)</bold> the map showing 28 collection sites of the Korean rhizocephalans modified from <xref ref-type="bibr" rid="B11">Jung et&#xa0;al. (2019</xref>; <xref ref-type="bibr" rid="B12">2021</xref>)<sup>1</sup>, 21 sites in black letters: investigated in this study, 7 sites in blue letters: investigated only in previous studies; <bold>(B)</bold> the heatmap showing the density of 26 Korean rhizocephalans per each 28 collection sites.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1359503-g001.tif"/>
</fig>
<p>For molecular analysis, the lateral end of the externa tissue of each rhizocephalan specimen was excised for total genomic DNA extraction using the QIAamp DNA Micro Kit (QIAGEN, Hilden, Germany). Universal primers LCO1490 (5&#x2032;-GGTCAACAAATCATAAAGATATTGG-3&#x2032;) and HCO2198 (5&#x2032;-TAAACTTCAGGGTGACCAAAAAATCA-3&#x2032;) were used to amplify a fragment of mitochondrial cytochrome c oxidase subunit I (<italic>cox1</italic>) (<xref ref-type="bibr" rid="B6">Folmer et&#xa0;al., 1994</xref>). Polymerase chain reaction (PCR) was performed in reaction volumes of 50 &#xb5;L that included 2 &#xb5;L DNA template, 5 &#xb5;L 10 x Ex Taq Buffer, 2 &#xb5;L of each primer (10 &#xb5;M), 0.25 &#xb5;L Go Taq DNA polymerase (Promega, Madison City, WI, USA), 2.5 &#xb5;L dNTP mix (10 mM), and 35.75 &#xb5;L distilled H<sub>2</sub>O. PCR amplification was performed using the following steps: 5 min denaturation at 94&#xb0;C followed by 35 cycles of 30 sec at 94&#xb0;C, 1 min at 52&#xb0;C, 1 min at 72&#xb0;C, and a final extension of 7 min at 72&#xb0;C. PCR products were visualized on 1% agarose gels and sequenced with an ABI PRISM 3730xl DNA analyzer (Applied Biosystems, Foster City, CA, USA). Nucleotide sequences of mtDNA <italic>cox1</italic> fragments were analyzed and edited using Geneious v. 9.1.8 (<xref ref-type="bibr" rid="B15">Kearse et&#xa0;al., 2012</xref>) and aligned using ClustalW in the MEGA10 program (<xref ref-type="bibr" rid="B20">Kumar&#xa0;et&#xa0;al.,&#xa0;2018</xref>). The mtDNA <italic>cox1</italic> sequences of the Korean rhizocephalan species were deposited on GenBank (OR481955-OR482064). Thirty-six <italic>cox1</italic> and 16S rRNA rhizocephalan sequences were downloaded from GenBank and included in further phylogenetic analyses (<xref ref-type="supplementary-material" rid="ST1"><bold>Supplementary Table 1</bold></xref>).</p>
<p>Phylogenetic relationships among rhizocephalan species were inferred using maximum likelihood of RaxML version 8 (<xref ref-type="bibr" rid="B29">Stamatakis, 2014</xref>) and Bayesian inference of MrBayes version 3.2.6 (<xref ref-type="bibr" rid="B28">Ronquist et&#xa0;al., 2012</xref>), respectively. Maximum likelihood analyses of <italic>cox1</italic> sequences were performed using the general time reversible substitution model (<xref ref-type="bibr" rid="B31">Tavar&#xe9;, 1986</xref>), with a gamma distribution (+G) and invariable sites (+I) rate categories based on Bayesian Information Criterion (BIC) scores model using the Model Selection option of MEGA10 (<xref ref-type="bibr" rid="B20">Kumar et&#xa0;al., 2018</xref>). The Bayesian phylogenetic analysis was performed using the Markov chain Monte Carlo (MCMC) method, with two independent runs of 1 &#xd7; 10<sup>6</sup> generations with six chains, sampling every 100 generations. Bayesian posterior probability (BPP) values were estimated after discarding the 2 &#xd7; 10<sup>3</sup> initial trees as burn-in that reached a stationary stage of the parameters. The branch support in the ML trees was assessed by analysis of 1,000 bootstrap replicates. Interspecific and intraspecific sequence divergences were estimated based on the Kimura 2-parameter (K2P) distance matrix in MEGA10.</p>
</sec>
<sec id="s2_2">
<label>2.2</label>
<title>Analysis of host-parasite relationships (decapod-rhizocephalan species) and their distribution</title>
<p>To elucidate the ecological characteristics of Korean Rhizocephala, especially their distribution and relationships between Rhizocephala and their hosts, the author analyzed data of rhizocephalan species from 28 species and 11,016 individuals of decapod host across 28 sampling sites. This dataset includes both current study data and information from previous studies in Korea (<xref ref-type="bibr" rid="B11">Jung et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B12">2021</xref><sup>1</sup>). Vertical bar charts were generated to depict the frequency of decapod hosts infected by rhizocephalans. Furthermore, the author investigated the correlation between the distribution of rhizocephalans and their hosts, focusing on biogeographic patterns in the three Korean coast ecoregions, i.e., Yellow Sea, East China Sea, and East Sea (<xref ref-type="bibr" rid="B16">Kim et&#xa0;al., 2020</xref>; <xref ref-type="bibr" rid="B17">2022</xref>).</p>
</sec>
</sec>
<sec id="s3" sec-type="results">
<label>3</label>
<title>Results</title>
<sec id="s3_1">
<label>3.1</label>
<title>Sequence divergence and phylogenetic relationships among Korean rhizocephalan species</title>
<p>Because this sampling targeted the 4 families (Peltogasterellidae, Peltogastridae, Polyascidae, and Sacculinidae) which can cover the majority of whole Korean Rhizocephalan fauna, phylogenetic analysis in this study was focused on relationships among rhizocephalan species belonging to these four families. A total of 110 mtDNA <italic>cox1</italic> partial sequences (497 bp) were used for phylogenetic analysis, and the resulting phylogenies from maximum likelihood and Bayesian methods were generally consistent with each other in that four rhizocephalan families were monophyletic respectively, except <italic>Sacculina gracilis</italic> Boschma, 1931 is positioned in the Polyascidae clade (<xref ref-type="fig" rid="f2"><bold>Figure&#xa0;2</bold></xref>). In the inferred phylogenetic tree, the sequences of Korean rhizocephalan species were nested and/or clustered with their congeneric species retrieved from GenBank (<xref ref-type="fig" rid="f2"><bold>Figure&#xa0;2</bold></xref>). The individual sequence variations within rhizocephalan species were relatively low ranging from 0% (in <italic>S. gracilis</italic>, <italic>Parasacculina</italic> sp. 1) to 1.4% (in <italic>Sacculina confragosa</italic> Boschma, 1933), compared to interspecific differences [a minimum sequence divergence of 4.6% found between <italic>Polyascus</italic> aff. <italic>gregarius</italic> and <italic>Polyascus planus</italic> (Boschma, 1933)] (<xref ref-type="supplementary-material" rid="ST2"><bold>Supplementary Table 2</bold></xref>). One notable exception to low within-species variation was found in <italic>P.</italic> aff. <italic>gregarius</italic> (<xref ref-type="fig" rid="f2"><bold>Figure&#xa0;2</bold></xref>) in which sequence variation ranged from 4.6% to 5.7% between Korean and Chinese samples. From this phylogenetic analysis, the author discovered unidentified species (<italic>Peltogasterella</italic> sp., <italic>Peltogaster</italic> sp. 1, and <italic>Parasacculina</italic> sp. 1) of previous study (Jung and Park, in review) were separated from their congeneric species (<xref ref-type="fig" rid="f2"><bold>Figure&#xa0;2</bold></xref>), such as <italic>Peltogasterella gracilis</italic> (Boschma, 1927), <italic>Peltogaster postica</italic> Yoshida &amp; Osawa in Yoshida, Osawa, Hirose &amp; Hirose, 2011, and <italic>Parasacculina yatsui</italic> (Boschma, 1936) and <italic>P.