Ceciamaralia, a new genus of Dorvilleidae (Annelida) from deep waters of the Southwest Atlantic Ocean and an insight into its relationship within the family.

Dorvilleidae Chamberlin, 1919 is a family of Annelida containing some of the smallest 'polychaetes' species, being poorly studied worldwide, and with little knowledge regarding its diversity and occurrence. Samples obtained in oceanographic campaigns performed in the Southwest Atlantic Ocean (Brazilian coast) revealed a high number of specimens of dorvilleids, adding to our knowledge of the family's biodiversity. A detailed morphological analysis of these organisms has revealed a new genus, Ceciamaralia gen. nov., with two new species. The new genus differs from other Dorvilleidae genera in (i) the robust and enlarged pharynx which are frequently everted, (ii) unique composition of maxillae, with an elongated pair of serrated basal plates and one pair of anterior free maxillary plates with a long and thin anterior spine and (iii) ventral cirri present only in few first chaetigers. Ceciamaralia lanai gen. et sp. nov. is characterized by the presence of a broad and large dorsal cirrus on a few anterior parapodia and by furcate chaeta in supra-acicular fascicles. While Ceciamaralia nonatoi gen. et sp. nov. presents one geniculate chaeta instead of one furcate, the absence of dorsal cirri and, in some specimens, the absence of palps. A cladistic analysis supported the monophyly of Ceciamaralia gen. nov. by four synapomorphies related to the unique morphology of its maxillae, pharynx and appendages. This study is part of several recent taxonomic studies aiming to elucidate and increase the knowledge of Dorvilleidae, since it is part of a Ph.D project focused on the family.

A new species of Thoracophelia (Annelida, Opheliidae) from the Yellow Sea of South Korea.

Background: Thoracophelia Ehlers, 1897 is a genus of Opheliidae characterised by the body divided into three distinct regions, modified parapodia in chaetiger 10 and a ventral groove restricted to the posterior half of the body. To date, 18 species have been described in the genus. Amongst them, six species have been recorded in northeast Asia. New information: A new species, Thoracopheliafoliformis sp. nov., was discovered in the intertidal zone of the Yellow Sea, South Korea. This is the first Thoracophelia species report from the Yellow Sea. This new species is closely related to T.dillonensis (Hartman, 1938) from California and T.ezoensis Okuda, 1936 from Japan in having pectinate branchiae. However, the new species can be distinguished from the two species by the unique combination of the following characteristics: 15 pairs of wrinkled pectinate branchiae with 12-15 filaments at best development and a foliaceous mid-ventral plate in the pygidium instead of one or two thick ventral cirri. Detailed descriptions and illustrations of T.foliformis sp. nov. are provided. Sequences of the mitochondrial cytochrome c oxidase subunit I (COI), nuclear 18S ribosomal DNA (rDNA) and 28S rDNA of the new species were determined and analysed.

Transfer and bioaccumulation of chemical and biological contaminants in the marine polychaete Hediste diversicolor (OF müller 1776) when reared on salmon aquaculture sludge.

Side streams from aquaculture production such as fish sludge poses ample opportunities for biological upcycling, as the sludge contains high amounts of nutrients, energy and valuable biochemicals, making it an ideal food for extractive species. Sludge has been proposed as a feed stock for polychaete production, which in turn can be utilized live in shrimp aquaculture or as an aquafeed ingredient. However, the biosafety of such value chains has not yet been addressed. We conducted an experiment exposing the polychaete Hediste diversicolor to aquaculture sludge spiked with four different fish pathogens (Mycobacterium salmoniphilum, Yersinia ruckeri, Infectious Pancreatic Necrosis (IPN) and Infectious Salmon Anaemia (ISA)) known to cause diseases in Atlantic salmon (Salmo salar L.). Moreover, we assessed whether heavy metals and other potentially hazardous elements present in fish sludge bioaccumulates in the polychaetes. Neither of the bacteria nor viruses could be detected in the polychaetes after 14 days of continuous exposure. Seven of the 15 elements we analysed showed bioaccumulation factors significantly below one, meaning biodilution, while the other eight did not differ from one, meaning no bioaccumulation. None of the elements showed a significant bioaccumulation. Further on, none of the heavy metals found in the polychaetes at the end of our experiment exceeded the EU regulatory maximum levels for fish feed ingredients. The current results suggest that a H. diversicolor can reared on aquaculture sludge, and aquaculture sludge may serve as feed stock for polychaete production without the product exceeding EU regulations for contaminants in animal feed.

