Individual organisms can host multiple species of parasites (or symbionts), and one species of parasite can infect different host species, creating complex interactions among multiple hosts and parasites. When multiple parasite species coexist in a host, they may compete or use strategies, such as spatial niche partitioning, to reduce competition. Here, we present a host–symbiont system with two species of Selenidium (Apicomplexa, Gregarinida) and one species of astome ciliate co-infecting two different species of slime feather duster worms (Annelida, Sabellidae, Myxicola) living in neighbouring habitats. We examined the morphology of the endosymbionts with light and scanning electron microscopy (SEM) and inferred their phylogenetic interrelationships using small subunit (SSU) rDNA sequences. In the host ‘Myxicola sp. Quadra’, we found two distinct species of Selenidium; S. cf. mesnili exclusively inhabited the foregut, and S. elongatum n. sp. inhabited the mid to hindgut, reflecting spatial niche partitioning. Selenidium elongatum n. sp. was also present in the host M. aesthetica, which harboured the astome ciliate Pennarella elegantia n. gen. et sp. Selenidium cf. mesnili and P. elegantia n. gen. et sp. were absent in the other host species, indicating host specificity. This system offers an intriguing opportunity to explore diverse aspects of host–endosymbiont interactions and competition among endosymbionts.

Historic species records of the families Onuphidae and Eunicidae, from the Falkland Islands region, are reviewed and updated, new records added from intertidal and nearshore localities on and around the Falkland Islands, and a new species of Hyalinoecia described. Eight genera are reported from around the region including eight taxa of Onuphidae and two of Eunicidae, although most are only known from deep offshore waters. Kinbergonuphis dorsalis is re-described from type material, Kinbergonuphis sp. from the Falkland Islands is described and discrepancies between the two descriptions are examined. Hyalinoecia falklandica sp. nov. is described and the history of the genus and its misidentification in the region is discussed. The new species is distinguished from all other species in the genus through a combination of simple, unidentate falcigers on chaetigers 1 and 2, ventral cirri present to chaetigers 3 or 4, and branchiae present from chaetigers 26 or 27 to the end of the body. The historic record of Marphysa aenea from the Falkland Islands is also shown to be a misidentification and the actual, as-yet-undetermined species present is described. Problems surrounding a correct identification of Marphysa species, other species that do not quite fit current descriptions, and additional Eunicoidea taxa that might be expected to occur in the region are also discussed.

Despite an increase in the number of new lumbrinerid descriptions globally and over recent decades, no new lumbrinerid species have been described from southern Africa over the last 40 years. This suggests that species richness in the region is currently underestimated, and that continued research would result in the discovery of more undescribed species. Here, a new species is described from grab samples collected off southern Namibia at >100 m depth. Ninoe namibiensis sp. nov. is characterized by simple multidentate hooded hooks with a long hood that only appears after chaetiger 18, branchiae from chaetiger 1–2 to 38 (with up to 7 filaments) and maxillary apparatus: MX III with one prominent tooth followed by a knob and MX IV unidentate with finely denticulate cutting edge (11 small teeth). A key to the Ninoe from southern Africa is provided.

The annelid mitochondrial genomes (mitogenomes) have been well documented, and phylogenetic analyses based on the mitogenomes provide insightful implications for annelid evolution. However, the mitogenomes of some families remain unknown. Herein, we determined the complete mitogenome of the lugworm Abarenicola claparedi oceanica (15,524 bp), representing the first mitogenome from the family Arenicolidae. The gene order of this species is the same as the various lineages in Sedentaria. The maximum likelihood phylogenetic analyses were performed based on six different datasets, including 43 ingroups (oligochaetes, hirudineans, echiurans and closely related polychaetes) and two outgroups (Siboglinidae), namely, aligned and trimmed datasets consisting of the nucleotide sequences of protein-coding genes (PCGs) and rRNAs, and amino acid sequences of PCGs. Phylogenetic analyses based on the nucleotide sequences yielded trees with better support values than those based on the amino acid sequences. Arenicolidae is clustered with Maldanidae in all analyses. Analyses based on nucleotide sequences confirm the monophyly of Terebellidae, which was paraphyletic in recent mitogenomic phylogenetic studies. We also performed the phylogenetic analysis based on the RY-coding of the nucleotide sequences of PCGs only to yield phylogeny with generally low support values. Additional mitogenome sequences of related ingroup species would be needed to comprehensively understand the phylogenetic relationship, which was not present in this study.