</italic> aff. <italic>gregarius</italic> having the same family of hosts and similar morphological features (<xref ref-type="fig" rid="f2"><bold>Figure&#xa0;2</bold></xref>). Interspecific sequence differences of these three unidentified species from their congeneric species ranged from 18.5 to 35.8%, more or less comparable to an average distance (23.9%) of the congeneric comparison. By contrast, intraspecific variations of unidentified species were very low, from 0% (<italic>Parasacculina</italic> sp. 1) to 1.2% (in <italic>Peltogaster</italic> sp.). Unexpectedly, the phylogenetic tree depicted the family Sacculinidae as non-monophyletic because <italic>S. gracillis</italic> sequences (OR482063, OR482064) were not grouped with all the other Sacculinidae species (including <italic>Sacculina</italic> species), but instead with some <italic>Parasacculina</italic> species (including <italic>Parasacculina shiinoi</italic> (L&#xfc;tzen, Itani, Jespersen, Hong, Rees &amp; Glenner, 2016), <italic>Parasacculina</italic> sp. 1, and <italic>Parasacculina</italic> sp. 3), (<xref ref-type="fig" rid="f2"><bold>Figure&#xa0;2A</bold></xref>). Note that relationships at the deep branches (i.e., higher than generic level) are not strongly supported by bootstrap values (mostly &#x2264;50%). These unresolved relationships at higher taxonomic levels are likely due to the poor resolution of mtDNA <italic>cox1</italic> partial sequences as a molecular marker for deeper node Rhizocepala phylogeny. In the maximum likelihood phylogenetic tree of Rhizocephala combining mtDNA <italic>cox</italic>1 and 16S rRNA, deep branches were strongly supported by high bootstrap values (<xref ref-type="fig" rid="f2"><bold>Figure&#xa0;2B</bold></xref>). Indeed, sequence divergences between <italic>S. gracillis</italic> and <italic>Parasacculina</italic> species belonging to different families, ranging from 21.0% to 31.5%, are not noticeably higher than that detected between congeneric species (<italic>S. gracillis</italic> and other <italic>Sacculina</italic> species) ranging from 28.1% to 32.4%.</p>
<fig id="f2" position="float">
<label>Figure&#xa0;2</label>
<caption>
<p>Phylogenetic relationships among Korean rhizocephalan species. Maximum likelihood and Bayesian inference methods were used to reconstruct phylogenetic tree using the mtDNA sequence data. Numerical values on each node indicate maximum likelihood bootstrap support/Bayesian posterior probability. Multiple sequences representing <italic>Parasacculina imberbis</italic>, <italic>Parasacculina yatsui, Polyascus</italic> aff. <italic>gregarius</italic>, and <italic>Sacculina confragosa</italic>, are presented as black triangles. Sequences of <italic>Sacculina gracilis</italic> that assumed to be belonging to the genus <italic>Parasacculina</italic> are indicated in blue. *: sequences derived from GenBank. <bold>(A)</bold> <italic>cox1</italic> maximum likelihood and Bayesian inference tree; <bold>(B)</bold> <italic>cox1</italic> and 16S rDNA maximum likelihood tree.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1359503-g002.tif"/>
</fig>
</sec>
<sec id="s3_2">
<label>3.2</label>
<title>Host-parasite relationships between rhizocephalans, decapod hosts, and their distribution</title>
<p>Among all 28 species and 11,016 individuals of the investigated decapod host, 2.69% (296 individuals) were infected by Rhizocephala (<xref ref-type="table" rid="T1"><bold>Table&#xa0;1</bold></xref>). Combining the data on host-parasite association of the present study with results from the previous study (<xref ref-type="bibr" rid="B11">Jung et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B12">2021</xref>)<sup>1</sup> provides updated, synthetic information on the relationships between 26 parasitic barnacle species and 28 decapod host species discovered in Korea (<xref ref-type="fig" rid="f3"><bold>Figure&#xa0;3</bold></xref>, <xref ref-type="table" rid="T2"><bold>Table&#xa0;2</bold></xref>). From this, the author newly found four host-parasite relationships that were not previously reported from Korean rhizocephalan species. <italic>Microcassiope orientalis</italic> Takeda &amp; Miyake, 1969 was infected by <italic>Parasacculina</italic> sp. 4, <italic>Pachycheles stevensii</italic> Stimpson, 1858 was infected by <italic>Lernaeodiscus</italic> cf. <italic>rybakovi</italic>, and <italic>Areopaguristes hirsutimanus</italic> (Kobjakova, 1971) was infected by <italic>Peltogaster</italic> aff. <italic>ovalis</italic>. <italic>Pagurus ochotensis</italic> Brandt, 1851 was found infected by <italic>P. gracilis</italic> and an unknown <italic>Peltogaster</italic> sp. 3 (<xref ref-type="fig" rid="f3"><bold>Figure&#xa0;3A</bold></xref>, <xref ref-type="table" rid="T2"><bold>Table&#xa0;2</bold></xref>). Based on the host-parasite associations updated from this study, it turns out that three rhizocephalan species (<italic>P. gracillis</italic>, <italic>P. yatsui</italic>, and <italic>S. confragosa</italic>) were found from more than three host species, consistent with previous studies (<xref ref-type="bibr" rid="B3">Boschma, 1955b</xref>; <xref ref-type="bibr" rid="B32">Tsuchida et&#xa0;al., 2006</xref>; <xref ref-type="bibr" rid="B36">Yoshida et&#xa0;al., 2014</xref>) and 23 species were found in one or two host species (<xref ref-type="fig" rid="f3"><bold>Figure&#xa0;3A</bold></xref>, <xref ref-type="table" rid="T2"><bold>Table&#xa0;2</bold></xref>). Among these 23 rhizocephalans, five species [<italic>Peltogaster lineata</italic> Shiino, 1943<italic>, P. postica</italic>, <italic>Parasacculina pilosella</italic> (Van Kampen &amp; Boschma, 1925), <italic>Heterosaccus papillosus</italic> (Boschma, 1933), and <italic>S. gracilis</italic>] were previously reported to use more than 3 host species in Thailand, Singapore, Indonesia, Philippines, China, and Japan (<xref ref-type="bibr" rid="B2">Boschma, 1955a</xref>; <xref ref-type="bibr" rid="B3">b</xref>; <xref ref-type="bibr" rid="B36">Yoshida et&#xa0;al., 2014</xref>). The majority of host hermit crabs in Korea were infected by one or two rhizocephalan species, while <italic>Pagurus filholi</italic> (De Man, 1887), <italic>Hemigrapsus sanguineus</italic> (De Haan, 1835), and <italic>Pachygrapsus crassipes</italic> Randall, 1840 were infected by more than three rhizocephalan species (<xref ref-type="fig" rid="f3"><bold>Figure&#xa0;3B</bold></xref>). Extending the host-parasite association to higher taxonomic level, the host range of the rhizocephalan families is correlated with the infraorder level of host hermit crabs: The family Peltogastridae and Peltogasterellidae rhizocephalan species parasitize the Anomura species and the Sacculinidae and Polyascidae species were found in the Brachyura and Gebiidea species (<xref ref-type="table" rid="T3"><bold>Table&#xa0;3</bold></xref>).</p>
<table-wrap id="T1" position="float">
<label>Table&#xa0;1</label>
<caption>
<p>Number of individuals and infestation rate (%) of 28 Korean decapod hosts by Rhizocephala as examined in this study and previous studies (<xref ref-type="bibr" rid="B11">Jung et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B12">2021</xref>)<sup>1</sup>.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="left">Host decapod species</th>
<th valign="middle" align="left">Total number of individuals examined</th>
<th valign="middle" align="left">Number of individuals infested</th>
<th valign="middle" align="left">Infestation rate</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="left"><italic>Pachycheles stevensii</italic>
</td>
<td valign="middle" align="left">168</td>
<td valign="middle" align="left">2</td>
<td valign="middle" align="left">1.19%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Areopaguristes hirsutimanus</italic>
</td>
<td valign="middle" align="left">7</td>