A new genus and species of nudibranch-mimicking Syllidae (Annelida, Polychaeta).

Nudibranch mollusks, which are well-known for their vivid warning coloration and effective defenses, are mimicked by diverse invertebrates to deter predation through both Müllerian and Batesian strategies. Despite extensive documentation across different taxa, mimickers have not been detected among annelids, including polychaetes, until now. This study described a new genus and species of polychaete living on Dendronephthya octocorals in Vietnam and Japan. Belonging to Syllidae, it exhibits unique morphological adaptations such as a low number of body segments, simple chaetae concealed within the parapodia and large and fusiform antennae and cirri. Moreover, these appendages are vividly colored, featuring an internal dark red area with numerous terminal white spots and bright yellow tips, effectively contributing to mimicking the appearance of a nudibranch. This discovery not only documents the first known instance of such mimicry among annelids, but also expands our understanding of evolutionary adaptation and ecological strategies in marine invertebrates.

Discovery and characterization of a transient chaetal gland during the development of Capitella teleta (Sedentaria: Annelida).

Chaetae are among the most extensively studied structures in polychaetes, serving as a defining morphological trait for annelids. Capitella teleta stands out as one of the few established annelid models for developmental and morphological studies, thus receiving significant scholarly attention. In this study, we unveil a previously unnoticed glandular structure associated with chaetae within the larvae of C. teleta. Our investigations demonstrate the absence of comparable structures in the chaetal follicles of adults and juveniles (older than 1 week), as well as during active chaetogenesis, underscoring the transient nature of these glands. This indicates that larval chaetal follicles transform into a gland that later disappears. Utilizing histology and transmission electron microscopy, we characterized these glands. Our findings underscore the diversity of chaetal ultrastructure in annelids and show that, even in well-studied species, novel morphological details can be found. We emphasize the importance of examining various life-history stages to capture such transient morphological features. This work lays a crucial morphological foundation and deepens our understanding of chaetae and chaetogenesis in C. teleta, paving the way for more accurate interpretations of future experimental studies on chaetogenesis in this species.

Two new records and description of a new Perinereis (Annelida, Nereididae) species for the Saudi Arabian Red Sea region.

Annelid biodiversity studies in the Red Sea are limited and integrative taxonomy is needed to accurately improve reference libraries in the region. As part of the bioblitz effort in Saudi Arabia to assess the invertebrate biodiversity in the northern Red Sea and Gulf of Aqaba, Perinereis specimens from intertidal marine and lagoon-like rocky environments were selected for an independent assessment, given the known taxonomic ambiguities in this genus. This study used an integrative approach, combining molecular with morphological and geographic data. Our results demonstrate that specimens found mainly in the Gulf of Aqaba are not only morphologically different from other five similar Perinereis Group I species reported in the region, but phylogenetic analysis using available COI sequences from GenBank revealed different molecular operational taxonomic units, suggesting an undescribed species, P.kaustianasp. nov. The new species is genetically close and shares a similar paragnath pattern to the Indo-Pacific distributed P.helleri, in particular in Area III and Areas VII-VIII. Therefore, we suggest it may belong to the same species complex. However, P.kaustianasp. nov. differs from the latter mainly in the shorter length of the postero-dorsal tentacular cirri, median parapodia with much longer dorsal Tentacular cirri, posteriormost parapodia with much wider and greatly expanded dorsal ligules. Additionally, two new records are reported for the Saudi Neom area belonging to P.damietta and P.suezensis, previously described only for the Egyptian coast (Suez Canal) and are distributed sympatrically with the new species, but apparently not sympatric with each other.