Lacydonia Marion & Bobretsky, 1875 is the sole genus in the family Lacydoniidae Bergström, 1914. We herein describe the new species of Lacydonia shohoensis sp. nov. from 2042-m deep bottoms at Shoho Seamount of the Nishi-Shichito Ridge, the Northwest Pacific Ocean. It is most similar to L. anapaulae Rizzo et al., 2016 in having a depression on the median anterior region and lacking lateral lobes on the posterior margin of prostomium whereas it is distinguished by possessing pygidium dorsally pigmented with three reddish spots and non-pigmented pygidial lateral cirri equally elongated.

Bioluminescence is widespread in the marine environment. The bioluminescence of some species of the fireworm Odontosyllis (Annelida: Syllidae: Eusyllinae) has been well studied, although the presence or absence of bioluminescence in most species of this genus is yet to be revealed. The bioluminescent worms were observed after sunset around the new moon day in July and October 2020 and in July to October 2021 in Nagasaki Prefecture, Japan. Molecular phylogenetic analysis based on two mitochondrial and one nuclear gene sequence showed that the worms were closely related to Odontosyllis australiensis, but the partial 16S rRNA gene sequences differed by 2% between those of the Japanese and Australian material. Because only epitokes, i.e. morphologically modified sexually mature worms, were collected, further studies on morphological characters of atokes would be required in the future. We therefore tentatively refer to them as Odontosyllis cf. australiensis. Molecular phylogenetic analysis also showed that known bioluminescent Odontosyllis species belong to various lineages.

Metchnikovellids are a deep-branching group of microsporidia, parasites of gregarines inhabiting the alimentary tract of polychaetes and some other invertebrates. The diversity and phylogeny of these hyperparasites remain poorly studied. Modern descriptions and molecular data are still lacking for many species. The results of a light microscopy study and molecular data for Metchnikovella spiralis Sokolova et al., 2014, a hyperparasite of the eugregarine Polyrhabdina sp., isolated from the polychaete Pygospio elegans, were obtained. The original description of M. spiralis was based primarily on the analysis of stained preparations and transmission electron microscopy images. Here, the species description was complemented with the results of in vivo observations and phylogenetic analysis based on the SSU rRNA gene. It was shown that in this species, free sporogony precedes sac-bound sporogony, as it occurs in the life cycle of most other metchnikovellids. Spore sacs are entwined with spirally wound cords, and possess only one polar plug. Phylogenetic analyses did not group M. spiralis with M. incurvata, another metchnikovellid from the same gregarine species, but placed it as a sister branch to Amphiacantha. The paraphyletic nature of the genus Metchnikovella was discussed. The taxonomic summary for M. spiralis was emended.

The invasive sabellid Branchiomma bairdi (McIntosh, 1885) was collected in the Estany des Peix lagoon (Formentera Island, Balearic Sea, Western Mediterranean). This species is native to Bermuda (Western Atlantic Ocean), being widely distributed through the Caribbean Sea. However, it has been described as invasive worldwide, being also present in the Indo-Pacific region, the Eastern Atlantic Ocean and the Mediterranean Sea. We here provide the first record of the species for the Balearic Sea, together with a comparison to other morphologically similar alien species, Branchiomma boholense (Grube, 1878), which is supposed to have also been introduced in the Mediterranean basin. We also include comprehensive taxonomic descriptions of both taxa based on the type material, a dichotomous key for the Mediterranean species of Branchiomma and hypothesize the most likely way of introduction of B. bairdi in the Balearic region.

During several sampling periods in the Northern Adriatic Sea (Mediterranean) from 2003–2010, one species of Pista and one of Pistella were found. They were initially identified as Pista cf. lornensis (Pearson, 1969), but detailed examination revealed the presence of two new species, one belonging to the genus Pistella Hartmann-Schröder, 1996, P. rovignensis sp. nov. and one belonging to the genus Pista, P. adriatica sp. nov. These two species differ from each other in the structure of the branchiae, shape of lateral lobes and the structure of the anterior thoracic uncini. The two new species are compared with other species of Pistella and Pista currently known. The relationship of Pistella and Pista is confusing and a complete revision of the two genera is urgently needed together with other genera characterized by having long-handled thoracic uncini.

Among the polychaetes, the family Cirratulidae is one of the most challenging taxonomically because most of the currently used diagnostic characters change during ontogeny. It is therefore necessary to use a combination of characters to make an accurate identification. The present work is the first of a planned revision of the genus Cirratulus from coastal zones of Argentina. The examined material came from the two largest natural history museums of the country, and showed several morphotypes. Some specimens corresponded to described species, but others were undoubtedly undescribed. Among the previously known Cirratulidae, Cirratulus jucundus (Kinberg, 1866) and Cirratulus patagonicus (Kinberg, 1866), formerly distributed for Antarctica are now also present in coastal continental areas of Argentina. The new species Cirratulus mianzanii sp. nov. is described from subtidal areas off Peninsula Valdés, Patagonia. This species is distinguished from its congeners by the segmental origin of both tentacular filaments and first pair of branchiae in the junction between the peristomium and first chaetiger. Branchiae arise some distance from the notopodium but not at the dorsal midline of the body. Ventral groove present along the body, with a fine dark midline. Capillary chaetae are serrate seen under an optical microscope, and the segmental origin of neuro- and notopodial spines is different from other species of the genus.