<td valign="middle" align="left">1</td>
<td valign="middle" align="left">14.29%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Paguristes ortmanni</italic>
</td>
<td valign="middle" align="left">382</td>
<td valign="middle" align="left">1</td>
<td valign="middle" align="left">0.26%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus brachiomastus</italic>
</td>
<td valign="middle" align="left">100</td>
<td valign="middle" align="left">1</td>
<td valign="middle" align="left">1.00%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus filholi</italic>
</td>
<td valign="middle" align="left">721</td>
<td valign="middle" align="left">9</td>
<td valign="middle" align="left">1.25%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus japonicus</italic>
</td>
<td valign="middle" align="left">106</td>
<td valign="middle" align="left">4</td>
<td valign="middle" align="left">3.77%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus lanuginosus</italic>
</td>
<td valign="middle" align="left">603</td>
<td valign="middle" align="left">5</td>
<td valign="middle" align="left">0.83%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus maculosus</italic>
</td>
<td valign="middle" align="left">502</td>
<td valign="middle" align="left">13</td>
<td valign="middle" align="left">2.59%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus middendorffii</italic>
</td>
<td valign="middle" align="left">47</td>
<td valign="middle" align="left">1</td>
<td valign="middle" align="left">2.13%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus minutus</italic>
</td>
<td valign="middle" align="left">1244</td>
<td valign="middle" align="left">2</td>
<td valign="middle" align="left">0.16%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus ochotensis</italic>
</td>
<td valign="middle" align="left">148</td>
<td valign="middle" align="left">8</td>
<td valign="middle" align="left">5.41%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus pectinatus</italic>
</td>
<td valign="middle" align="left">313</td>
<td valign="middle" align="left">3</td>
<td valign="middle" align="left">0.96%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus proximus</italic>
</td>
<td valign="middle" align="left">175</td>
<td valign="middle" align="left">1</td>
<td valign="middle" align="left">0.57%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pagurus spina</italic>
</td>
<td valign="middle" align="left">28</td>
<td valign="middle" align="left">1</td>
<td valign="middle" align="left">3.57%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Porcellanopagurus nihonkaiensis</italic>
</td>
<td valign="middle" align="left">25</td>
<td valign="middle" align="left">2</td>
<td valign="middle" align="left">8.00%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Arcotheres sinensis</italic>
</td>
<td valign="middle" align="left">24</td>
<td valign="middle" align="left">4</td>
<td valign="middle" align="left">16.67%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Gaetice depressus</italic>
</td>
<td valign="middle" align="left">2105</td>
<td valign="middle" align="left">18</td>
<td valign="middle" align="left">0.86%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Cyclograpsus intermedius</italic>
</td>
<td valign="middle" align="left">72</td>
<td valign="middle" align="left">18</td>
<td valign="middle" align="left">25.00%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Hemigrapsus penicillatus</italic>
</td>
<td valign="middle" align="left">723</td>
<td valign="middle" align="left">2</td>
<td valign="middle" align="left">0.28%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Hemigrapsus sanguineus</italic>
</td>
<td valign="middle" align="left">1742</td>
<td valign="middle" align="left">103</td>
<td valign="middle" align="left">5.91%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Hemigrapsus takanoi</italic>
</td>
<td valign="middle" align="left">354</td>
<td valign="middle" align="left">1</td>
<td valign="middle" align="left">0.28%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Macromedaeus distinguendus</italic>
</td>
<td valign="middle" align="left">243</td>
<td valign="middle" align="left">13</td>
<td valign="middle" align="left">5.35%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Microcassiope orientalis</italic>
</td>
<td valign="middle" align="left">237</td>
<td valign="middle" align="left">7</td>
<td valign="middle" align="left">2.95%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pachygrapsus crassipes</italic>
</td>
<td valign="middle" align="left">379</td>
<td valign="middle" align="left">58</td>
<td valign="middle" align="left">15.30%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Charybdis (Charybdis) japonica</italic>
</td>
<td valign="middle" align="left">81</td>
<td valign="middle" align="left">3</td>
<td valign="middle" align="left">3.70%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Thalamita sima</italic>
</td>
<td valign="middle" align="left">129</td>
<td valign="middle" align="left">5</td>
<td valign="middle" align="left">3.88%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Pugettia intermedia</italic>
</td>
<td valign="middle" align="left">35</td>
<td valign="middle" align="left">4</td>
<td valign="middle" align="left">11.43%</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Upogebia major</italic>
</td>
<td valign="middle" align="left">323</td>
<td valign="middle" align="left">6</td>
<td valign="middle" align="left">1.86%</td>
</tr>
<tr>
<td valign="middle" align="left"><bold>Total</bold>
</td>
<td valign="middle" align="left"><bold>11,016</bold>
</td>
<td valign="middle" align="left"><bold>296</bold>
</td>
<td valign="middle" align="left"><bold>2.69%</bold>
</td>
</tr>
</tbody>
</table>
</table-wrap>
<fig id="f3" position="float">
<label>Figure&#xa0;3</label>
<caption>
<p>A vertical bar chart showing the correlation of 26 Korean rhizocephalan species and their 28 decapod hosts. <bold>(A)</bold> decapod host per Rhizocephala; <bold>(B)</bold> Rhizocephala per decapod host.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-11-1359503-g003.tif"/>
</fig>
<table-wrap id="T2" position="float">
<label>Table&#xa0;2</label>
<caption>
<p>Known hosts species of 26 Korean Rhizocephala.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="left">Rhizocephalan species</th>
<th valign="middle" align="left">First major host in Korea</th>
<th valign="middle" align="left">Second major host in Korea</th>
<th valign="middle" align="left">Other Korean hosts</th>
<th valign="middle" align="left">Hosts with no parasitic relationships found in Korea</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="left"><italic>Lernaeodiscus</italic> cf. <italic>rybakovi</italic>.</td>
<td valign="middle" align="left"><italic>Pachycheles stevensii</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster lineata</italic>
</td>
<td valign="middle" align="left"><italic>Pagurus filholi</italic>
</td>
<td valign="middle" align="left"><italic>Pagurus brachiomastus</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>Pagurus nigrivittatus*, Pagurus maculosus*</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster postica</italic>
</td>
<td valign="middle" align="left"><italic>P. filholi</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>P. nigrivittatus*, Pagurus angustus</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster reticulata</italic>
</td>
<td valign="middle" align="left"><italic>Pagurus minutus</italic>
</td>
<td valign="middle" align="left"><italic>Pagurus proximus</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster</italic> aff. <italic>ovalis</italic>
</td>