Ultrastructure of the Jurassic serpulid tubes-phylogenetic and paleoecological implications.

The ultrastructural diversity of the Middle and Late Jurassic serpulid tubes from the Polish Basin has been investigated. The inspection of 12 taxa representing the two major serpulid clades allowed for the identification of three ultrastructure types-irregularly oriented prismatic structure (IOP), spherulitic prismatic structure (SPHP), and simple prismatic structure (SP). Six of the studied species are single-layered and six species possess two distinct layers. Ultrastructural diversity corresponds to certain serpulid clades. The members of Filograninae have single-layered tube walls composed of possibly plesiomorphic, irregularly oriented prismatic structure (IOP). Two-layered tubes occur solely within the clade Serpulinae, where the external, denser layer is built of either the ordered spherulitic (SPHP) or simple prismatic microstructure (SP), and the internal layer is composed of irregularly oriented prismatic structure (IOP). Apart from phylogenetic signals provided by the tube ultrastructure, it can be used in analyzing paleoecological aspects of tube-dwelling polychaetes. Compared to the more primitive, irregularly oriented microstructures of Filograninae, the regularly oriented microstructures of Serpulinae need a higher level of biological control over biomineralization. The advent of the dense outer protective layer (DOL) in serpulids, as well as the general increase in ultrastructure diversity, was likely a result of the evolutionary importance of the tubes for serpulids. Such diversity of the tube ultrastructural fabrics allowed for maximizing functionality by utilizing a variety of morphogenetic programs. The biomineralization system of serpulids remains more complex compared to other tube-dwelling polychaetes. Physiologically more expensive tube formation allows for mechanical strengthening of the tube by building robust, strongly ornamented tubes and firm attachment to the substrate. Contrary to sabellids, which perform a fugitive strategy, an increased tube durability allows serpulids a competitive advantage over other encrusters.

Morphometric and taxonomic approach to describe Heterospio variabilis (Annelida, Longosomatidae), a new species with three size-dependent morphotypes, from the Gulf of California, Eastern Pacific.

The Longosomatidae, a poorly known polychaete family, includes only 23 recognized species; in this study, based on morphometric and taxonomic analyses, we describe a new species with three morphotypes: Heterospio variabilis from the Gulf of California, Mexico. The specimens examined exhibit large morphological variations but were clearly separated from close species due to a unique combination of morphological characters: chaetiger 9 as the first elongated chaetiger, four to eight branchial pairs; chaetae from chaetiger 10 forming rings in two rows, posterior row with thin and robust capillaries, anterior row with subuluncini, aristate spines, acicular spines and thick acicular spines. With the discriminant analysis, carried out on 11 morphometric characters, the presence of three morphological groups were recognized (Wilks' lambda= 0.093, p = 0.0001). However, the variables selected to discriminate the specimens (partial Wilks' lambda > 0.57) were correlated to their size: number of branchiae, body width, prostomium width, rate length CH9/CH1-CH8, length CH1-CH8 and length CH9 (r > 0.5). So, we concluded that they belong to a single species with three morphotypes: morpho A with eight branchial pairs, morpho B with 5-6-7 pairs and morpho C with 4 pairs. No correlations between the distribution of the distinct morphotypes along the eastern gulf shelf and the environmental conditions where they settle were detected.

Re-description of Parasphaerosyllisindica Monro, 1937 (Annelida, Syllidae), with the establishment of a new species from western Mexico.

