An undescribed species belonging to the family Eunicidae was detected in a sublittoral habitat of the southern coast of Korea. This Korean eunicid species was initially thought to belong to the genus Lysidice based on its general appearance, including the absence of prostomial lateral palps and peristomial cirri. However, a more detailed characterization of the morphological features of the maxillae and mandible coupled with mtCOI and 18S rRNA gene sequence analyses confirmed that this species is a member of the genus Paucibranchia. The absence of lateral palps found in the intact adult specimens with 153 segments is a unique feature not previously reported in species belonging to Paucibranchia. Thus, the new species, Paucibranchia triantennata sp. nov., can be easily distinguished from other known congeneric species. Except for the absence of lateral palps, P. triantennata sp. nov. resembled P. conferta, P. gathofi, and P. patriciae in the shape of the prostomium, brief location and shape of branchiae, and absence of compound spinigers. However, P. triantennata could be clearly distinguished from these species by the shorter prostomial antennae, a large number of subacicular hooks, and the morphological features of the maxillae and compound falcigers.

This study assessed the taxonomic statuses of three lesser-known Magelona F. Müller, 1858 species collected from intertidal to sublittoral habitats in Korean coastal waters, basing identification on morphological features and comparing them with their closest congeners. We present a comprehensive description and illustration of taxonomically significant and standardized characters, covering all thoracic chaetigers of three newly discovered Magelona species from Korea. Within the documented Korean Magelona species, these three species exhibit the following distinctive characteristics. Magelona sachalinensis Buzhinskaja, 1985, possess diminutive superior dorsal lobes in the initial four chaetigers and specialized chaetae on chaetiger 9; Magelona lenticulata Gallardo, 1968, is characterized by foliaceous postchaetal superior dorsal lobes in chaetigers 1–8; and Magelona cf. longicornis Johnson, 1901, is distinguished by notably elongated noto- and neuropodial postchaetal lamellae on chaetiger 9. Methyl green staining patterns showed species-specific characteristics and were confirmed to be effective in distinguishing the examined Korean species from each other and useful for making comparisons with previously reported Magelona species. Our study suggests that further comprehensive research on the morphological and genetic characteristics of Magelona species will enhance our understanding of their diversity.

This study reports two new species of Ampharete and one new record of a species of Amphicteis (Ampharetidae) from the Korean subtidal zone, based on an integrative approach. Ampharete koreana sp. nov. is characterized by paleal chaetae several times thicker than notochaetae, with short filamentous tips. It more closely resembles A. finmarchica but differs in the shape of its paleae, interbranchial gap, thoracic uncini, and pygidial cirri and shows a 2% genetic divergence in histone H3 and in 18% of mtCOI genes. Ampharete namhaensis sp. nov. shares with A. petersenae the absence of paleae and the presence of 16 abdominal uncinigers, a rare combination of traits among known Ampharete species. However, A. petersenae differs in having a marked interbranchial gap, prominent eyes on the prostomium, double the number of abdominal uncini and buccal tentacles, and in its distribution (southern Korea for A. namhaensis sp. nov. vs. North Atlantic and Arctic-adjacent waters for A. petersenae). The Korean Amphicteis specimens examined in this study closely resemble A. glabra in key morphological features, including fine paleae tips, 14 thoracic and 15 abdominal uncinigers, and approximately eight paleae. However, given the need for a comprehensive revision of Amphicteis and the potential for cryptic diversity within the genus, these specimens are provisionally designated as Amphicteis sp. cf. A. glabra.

Phyllodoce koreana was first described in 1985 in Gwangyang Bay, a semi-enclosed bay in Korea affected by significant organic input from the Seomjin River and dredging activities near the Gwangyang Port. Since then, this Korean endemic species has received limited attention in taxonomic and ecological studies. Phyllodoce koreana is known for its resilience to mild disturbances but is vulnerable to severe environmental changes. In this study, P. koreana specimens were collected from organically polluted Asian stalked tunicate aquaculture farms at eight sites in Jindong Bay, a location with environmental conditions similar to those of Gwangyang Bay, over the course of five sampling events from March to November. Both bays experience benthic hypoxia in summer due to elevated water temperatures and organic matter accumulation. Phyllodocid specimens were primarily collected in March and November 2023, non-hypoxic periods, suggesting potential seasonal adaptations to environmental fluctuations. The morphological features of the collected specimens were consistent with the original description of P. koreana, confirming their identification. Additionally, we reported previously overlooked morphological details, contributing to a more comprehensive taxonomic understanding of the species. We also present, for the first time, the complete mitochondrial genome of this species, comprising 15,559 bp, which provides essential genetic data for future taxonomic and phylogenetic studies. The phylogenetic analysis of protein-coding genes shows that, among 17 related polychaete species, P. koreana (family Phyllodocidae) is closely related to the family Goniadidae. Future research should expand our knowledge of polychaete taxonomy by integrating additional mitochondrial genomes and investigating the role of conserved gene synteny within Polychaeta.

The family Capitellidae performs critical roles in bioturbation and sediment remediation within global marine benthic ecosystems. However, they are a taxonomically challenging group due to their simple morphology and a ‘morphological mosaic’, where traditional classificatory traits, such as thoracic chaetiger counts, appear convergently across genera. Previous multi-locus studies (using 18S, 28S, H3, and COI) first highlighted this conflict, revealing the polyphyly of major genera like Notomastus and even Leiochrides itself (based on unidentified specimens). More recently, mitogenomic studies uncovered massive gene order rearrangements and a conflicting topology but did not include Leiochrides. Critically, with no complete mitogenome reported for a formally identified Leiochrides species, its true phylogenetic position and the validity of its polyphyly remain unresolved. To address this critical gap, we sequenced and characterized the first complete mitochondrial genome from a formally identified species, Leiochrides yokjidoensis, recently described from Korean waters. The complete mitogenome was 17,933 bp in length and included the typical 13 protein-coding genes (PCGs), 2 ribosomal RNAs (rRNAs), and 22 transfer RNAs (tRNAs). Gene order (GO) analysis revealed the occurrence of gene rearrangements in Capitellidae and in its sister clade, Opheliidae. A phylogenomic analysis using the amino acid sequences of 13 PCGs from 30 species established the first robust systematic position for the genus Leiochrides (based on this formally identified species). Phylogenetic results recovered Leiochrides as a sister group to the clade comprising Mediomastus, Barantolla, Heteromastus, and Notomastus hemipodus (BS 99%). This distinct placement confirms that Leiochrides represents an independent evolutionary lineage, phylogenetically separate from the polyphyletic Notomastus complex, despite their morphological similarities. Furthermore, our analysis confirmed the polyphyly of Notomastus, with N. hemipodus clustering distinctly from other Notomastus species. Additionally, signatures of positive selection were detected in ND4, and ND5 genes, suggesting potential adaptive evolution to the subtidal environment. This placement provides a critical, high-confidence anchor point for the genus Leiochrides. It provides a reliable reference to investigate the unresolved polyphyly suggested by previous multi-locus studies and provides compelling evidence for the hypothesis that thoracic chaetiger counts are of limited value for inferring phylogenetic relationships. This study provides the foundational genomic cornerstone for Leiochrides, representing an essential first step toward resolving the systematics of this taxonomically challenging family.