<td valign="middle" align="left"><italic>Pagurus japonicus</italic>
</td>
<td valign="middle" align="left"><italic>Areopaguristes hirsutimanus</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster</italic> sp. 1</td>
<td valign="middle" align="left"><italic>Pagurus lanuginosus</italic>
</td>
<td valign="middle" align="left"><italic>P. maculosus</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster</italic> sp. 2</td>
<td valign="middle" align="left"><italic>Paguristes ortmanni</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster</italic> sp. 3</td>
<td valign="middle" align="left"><italic>Pagurus ochotensis</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogasterella gracilis</italic>
</td>
<td valign="middle" align="left"><italic>P. maculosus</italic>
</td>
<td valign="middle" align="left"><italic>P. lanuginosus</italic>
</td>
<td valign="middle" align="left"><italic>P. filholi, P. ochotensis</italic>,<break/><italic>Pagurus middendorffii</italic>,<break/><italic>Pagurus pectinatus, Pagurus spina</italic>
</td>
<td valign="middle" align="left"><italic>Discorsopagurus schmitti, Labidochirus splendescens</italic>,<break/><italic>Pagurus aleuticus, P. dalli, P. edwardsi, P. hemphilli</italic>,<break/><italic>P. hirsutiusculus, P. nigrivittatus*, P. brachiomastus*</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogasterella</italic> sp.</td>
<td valign="middle" align="left"><italic>Porcellanopagurus nihonkaiensis</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina shiinoi</italic>
</td>
<td valign="middle" align="left"><italic>Upogebia major</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina imberbis</italic>
</td>
<td valign="middle" align="left"><italic>Pachygrapsus crassipes</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>Metopograpsus quadridentatus</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina oblonga</italic>
</td>
<td valign="middle" align="left"><italic>Cyclograpsus intermedius</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina pilosella</italic>
</td>
<td valign="middle" align="left"><italic>Pugettia intermedia</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>Quadrella coronata, Pugettia quadridens*</italic>,<break/><italic>Menaethius monoceros</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina pinnotherae</italic>
</td>
<td valign="middle" align="left"><italic>Arcotheres sinensis</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>Pinnotheres parvulus</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina yatsui</italic>
</td>
<td valign="middle" align="left"><italic>P. crassipes</italic>
</td>
<td valign="middle" align="left"><italic>Hemigrapsus sanguineus</italic>
</td>
<td valign="middle" align="left"><italic>H. penicillatus, C. intermedius</italic>
</td>
<td valign="middle" align="left"><italic>Metopograpsus messor, Metopograpsus oceanicus</italic>,<break/><italic>Metopograpsus quadridentatus</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina</italic> sp. 1</td>
<td valign="middle" align="left"><italic>H. sanguineus</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina</italic> sp. 2</td>
<td valign="middle" align="left"><italic>Macromedaeus distinguendus</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina</italic> sp. 3</td>
<td valign="middle" align="left"><italic>M. distinguendus</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina</italic> sp. 4</td>
<td valign="middle" align="left"><italic>Microcassiope orientalis</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
<tr>
<td valign="middle" align="left"><italic>Polyascus</italic> aff. <italic>gregarius</italic>
</td>
<td valign="middle" align="left"><italic>H. sanguineus</italic>
</td>
<td valign="middle" align="left"><italic>H. takanoi</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>Eriocheir japonica*</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Heterosaccus papillosus</italic>
</td>
<td valign="middle" align="left"><italic>Charybdis</italic> (<italic>Charybdis</italic>) <italic>japonica</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>Ch.</italic> (<italic>Gonioneptunus</italic>) <italic>bimaculata*, Ch. (Archias)</italic>
<break/><italic>truncata, Ch. (Charybdis) anisodon</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Sacculina confragosa</italic>
</td>
<td valign="middle" align="left"><italic>P. crassipes</italic>
</td>
<td valign="middle" align="left"><italic>H. sanguineus</italic>
</td>
<td valign="middle" align="left"><italic>Gaetice depressus</italic>
</td>
<td valign="middle" align="left"><italic>C. intermedius*</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Sacculina angulata</italic>
</td>
<td valign="middle" align="left"><italic>Thalamita sima</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>Xiphonectes longispinosus</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Sacculina gracilis</italic>
</td>
<td valign="middle" align="left"><italic>T. sima</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"><italic>X. longispinosus, T. picta</italic>, <italic>T. danae</italic>, <italic>Heteropilumnus setosus</italic>,<break/><italic>Ser fukiensis</italic>, <italic>Notonyx vitreu</italic>
</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Sacculina upogebiae</italic>
</td>
<td valign="middle" align="left"><italic>Upogebia major</italic>
</td>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
<td valign="middle" align="left"/>
</tr>
</tbody>
</table>
<table-wrap-foot>
<p>*: The species inhabiting Korea was confirmed, but Rhizocephala was not found in Korea.</p>
</table-wrap-foot>
</table-wrap>
<table-wrap id="T3" position="float">
<label>Table&#xa0;3</label>
<caption>
<p>Host family and their infraorder of 4 families and 28 species of Korean Rhizocephala.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="left">Rhizocephala species</th>
<th valign="middle" align="left">The family of rhizocephalan species</th>
<th valign="middle" align="left">The infraorder<break/>of hosts</th>
<th valign="middle" align="left">The family of hosts</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" align="left"><italic>Lernaeodiscus</italic> cf. <italic>rybakovi</italic>
</td>
<td valign="middle" align="left">Peltogasteridae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Porcellanidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster lineata</italic>
</td>
<td valign="middle" align="left">Peltogasteridae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Paguridae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster postica</italic>
</td>
<td valign="middle" align="left">Peltogasteridae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Paguridae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster reticulata</italic>
</td>
<td valign="middle" align="left">Peltogasteridae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Paguridae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster</italic> aff. <italic>ovalis</italic>
</td>
<td valign="middle" align="left">Peltogasteridae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Paguridae, Diogenidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster</italic> sp. 1</td>
<td valign="middle" align="left">Peltogasteridae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Paguridae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster</italic> sp. 2</td>
<td valign="middle" align="left">Peltogasteridae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Diogenidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogaster</italic> sp. 3</td>
<td valign="middle" align="left">Peltogasteridae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Paguridae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogasterella gracilis</italic>