Background: Parasphaerosyllis Monro, 1937 is a syllid genus, currently composed of four species: P.indica Monro, 1937 from the Arabian Sea, P.uschakovi (Chlebovitsch, 1959) from the Kurile Islands, P.ezoensis Imajima & Hartman, 1964 from Japan and P.malimalii Capa, San Martín & López, 2001 from the Pacific coast of Panama. The distribution of P.indica is circum-tropical to temperate waters, but the presence of species complexes has been suggested. In order to clarify the distribution of P.indica in many areas of the world, a re-description, based on examination of the type material, is required as a first step to a better understanding of its diagnostic features. New information: Parasphaerosyllisindica is re-described, based on holotype examination, a new species is established from the Gulf of California and Parasphaerosyllismalimalii is reported for the first time since its description in 2001. Parasphaerosyllisirregulata sp. nov. is distinguished from its congeners by the following features: 1) Palps are free at their base; 2) Two types of dorsal cirri are present: spherical to bulbous and moniliform cirri; 3) Both types of cirri are distributed irregularly. A spherical/bulbous and moniliform cirrus may appear together within the same segment (asymmetrical segment) or only a spherical/bulbous cirrus may appear in several consecutive segments (not alternating as occurs in congeners); 4) The spherical/bulbous cirri may have distal knobs with 1-3 terminal articles; and 5) Bidentate falcigers with short, sub-triangular blades with a proximal tooth slightly larger that the distal one. A taxonomic key to species of Parasphaerosyllis species is included.

Novel BRICHOS-Related Antimicrobial Peptides from the Marine Worm Heteromastus filiformis: Transcriptome Mining, Synthesis, Biological Activities, and Therapeutic Potential.

Marine polychaetes represent an extremely rich and underexplored source of novel families of antimicrobial peptides (AMPs). The rapid development of next generation sequencing technologies and modern bioinformatics approaches allows us to apply them for characterization of AMP-derived genes and the identification of encoded immune-related peptides with the aid of genome and transcriptome mining. Here, we describe a universal bioinformatic approach based on the conserved BRICHOS domain as a search query for the identification of novel structurally unique AMP families in annelids. In this paper, we report the discovery of 13 novel BRICHOS-related peptides, ranging from 18 to 91 amino acid residues in length, in the cosmopolitan marine worm Heteromastus filiformis with the assistance of transcriptome mining. Two characteristic peptides with a low homology in relation to known AMPs-the α-helical amphiphilic linear peptide, consisting of 28 amino acid residues and designated as HfBRI-28, and the 25-mer ß-hairpin peptide, specified as HfBRI-25 and having a unique structure stabilized by two disulfide bonds-were obtained and analyzed as potential antimicrobials. Interestingly, both peptides showed the ability to kill bacteria via membrane damage, but mechanisms of their action and spectra of their activity differed significantly. Being non-cytotoxic towards mammalian cells and stable to proteolysis in the blood serum, HfBRI-25 was selected for further in vivo studies in a lethal murine model of the Escherichia coli infection, where the peptide contributed to the 100% survival rate in animals. A high activity against uropathogenic strains of E. coli (UPEC) as well as a strong ability to kill bacteria within biofilms allow us to consider the novel peptide HfBRI-25 as a promising candidate for the clinical therapy of urinary tract infections (UTI) associated with UPEC.

Two new species of Parahesione (Annelida: Hesionidae) associated with ghost shrimps (Crustacea: Decapoda) and their phylogenetic relationships.

Two new species of Hesionidae, Parahesione pulvinata sp. nov. and Parahesione apiculata sp. nov. are described based on materials collected at tidal flats in Okinawa (Japan) from burrows of the ghost shrimps Neocallichirus jousseaumei and Glypturus armatus. The two new species are characterized by having eight enlarged cirri, dorsal cirrophores with dorsal foliose lobe and biramous parapodia, and by lacking median antenna. Parahesione apiculata sp. nov. has digitate lobes on the posterior margin of the dorsal foliose lobe (absent in P. pulvinata sp. nov.). The two new species were never found outside the ghost shrimp burrows, suggesting they are obligate symbionts. Phylogenetic analyses based on four concatenated genes suggest that the symbiotic lifestyle has evolved several times in Hesionidae.