This study recognizes six species of onuphid polychaetes associated with the Cymodocea nodosa meadows of the Canary Islands. Of these, three species of the genus Aponuphis, A. bilineata, A. brementi and A. ornata have been previously reported from the eastern North Atlantic, whilst A. willsiei was only known from the western Mediterranean Sea. We describe two new species: Onuphis erici sp. nov. and Kinbergonuphis sanmartini sp. nov., of which the latter represents the first discovery of the genus Kinbergonuphis in the eastern North Atlantic Ocean. Furthermore, we present brief notes on their ontogeny and ecology and remarks on the presence and microstructure of lateral organs observed in one of the new species.

Two new species of Cossura Webster & Benedict, 1887 were found in material collected during sampling from the terminal lobes of the Congo deep-sea fan. They were described using light and scanning electron microscopy. Cossura platypus sp. nov. has 15–17 thoracic chaetigers, a prostomium longer than it is wide, with a widely rounded anterior margin, an abruptly expanded posterior prostomial ring the same length as the peristomium, without a mid-ventral notch, a branchial filament attached to the midlength of chaetiger 3, and a pygidium with three anal cirri. Cossura platypus sp. nov. is similar to C. brunnea Fauchald, 1972 but differs in the shape of the prostomium, which is widely rounded anteriorly in C. platypus sp. nov. and is broadly triangular in C. brunnea; furthermore, C. platypus sp. nov.is uniformly pale, whereas C. brunnea has dark pigmentation. Cossura candida Hartman, 1955 differs from C. platypus sp. nov. in the conical shape of the prostomium and 24–35 thoracic chaetigers. Cossura flabelligera sp. nov. has 16–19 thoracic chaetigers, a conical prostomium, and a branchial filament arising from the posterior part of chaetiger 2; the entire body, including the chaetae, is covered by a thick mucous sheath similar to the tunic of flabelligerids. Cossura flabelligera sp. nov. resembles C. longocirrata Webster & Benedict, 1887 in the position of the branchial filament, the shape of the prostomium, and the number of thoracic chaetigers; it differs in having a thick mucous sheath. This character seems to be unique for the Cossuridae.

During quantitative surveys two new species of Cirratulidae from subtidal bottoms of the Atlantic coast of Argentina were collected. They belong to the genera Monticellina and Chaetozone. Monticellina morae sp. nov. is characterized by the presence of a long peristomium of smooth appearance, but triannulated at SEM, bearing the dorsal tentacles in the posterior part, with the first pair of branchiae posterolateral to the dorsal tentacles in chaetiger 1. The body is divided into three parts, an anterior region laterally expanded and with crowded segments, a middle region with beadlike chaetigers and an inflated and crowded posterior region. Chaetozone larae sp. nov. is characterized by an inflated dorsally and laterally expanded anterior crowded region. Peristomium biannulated laterally, with a dorsal crest on the first and second annuli that occupy the entire width, and then a narrow crest in the following annulus (interpreted as an achaetous segment). Capillaries are lanceolate, short and long, with acicular spines curved, forming partial cinctures. In addition, a comparative table of all described species of Monticellina is included and the distinctive characters are discussed.

In this research, the feeding guild composition of the polychaete community was described and analysed in the northern Yellow Sea, China. The polychaete feeding guilds also showed a distinct seasonal abundance fluctuation that was similar to that observed for macrofauna. Burrowers were the most predominant feeding guild, whereas filter feeders and herbivores only constituted a negligible proportion of the total abundance. A Spearman rank correlation was performed to analyse the relationships between the feeding guilds and the environmental parameters. Surface deposit feeders and carnivores appeared to be vulnerable to variations of environmental conditions. However, burrowers and filter feeders were not susceptible to the natural or anthropogenic disturbances, with grain size determined to be the only conclusive factor. These results revealed that the feeding guild of polychaetes showed significant spatial and temporal changes, and spatial heterogeneity was a stronger predictor than temporal variation. The feeding guilds of polychaetes can be applied alone to discern the environmental change, which would be faster and more cost-effective with best equilibrium between the precision of the results and a decrease in taxonomic effort. Furthermore, bimonthly sampling was proved to be unessential and reduction of the sampling frequency was necessary to save expense and effort.