</td>
<td valign="middle" align="left">Peltogasterellidae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Paguridae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Peltogasterella</italic> sp.</td>
<td valign="middle" align="left">Peltogasterellidae</td>
<td valign="middle" align="left">Anomura</td>
<td valign="middle" align="left">Paguridae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina shiinoi</italic>
</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Gebiidea</td>
<td valign="middle" align="left">Upogebiidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina imberbis</italic>
</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Grapsidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina oblonga</italic>
</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Varunidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina pilosella</italic>
</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Epialtidae, Trapeziidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina pinnotherae</italic>
</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Pinnotheridae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina yatsui</italic>
</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Grapsidae, Varunidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina</italic> sp. 1</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Varunidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina</italic> sp. 2</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Xanthidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina</italic> sp. 3</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Xanthidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Parasacculina</italic> sp. 4</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Nanocassiopidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Polyascus</italic> aff. <italic>gregarius</italic>
</td>
<td valign="middle" align="left">Polyascidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Varunidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Heterosaccus papillosus</italic>
</td>
<td valign="middle" align="left">Sacculinidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Portunidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Sacculina confragosa</italic>
</td>
<td valign="middle" align="left">Sacculinidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Grapsidae, Varunidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Sacculina angulata</italic>
</td>
<td valign="middle" align="left">Sacculinidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Portunidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Sacculina gracilis</italic>
</td>
<td valign="middle" align="left">Sacculinidae</td>
<td valign="middle" align="left">Brachyura</td>
<td valign="middle" align="left">Portunidae, Goneplacidae, Pilumnidae</td>
</tr>
<tr>
<td valign="middle" align="left"><italic>Sacculina upogebiae</italic>
</td>
<td valign="middle" align="left">Sacculinidae</td>
<td valign="middle" align="left">Gebiidea</td>
<td valign="middle" align="left">Upogebiidae</td>
</tr>
</tbody>
</table>
</table-wrap>
<p>To analyze the frequency of occurrence of rhizocephalan species and their host species in different locations and marine ecoregions in Korea, a total of 26 rhizocephalan species and 28 host species were examined in 28 locations and three marine ecoregions (<xref ref-type="supplementary-material" rid="ST3"><bold>Supplementary Table 3</bold></xref>). In the Yellow Sea ecoregion, 46 individuals of 11 rhizocephalan species were found from 10 decapod host species. Notably, <italic>Sacculina upogebiae</italic> Shiino, 1943 was exclusively found in this ecoregion. In the East China Sea ecoregion, 182 individuals representing 24 species of Rhizocephala were found from 21 host species. Among them, 10 rhizocephalan species were found solely in this region. In the East Sea ecoregion, 68 individuals of 11 rhizocephalan species were found to infect 17 host species. <italic>Peltogaster</italic> sp. 2 was specifically found only in the East Sea ecoregion. Out of the 26 rhizocephalan species, six species were found in common from three ecoregions. Notably, <italic>P. gracilis</italic>, <italic>P. yatsui</italic>, <italic>P.</italic> aff. <italic>gregarius</italic>, and <italic>S. confragosa</italic> had the highest occurrence rate among these species (<xref ref-type="fig" rid="f1"><bold>Figure&#xa0;1B</bold></xref>). However, <italic>P.</italic> aff. <italic>gregarius</italic> was not observed in the upper East Sea ecoregion. Furthermore, the author examined the relationship between Rhizocephala and host species that appeared in only one ecoregion, differentiating it from the case of the rhizocephalan species that appeared in two or more ecoregions. As a result, a specific relationship was found in one instance in the Yellow Sea ecoregion, five instances in the East China Sea ecoregion, and seven instances in the East Sea ecoregion (<xref ref-type="table" rid="T4"><bold>Table&#xa0;4</bold></xref>). Despite being present in multiple ecoregions, six decapod host species were found to be infected by six or more individuals of a single Rhizocephalan species in only one specific ecoregion (<xref ref-type="table" rid="T4"><bold>Table&#xa0;4</bold></xref>). For instance, <italic>Gaetice depressus</italic> (De Haan, 1833) was found to be infected with <italic>S. confragosa</italic> exclusively in the East China Sea ecoregion, despite that <italic>G. depressus</italic> is common in all three marine ecoregions of Korea.</p>
<table-wrap id="T4" position="float">
<label>Table&#xa0;4</label>
<caption>
<p>Relationships between Rhizocephala species and host appear in each three Korean marine ecoregions.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="top" rowspan="2" align="left">Rhizocephala species</th>
<th valign="top" rowspan="2" align="left">Host</th>
<th valign="top" colspan="3" align="left">Number of Rhizocephala individual<break/>in the ecoregion</th>
</tr>
<tr>
<th valign="top" align="left">Yellow Sea</th>
<th valign="top" align="left">East China Sea</th>
<th valign="top" align="left">East Sea</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="top" align="left"><italic>Lernaeodiscus</italic> cf. <italic>rybakovi</italic>
</td>
<td valign="top" align="left"><italic>Pachycheles stevensii</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" rowspan="2" align="left"><italic>Peltogaster lineata</italic>
</td>
<td valign="top" align="left"><italic>Pagurus brachiomastus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus filholi</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Peltogaster postica</italic>
</td>
<td valign="top" align="left"><italic>Pagurus filholi</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">6</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" rowspan="2" align="left"><italic>Peltogaster reticulata</italic>
</td>
<td valign="top" align="left"><italic>Pagurus minutus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">2</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus proximus</italic>
</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" rowspan="2" align="left"><italic>Peltogaster</italic> aff. <italic>ovalis</italic>
</td>
<td valign="top" align="left"><italic>Areopaguristes hirsutimanus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus japonicus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">4</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" rowspan="2" align="left"><italic>Peltogaster</italic> sp. 1</td>