Gilsonicaris from the Lower Devonian Hunsrück slate is a eunicidan annelid and not the oldest crown anostracan crustacean.

The Lower Devonian (Lower Emsian, -400 Myr) roof slates of the Hunsrück in southeastern Germany have delivered a highly diverse and exceptionally preserved marine fauna that provides a unique snapshot into the anatomy and ecology of a wide range of Palaeozoic animals. Several of the described taxa, however, remain enigmatic in their affinity, at least until new pyritized features hidden under the surface of the slate are revealed using X-ray radiography or micro-computed tomography (µCT). Here, we redescribe such an enigmatic fossil, the putative anostracan crustacean Gilsonicaris rhenanus Van Straelen, 1943. Using µCT scanning, we unveil unprecedented details of its anatomy, including a ventral oral opening and four pairs of recalcitrant jaw elements. These jaws are morphologically consistent with the scolecodonts of eunicidan polychaetes, which along with the gross anatomy of the body and head unambiguously identifies G. rhenanus as a polychaete rather than an arthropod. While this discovery firmly discards the Early Devonian record of crown anostracans in the fossil record, it adds a new record of eunicidan soft tissues, which are surprisingly rare considering the abundant microfossil record of scolecodonts.

Third-Generation Sequencing Reveals the Adaptive Role of the Epigenome in Three Deep-Sea Polychaetes.

The roles of DNA methylation in invertebrates are poorly characterized, and critical data are missing for the phylum Annelida. We fill this knowledge gap by conducting the first genome-wide survey of DNA methylation in the deep-sea polychaetes dominant in deep-sea vents and seeps: Paraescarpia echinospica, Ridgeia piscesae, and Paralvinella palmiformis. DNA methylation calls were inferred from Oxford Nanopore sequencing after assembling high-quality genomes of these animals. The genomes of these worms encode all the key enzymes of the DNA methylation metabolism and possess a mosaic methylome similar to that of other invertebrates. Transcriptomic data of these polychaetes support the hypotheses that gene body methylation strengthens the expression of housekeeping genes and that promoter methylation acts as a silencing mechanism but not the hypothesis that DNA methylation suppresses the activity of transposable elements. The conserved epigenetic profiles of genes responsible for maintaining homeostasis under extreme hydrostatic pressure suggest DNA methylation plays an important adaptive role in these worms.

Deep-sea benthic crustacean and annelid data from the Bering Sea.

Samples of Crustacea and Annelida (Polychaeta, Sipuncula, and Hirudinea) were collected in the Bering Sea and the northwestern Pacific Ocean during scientific cruise SO-249 BERING in 2016. Biological samples were collected from 32 locations by the team on-board RV Sonne using a chain bag dredge at depths ranging between 330-5,070 m, and preserved in 96% ethanol. Specimens were morphologically identified to the lowest taxonomic level possible using a Leica M60 stereomicroscope. The generated data here comprise taxonomic information as well as annotated bathymetric and biogeographic information from a total of 78 samples (26 Crustacea, 47 Polychaeta, 4 Sipuncula, and 1 Hirudinea). The dataset was prepared following Darwin Core Biodiversity standards for FAIR data sharing based on Ocean Biodiversity Information System (OBIS) and Global Biodiversity Facility (GBIF) guidelines. The standardised digitised data were then mobilised to both OBIS and GBIF under CC BY 4.0 licence to publicly share and adopt the data. As records of these important marine taxa from bathyal and abyssal depths are sparse, especially from the deep Bering Sea, the herein generated and digitised data aid in filling existing knowledge gaps on their diversity and distribution in that region. As part of the "Biogeography of the NW Pacific deep-sea fauna and their possible future invasions into the Arctic Ocean" (BENEFICIAL) project, this dataset thus not only increases our knowledge in re-assessing and uncovering the deep-sea diversity of these taxa, but also serves policy and management sectors by providing first-hand data for global report assessments.