<td valign="top" align="left"><italic>Pagurus lanuginosus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus maculosus</italic>
</td>
<td valign="top" align="left">5</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Peltogaster</italic> sp. 2</td>
<td valign="top" align="left"><italic>Paguristes ortmanni</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Peltogaster</italic> sp. 3</td>
<td valign="top" align="left"><italic>Pagurus ochotensis</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">6</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" rowspan="7" align="left"><italic>Peltogasterella gracilis</italic>
</td>
<td valign="top" align="left"><italic>Pagurus filholi</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus lanuginosus</italic>
</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left">2</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus maculosus</italic>
</td>
<td valign="top" align="left">3</td>
<td valign="top" align="left"/>
<td valign="top" align="left">4</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus middendorffii</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus ochotensis</italic>
</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus pectinatus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
<td valign="top" align="left">2</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pagurus spina</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Peltogasterella</italic> sp.</td>
<td valign="top" align="left"><italic>Porcellanopagurus nihonkaiensis</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">2</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina shiinoi</italic>
</td>
<td valign="top" align="left"><italic>Upogebia major</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">3</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina imberbis</italic>
</td>
<td valign="top" align="left"><italic>Pachygrapsus crassipes</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">11</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina oblonga</italic>
</td>
<td valign="top" align="left"><italic>Cyclograpsus intermedius</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">4</td>
<td valign="top" align="left">8</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina pilosella</italic>
</td>
<td valign="top" align="left"><italic>Pugettia intermedia</italic>
</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left">2</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina pinnotherae</italic>
</td>
<td valign="top" align="left"><italic>Arcotheres sinensis</italic>
</td>
<td valign="top" align="left">2</td>
<td valign="top" align="left">2</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" rowspan="4" align="left"><italic>Parasacculina yatsui</italic>
</td>
<td valign="top" align="left"><italic>Cyclograpsus intermedius</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
<td valign="top" align="left">6</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Hemigrapsus penicillatus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
<td valign="top" align="left">2</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Hemigrapsus sanguineus</italic>
</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left">9</td>
<td valign="top" align="left">3</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pachygrapsus crassipes</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">6</td>
<td valign="top" align="left">9</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina</italic> sp. 1</td>
<td valign="top" align="left"><italic>Hemigrapsus sanguineus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">3</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina</italic> sp. 2</td>
<td valign="top" align="left"><italic>Macromedaeus distinguendus</italic>
</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left">11</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina</italic> sp. 3</td>
<td valign="top" align="left"><italic>Macromedaeus distinguendus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Parasacculina</italic> sp. 4</td>
<td valign="top" align="left"><italic>Microcassiope orientalis</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">7</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" rowspan="2" align="left"><italic>Polyascus</italic> aff. <italic>gregarius</italic>
</td>
<td valign="top" align="left"><italic>Hemigrapsus sanguineus</italic>
</td>
<td valign="top" align="left">16</td>
<td valign="top" align="left">52</td>
<td valign="top" align="left">1</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Hemigrapsus takanoi</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">1</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Heterosaccus papillosus</italic>
</td>
<td valign="top" align="left"><italic>Charybdis</italic> (<italic>Charybdis</italic>) <italic>japonica</italic>
</td>
<td valign="top" align="left">2</td>
<td valign="top" align="left">1</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Sacculina angulata</italic>
</td>
<td valign="top" align="left"><italic>Thalamita sima</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">3</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" rowspan="3" align="left"><italic>Sacculina confragosa</italic>
</td>
<td valign="top" align="left"><italic>Gaetice depressus</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">18</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Hemigrapsus sanguineus</italic>
</td>
<td valign="top" align="left">9</td>
<td valign="top" align="left">7</td>
<td valign="top" align="left">2</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Pachygrapsus crassipes</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">14</td>
<td valign="top" align="left">18</td>
</tr>
<tr>
<td valign="top" align="left"><italic>Sacculina gracilis</italic>
</td>
<td valign="top" align="left"><italic>Thalamita sima</italic>
</td>
<td valign="top" align="left"/>
<td valign="top" align="left">2</td>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" align="left"><italic>Sacculina upogebiae</italic>
</td>
<td valign="top" align="left"><italic>Upogebia major</italic>
</td>
<td valign="top" align="left">3</td>
<td valign="top" align="left"/>
<td valign="top" align="left"/>
</tr>
<tr>
<td valign="top" colspan="2" align="left">Total</td>
<td valign="top" align="left">46</td>
<td valign="top" align="left">182</td>
<td valign="top" align="left">68</td>
</tr>
</tbody>
</table>
</table-wrap>
</sec>
</sec>
<sec id="s4" sec-type="discussion">
<label>4</label>
<title>Discussion</title>
<sec id="s4_1">
<label>4.1</label>
<title>The overview of the taxonomy and phylogenetic relationships of Korean rhizocephalan</title>
<p>Morphological and molecular analyses provided insights into the taxonomy and phylogenetic relationships of Korean rhizocephalan species in this study. Phylogenetic trees recognized four rhizocephalan families, i.e., Polyascidae, Sacculinidae, Peltogastridae, and Peltogasterellidae, consistent with previous molecular analysis (<xref ref-type="bibr" rid="B10">H&#xf8;eg et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B11">Jung et&#xa0;al., 2019</xref>; <xref ref-type="bibr" rid="B12">2021</xref>). Three unidentified species (<italic>Lernaeodiscus</italic> cf. <italic>rybakovi</italic>, <italic>Peltogaster</italic> sp. 3, and <italic>Parasacculina</italic> sp. 4) were distinguished from their congeneric species based on their hosts and their morphological characteristics. In addition, the author assumed that <italic>P.</italic> aff. <italic>gregarius</italic> with different geographic origins (i.e., Korea and China) might represent two different cryptic species because they have different host species and high sequence divergence ranging from 4.6% to 5.7% in molecular analysis (<xref ref-type="supplementary-material" rid="ST2"><bold>Supplementary Table 2</bold></xref>, <xref ref-type="fig" rid="f2"><bold>Figure&#xa0;2</bold></xref>).</p>
</sec>
<sec id="s4_2">
<label>4.2</label>
<title>The characteristics of host-parasite relationships between Korean rhizocephalan and decapod hosts</title>
<p>From a combined analysis of host-parasite association of 26 rhizocephalans and 28 host decapods including previous studies, this study revealed that Korean rhizocephalan species have different patterns in their host range, compared to the previously reported in Japan (<xref ref-type="bibr" rid="B32">Tsuchida et&#xa0;al., 2006</xref>; <xref ref-type="bibr" rid="B36">Yoshida et&#xa0;al., 2014</xref>). Three unreported relationships between Rhizocephala and the host, i.e., <italic>M. orientalis</italic> and <italic>Parasacculina</italic> sp. 4, <italic>A. hirsutimanus</italic> and <italic>P.</italic> aff. <italic>ovalis</italic>, and <italic>P.&#xa0;ochotensis</italic> and <italic>Peltogaster</italic> sp. 3, were found in this study. The author also found 23 rhizocephalan species that show high host specificity, parasitizing only one or two host species in Korea (<xref ref-type="fig" rid="f3"><bold>Figure&#xa0;3A</bold></xref>, <xref ref-type="table" rid="T2"><bold>Table&#xa0;2</bold></xref>). In general, parasites are highly dependent on the availability and suitability of their hosts which is chance-dependent in their early establishment of host-parasite association. In case of <italic>S. gracilis</italic>, compared to other geographic areas (Thailand, Indonesia, Singapore, Philippines, and China; see <xref ref-type="bibr" rid="B2">Boschma, 1955a</xref>) where relatively rich data resources of host candidates have ever known [e.g., <italic>Xiphonectes longispinosus</italic> (Dana, 1852), <italic>Thalamita picta</italic> Stimpson, 1858, <italic>Thalamita sima</italic> H. Milne Edwards, 1834, <italic>Thalamita danae</italic> Stimpson, 1858, <italic>Heteropilumnus setosus</italic> (A. Milne-Edwards, 1873), <italic>Ser fukiensis</italic> Rathbun, 1931, and <italic>Notonyx vitreu</italic> Alcock, 1900], only one host candidate <italic>T. sima</italic> was thus far reported to occur in Korea. Therefore, it is reasonable to assume that <italic>T. sima</italic> can only be utilized as a host for Korean populations of <italic>S. gracilis</italic>. However, <italic>P. lineata</italic> and <italic>P. postica</italic> were found in <italic>Pagurus brachiomastus</italic> (Thallwitz, 1891) and <italic>P. filholi</italic>, but none of these rhizocephalans were found from <italic>Pagurus nigrivittatus</italic> Komai, 2003 (examined 47 individuals) and <italic>Pagurus maculosus</italic> Komai &amp; Imafuku, 1996 (examined 502 individuals) (<xref ref-type="table" rid="T4"><bold>Table&#xa0;4</bold></xref>) even though these hermit crab species inhabit Korea and were reported as a host of <italic>P. lineata</italic> or <italic>P. postica</italic> in Japan (<xref ref-type="bibr" rid="B36">Yoshida et&#xa0;al., 2014</xref>).</p>
<p>In contrast, this study provides additional information on the extensive range of host-parasite association at the higher taxonomic level between rhizocephalans and their host hermit crabs, i.e., between the family of rhizocephalans and the infraorder of the hermit crab hosts. Unlike host-parasite association at the species level which is host resource-dependent (availability and abundance of host resource), their relationships at the higher taxonomic level of host and parasite are relatively relaxed (<xref ref-type="table" rid="T3"><bold>Table&#xa0;3</bold></xref>) as previously documented in other metazoan groups (<xref ref-type="bibr" rid="B34">Williams, 2008</xref>; <xref ref-type="bibr" rid="B30">Sweet et&#xa0;al., 2018</xref>). Further characterization of host-parasite relationships based on extensive taxon samplings is needed to fill the gap in our current understanding of their relaxed relationship, host ranges, and preferences in diverse parasitic barnacle species.</p>
</sec>
<sec id="s4_3">
<label>4.3</label>
<title>The influence of geographical factors on the host range and distribution pattern of Korean rhizocephalans</title>
<p>In this study, the author compared species diversity and abundance of parasitic barnacles across the Korean sea coast that was characterized into three ecoregions (<xref ref-type="bibr" rid="B16">Kim et&#xa0;al., 2020</xref>; <xref ref-type="bibr" rid="B17">2022</xref>). From this, the author found that rhizocephalan species diversity and abundance are higher in the East China Sea ecoregion (182 individuals and 24 species) than in the Yellow Sea (46 individuals and 10 species) and the East Sea ecoregions (68 individuals and 11 species) (<xref ref-type="fig" rid="f1"><bold>Figure&#xa0;1</bold></xref>). Relatively lower species richness and a scanty of rhizocephalans in the Yellow Sea and the East Sea ecoregions are assumed relevant to the lower survival rate of their larva as surface water temperature decreases. <xref ref-type="bibr" rid="B13">Kashenko and Korn (2002</xref>; <xref ref-type="bibr" rid="B14">2003)</xref> reported that most larvae of three rhizocephalan species, i.e., <italic>P. gracilis</italic>, <italic>Peltogaster reticulata</italic> Shiino, 1943, and <italic>Polyascus polygeneus</italic> (L&#xfc;tzen &amp; Takahashi, 1997), were not able to survive lower than 12&#xb0;C. The lowest water temperature of the Yellow Sea and the East Sea ecoregions in winter often drops below 10&#xb0;C. On the other hand, the lowest water temperature in the East China Sea ecoregion in winter is approximately 11-12&#xb0;C (<xref ref-type="bibr" rid="B21">Ma et&#xa0;al., 2006</xref>; <xref ref-type="bibr" rid="B16">Kim et&#xa0;al., 2020</xref>; <xref ref-type="bibr" rid="B26">Park et&#xa0;al., 2020</xref>). It is assumed that Rhizocephala larvae in the Yellow Sea and the East Sea ecoregions with relatively lower surface water temperature during the winter season might have a lower survival rate, leading to relatively lower species richness of rhizocephalans compared to the East China Sea ecoregions. However, <italic>P. gracilis</italic> exhibits high densities with a 20% infection rate in <italic>Pagurus middendorffii</italic> Brandt, 1851 in Vostok Bay (<xref ref-type="bibr" rid="B18">Kornienko et&#xa0;al., 2018</xref>), where winter water temperatures can drop below zero (<xref ref-type="bibr" rid="B5">Fayman et&#xa0;al., 2019</xref>), which suggests that low temperatures may not universally limit Rhizocephalan diversity. Therefore, this necessitates further study to elucidate the specific impact of minimum winter water temperatures on the diversity of Rhizocephalans.</p>