Comparative analysis of the jaw apparatus of three marine annelids using scanning electron microscopy: Microstructure and elemental composition.

Polychaeta are highly diversified invertebrates that inhabit marine, brackish or freshwater environments. They have acquired a unique range of adaptative features for securing food. However, the jaw apparatus may reveal not only defence and predation mechanisms, but also its relation to environmental chemistry. The present work compared the structure and chemical profile of the jaws of different estuarine Polychaeta: Nephtys hombergii (Nephtyidae), Hediste diversicolor (Nereididae) and Glycera alba (Glyceridae) using Scanning Electron Microscopy (SEM) and Scanning Electron Microscopy with Energy Dispersive X-Ray (SEM-EDX). Analyses revealed that N. hombergii possesses a muscular jawless proboscis with terminal sensorial papillae for detecting prey, whereas the G. alba proboscis exhibits four delicately sharp jaws with perforations for venom delivery and H. diversicolor bears two blunt denticulated jaws to grasp a wide variety of food items. Melanin and metals like copper provide hardness to the slender jaws of Glycera, while, in the absence of heavier metallic elements, halogens contribute to H. diversicolor jaws robustness. The more specific chemistry of the jaws of glycerids is associated with its more refined venom injection, whereas Hediste is an opportunistic omnivore and Nepthys an agile forager. Altogether, the chemistry of jaws is an adaptive feature for feeding, locomotion and even resilience to complex and often adverse chemical profiles of estuaries.

A new species of the genus Lindaspio Blake & Maciolek, 1992 (Annelida, Spionidae) from a cold seep near Hainan Island, China.

A new species of the spionid genus Lindaspio Blake & Maciolek, 1992 was collected from a cold seep near the Hainan Island at a depth of 1758 m. Morphologically, Lindaspiopolybranchiatasp. nov. differs from the congeners in having a narrow, folded caruncle and more neuropodial branchiae (from chaetiger 20). The 18S, COI, and 16S sequences of the new species have been submitted to GenBank. It is the first record of the genus Lindaspio from Chinese waters. A key to all species of Lindaspio is given.

Investigating the diversity of bioluminescent marine worm Polycirrus (Annelida), with description of three new species from the Western Pacific.

Bioluminescence, a phenomenon observed widely in organisms ranging from bacteria to metazoans, has a significant impact on the behaviour and ecology of organisms. Among bioluminescent organisms, Polycirrus, which has unique emission wavelengths, has received attention, and advanced studies such as RNA-Seq have been conducted, but they are limited to a few cases. In addition, accurate species identification is difficult due to lack of taxonomic organization. In this study, we conducted comprehensive taxonomic survey of Japanese Polycirrus based on multiple specimens from different locations and described as three new species: Polycirrus onibi sp. nov., P. ikeguchii sp. nov. and P. aoandon sp. nov. The three species can be distinguished from the known species based on the following characters: (i) arrangement of mid-ventral groove, (ii) arrangement of notochaetigerous segments, (iii) type of neurochaetae uncini, and (iv) arrangement of nephridial papillae. By linking the bioluminescence phenomenon with taxonomic knowledge, we established a foundation for future bioluminescent research development. We also provide a brief phylogenetic tree based on cytochrome c oxidase subunit I (COI) sequences to discuss the evolution of bioluminescence and the direction of future research.

Small worms of Sthenelanellinae, Pholoinae and Pisioninae (Annelida, Sigalionidae) from the Tropical Northwestern Atlantic.