<p>This research unveiled distinct patterns in the specificity of Rhizocephala-host relationships across three Korean ecoregions (the East China Sea, the East Sea, and the Yellow Sea). In the East China Sea ecoregion, the author identified a remarkable diversity, with 10 ecoregion-specific rhizocephalan species and 5 unique host-parasite relationships that are not found in other ecoregions (<xref ref-type="fig" rid="f3"><bold>Figure&#xa0;3</bold></xref>). This contrasts starkly with the East Sea ecoregion, where the author found only one ecoregion-specific rhizocephalan species, alongside 7 unique host-parasite relationships. The Yellow Sea ecoregion showed a lower specificity with a simpler pattern of rhizocephalan fauna represented by only one ecoregion-specific rhizocephalan species and a single unique host-parasite relationship. These disparities appear to be linked to the environmental characteristics of each ecoregion, particularly the lowest water temperatures in winter and the host species diversity. In the East China Sea ecoregion, the relatively higher water temperatures in winter (&gt;11&#xb0;C) and a greater abundance of host species (21 species, <xref ref-type="table" rid="T4"><bold>Table&#xa0;4</bold></xref>) might foster an environment likely to increase the diversity of Rhizocephala, might lead to the settlement of specialist Rhizocephala parasitizing one or two hosts. Conversely, in the East Sea ecoregion, despite the secondary high diversity of hosts (17 species, <xref ref-type="table" rid="T4"><bold>Table&#xa0;4</bold></xref>), the lower water temperatures in winter (&lt;10&#xb0;C) might prevent the settlement of specialist Rhizocephala. This may result in the generalist Rhizocephala parasitizing more than three hosts might have adapted to parasitize unique hosts. In the Yellow Sea ecoregion, both the lower minimum sea temperatures and a limited host species pool seem to constrain the development of unique Rhizocephala-host relationships. The hypothesis receives limited support, as evidenced by the discovery of 6 Rhizocephalan species in southern Honshu, Japan, with a minimum winter water temperature above 11&#xb0;C and 7 host Paguroidea species (<xref ref-type="bibr" rid="B36">Yoshida et&#xa0;al., 2014</xref>), compared to only 3 Rhizocephalan species in Vostok Bay, Russia, where the minimum winter water temperature falls below zero and hosts 5 Paguroidea species (<xref ref-type="bibr" rid="B18">Kornienko et&#xa0;al., 2018</xref>).</p>
<p>In the present investigation, these findings reveal a geographically constrained parasitism of Rhizocephala on specific hosts. Notably, the parasitism of <italic>S. confragosa</italic> on <italic>G. depressus</italic> is exclusive to Namhae and its vicinities, i.e., Yeosu, Tongyeong, Sacheon, and Goheung, within the East China Sea ecoregion, despite the extensive prevalence of <italic>G. depressus</italic> along the entire Korean coastline. Similarly, <italic>Parasacculina imberbis</italic> Shiino, 1943 exhibits a unique parasitic relationship with <italic>P. crassipes</italic> solely in the southern extents of the East China Sea ecoregion, such as Jeju, Seogwipo, Namhae, and Tongyeong, similar to the distribution of Japanese <italic>P. imberbis</italic> limited to southern Honshu (<xref ref-type="bibr" rid="B32">Tsuchida et&#xa0;al., 2006</xref>). These observations resonate with the patterns documented by <xref ref-type="bibr" rid="B3">Boschma (1955b)</xref>, wherein <italic>Heterosaccus occidentalis</italic> (Boschma, 1928) demonstrates a limited distribution relative to the wider distribution of its three hosts: <italic>Mithraculus forceps</italic> A. Milne-Edwards, 1875, <italic>Omalacantha bicornuta</italic> (Latreille, 1825), and <italic>Pitho lherminieri</italic> (Desbonne in Desbonne &amp; Schramm, 1867). These results may be related to the gap in ecological parameters of each location, especially the difference in optimal water temperature and optimal salinity promote the development and survival of each larvae of rhizocephalan species (<xref ref-type="bibr" rid="B13">Kashenko and Korn, 2002</xref>; <xref ref-type="bibr" rid="B14">2003</xref>; <xref ref-type="bibr" rid="B33">Tu et&#xa0;al., 2009</xref>), which can influence the inhabit of Rhizocephala. These factors could serve as ecological barriers that delineate the range of these parasitic entities. This hypothesis warrants further investigation to unravel the complexities underlying these specialized parasitic relationships.</p>
</sec>
</sec>
<sec id="s5" sec-type="conclusions">
<label>5</label>
<title>Conclusions</title>
<p>The present study integrates multiple sources of information about the molecular phylogeny of Korean rhizocephalans and their distribution and host range based on recent collections of parasitic barnacles. Through this molecular analysis, the unidentified species of Korean Rhizocephala in the previous studies are different from other parasitic barnacles. In addition, each Rhizocephala family correlated with the infraorder of their host. The author found that the Korean rhizocephalan species show different patterns of host-parasite association from those previously documented in other geographic regions, which might be due to different host species resources depending on geographical origins. Meanwhile, in this study, the distribution of Rhizocephala in three Korean marine ecoregions was confirmed, and as a result, the East China Sea had higher diversity than other ecoregions, which might be related to the highest water temperature of the East China Sea in winter. An in-depth analysis from further extensive taxon samplings of rhizocephalans and crustacean host species is needed to complete our understanding of host-parasite association and distribution of Rhizocephala for their control and utilization.</p>
</sec>
<sec id="s6" sec-type="data-availability">
<title>Data availability statement</title>
<p>The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/<xref ref-type="supplementary-material" rid="ST1"><bold>Supplementary Material</bold></xref>.</p>
</sec>
<sec id="s7" sec-type="ethics-statement">
<title>Ethics statement</title>
<p>The manuscript presents research on animals that do not require ethical approval for their study.</p>
</sec>
<sec id="s8" sec-type="author-contributions">
<title>Author contributions</title>
<p>JJ: Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Visualization, Writing &#x2013; original draft, Writing &#x2013; review &amp; editing.</p>
</sec>
</body>
<back>
<sec id="s9" sec-type="funding-information">
<title>Funding</title>
<p>The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work was funded by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2021R1A6A3A01086374, 2021R1A6A1A10039823).</p>
</sec>
<ack>
<title>Acknowledgments</title>
<p>The author appreciates Dr. Joong-Ki Park (Ewha Womans University) who supervised many parts of this manuscript and Dr. Jongwoo Jung, Ms. Sung-Hyun Park (Ewha Womans University), and Dr. Hyunsoon Kim (Seoul National University) who helped sample collection of the materials used in this study.</p>
</ack>
<sec id="s10" sec-type="COI-statement">
<title>Conflict of interest</title>
<p>The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec id="s11" sec-type="disclaimer">
<title>Publisher&#x2019;s note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
<sec id="s12" sec-type="supplementary-material">
<title>Supplementary material</title>
<p>The Supplementary Material for this article can be found online at: <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/fmars.2024.1359503/full#supplementary-material">https://www.frontiersin.org/articles/10.3389/fmars.2024.1359503/full#supplementary-material</ext-link>
</p>
<supplementary-material xlink:href="Table_1.docx" id="ST1" mimetype="application/vnd.openxmlformats-officedocument.wordprocessingml.document"/>
<supplementary-material xlink:href="Table_2.xlsx" id="ST2" mimetype="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"/>
<supplementary-material xlink:href="Table_3.xlsx" id="ST3" mimetype="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"/>
</sec>
<fn-group>
<fn id="fn1">
<label>1</label>
<p>Jung, J., and Park, J. K. (2024). The fauna of parasitic barnacles (Cirripedia: Rhizocephala) in Korea. Zool. Stud 63, 01. in review.</p>
</fn>
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