The family Sigalionidae is characterized, among other features, by including scale worms with large bodies. However, among sigalionids, the subfamilies Sthenelanellinae, Pholoinae, and Pisioninae stand out by their small representatives with few segments and fragile bodies. In the Tropical Northwestern Atlantic, which includes part of the Gulf of Mexico and the Caribbean Sea, these subfamilies have been rarely studied, with few species recorded, and questionable records. This contribution aims to improve the knowledge of sthenelanellins, pholoins, and pisionins in the region through a faunistic study based on material from two Mexican scientific collections: the Reference Collection of Laboratorio de Biodiversidad y Cambio Climático (BIOMARCCA) and the Reference Collection of Benthos (ECOSUR) of El Colegio de la Frontera Sur. Pisione wolfi is confirmed from the Tropical Northwestern Atlantic, and three new species are described: Sthenelanella pechi sp. nov., S. sarae sp. nov. and Taylorpholoe anabelae sp. nov. A key to all Sthenelanella and Taylorpholoe species is also included.

The cellular 3D printer of a marine bristle worm-chaetogenesis in Platynereis dumerilii (Audouin & Milne Edwards, 1834) (Annelida).

Annelid chaetae are extracellular chitinous structures that are formed in an extracellular epidermal invagination, the chaetal follicle. The basalmost cell of this follicle, the chaetoblast, serves like a 3D-printer as it dynamically shapes the chaeta. During chaetogenesis apical microvilli of the chaetoblast form the template for the chaeta, any structural details result from modulating the microvilli pattern. This study describes this process in detail in the model organism Platynereis dumerilii and clarifies some aspects of chaetogenesis in its close relative Nereis vexillosa, the first annelid in which the ultrastructure of chaetogenesis had been described. Nereid species possess compound chaetae characteristic for numerous subgroups of errant annelids. The distal most section of these chaetae is movable; a hinge connects this part of the chaeta to the shaft. Modulation of the microvilli and differences in their structure, diameter and number of microvilli, and their withdrawal and reappearance determine the shape of these compound chaetae. Chaetal structure and pattern also change during life history. While larvae possess a single type of chaeta (in addition to internal aciculae), juveniles and adults possess two types of chaetae that are replaced by large paddle-shaped chaetae in swimming epitokous stages. Chaetogenesis is a continuous process that lasts during the entire lifespan. The detailed developmental sequence of chaetae and their site of formation are very similar within species and species groups. We expect that similarity results from a conserved gene regulatory network making this an optimal system to test the phylogenetic affinity of taxa and the homology of their chaetae.

Transcriptomic profiling of immune-associated molecules in the coelomocytes of lugworm Arenicola marina (Linnaeus, 1758).

Organization and functioning of immune system remain unevenly studied in different taxa of lophotrochozoan animals. We analyzed transcriptomic data on coelomocytes of the lugworm Arenicola marina (Linnaeus, 1758; Annelida, Polychaeta) to gain insights into the molecular mechanisms involved in polychaete immunity. Coelomocytes are specialized motile cells populating coelomic fluid of annelids, responsible for cellular defense reactions and providing humoral immune factors. The transcriptome was enriched with immune-related transcripts by challenging the cells in vitro with lipopolysaccharides of Escherichia coli and Zymosan from Saccharomyces cerevisiae. Our analysis revealed a multifaceted and complex internal defense system of the lugworm. A. marina possesses orthologs of proto-complement-like factors: six thioester-containing proteins, a complement-like receptor, and a MASP-related serine protease (MReM2). A. marina coelomocytes employ pattern-recognition receptors to detect pathogens and regulate immune responses. Among them, there are 18 Toll-like receptors and various putative lectin-like proteins with evolutionary conserved and taxa-specific domains. C-type lectins and a novel family of Gal-binding and CUB domains containing receptors were the most abundant in the transcriptome. The array of pore-forming proteins in the coelomocytes was surprisingly reduced compared to that of other invertebrate species. We characterized a set of conserved proteins metabolizing reactive oxygen species and nitric oxide and expanded the arsenal of potential antimicrobial peptides. Phenoloxidase activity in immune cells of lugworm is mediated only by laccase enzyme. The described repertoire of immune-associated molecules provides valuable candidates for further functional and comparative research on the immunity of annelids.