Polychaete worms are known to thrive in extreme environmental conditions, however little is known about how polychaete species will respond to major climatic stressors, such as ocean acidification. Here, we examined the distribution of Fabriciidae (Annelida, Sabellida) species along a gradient of ocean acidification, caused by carbon dioxide (CO2) vent emissions in a shallow, coastal system off the island of Ischia (Tyrrhenian Sea, Italy). A total of 265 specimens of Fabriciidae, representing six species from five genera, were collected along the gradient. Most of the species were found across the entire CO2 gradient, suggesting polychaetes may have a high tolerance for ocean acidification in the future. Two of the species were new to science, and two of the genera were previously unrecorded in the Mediterranean. A full description of the new species Brifacia aragonensis sp. nov. and Parafabricia mazzellae sp. nov. is given, both of which were most abundant in the most acidified areas (pH 6.6–7.2). The geographical distribution and ecology of the new taxa, as well as of the other fabriciid species collected, is discussed. Taxonomic keys to identify the Fabriciidae species currently recorded in the Mediterranean Sea are also provided.

The present paper reports results of a transmission electron microscopy study of a new metchikovellid microsporidium. It was isolated from gregarines Polyrhabdina sp. inhabiting guts of polychaetes Pygospio elegans sampled at the White Sea silt littoral zone. Free sporogony (FS) occurred in the life cycle of the microsporidium alongside sac-bound sporogony (BS). Free spores resided in a parasitophorous vacuole and were of typical metchnikovellidean structure, uninucleate and oblong. They measured on sections 2·0–3·2×1·3–1·9 μm. The life cycle included pre-sporogonial stages represented by dikaryotic cells and 4-nucleate cells with coupled nuclei. A multinucleate sporogonial plasmodium of FS split in numerous (>10) sporoblasts. In BS segregation of sporoblasts occurred within thick-walled cysts by internal budding. Spore sacs of this microsporidium, measuring on average 11·6×4·7 μm, were limited by a thick electron-dense wall, externally ornamented with spirally wound cords of dense material. These oval spore sacs contained eight barrel-shaped spores, comparable in size and ultrastructure to FS spores. Ultrastructure of both types of spores and intracellular development of the new microsporidium and Metchnikovella spp. were similar, suggesting they belong to the same genus. In this paper we describe a new species Metchnikovella spiralis and discuss morphology of metchnikovellids in the context of putative evolutionary history of Microsporidia.

Several poorly known species of the family Syllidae (Annelida) have been revised and re-described. New diagnosis and taxonomic remarks are provided for Erinaceusyllis erinaceus, Opisthosyllis neglecta comb. nov., Parapionosyllis parapionosylliformis comb. nov., Prosphaerosyllis semiverrucosa comb. nov. and Amblyosyllis granosa.

Nothria and Anchinothria, two sister genera of onuphid worms were studied from eastern Australian waters. Nothria abyssia, with a wide distribution in Pacific and Antarctic waters is reported from slope depths south-east of Australia, while N. otsuchiensis, described from Japanese waters, was found along the eastern coast of Australia from shallow subtidal to deep slope areas. Anchinothria parvula sp. nov. is described, based on the presence of uni- and bidentate pseudocompound falcigers in the first three pairs of parapodia, absence of branchiae and appearance of limbate chaetae from chaetiger 2, subacicular hooks from chaetigers 6–8 and pectinate chaetae from chaetiger 14. The new species represents the first record of the genus in Australian waters. Ontogenetic variation of chaetal composition and morphology of parapodia was estimated in two Nothria species examined. Three size-related morphotypes with different chaetal composition and morphology of anterior falcigers were revealed in N. abyssia and N. otsuchiensis. The number of chaetigers with pseudocompound falcigers is the only character independent of size of the specimens in both examined species. Distribution of subacicular hooks and postchaetal lobes is shown to be size-dependent. The following characters are considered of diagnostic value only when scored in adults: presence/absence of simple falcigers in anterior parapodia; chaetiger of origin of limbate chaetae, pectinate chaetae, and subacicular hooks; and number of chaetigers with auricular prechaetal lobes and digitiform postchaetal lobes. The use of adult specimens with the final set of definitive morphological structures for diagnoses of Nothria species is suggested.

A one-year study of the reproductive biology of a population of Diopatra marocensis at the Villaviciosa estuary, northern Spain, was undertaken with emphasis on brooding behaviour, larval development and gametogenesis. Field observations together with a histological study of monthly collected individuals revealed that the population was iteroparous, reproducing annually during a short breeding season extending from March to June. The study demonstrated that all individuals of the population at Villaviciosa estuary were producing at the same time eggs and sperm providing for the first time evidence for the occurrence of simultaneous hermaphroditism in D. marocensis. Mature sperm was of the ent-aquasperm type and was stored in modified nephridial chambers in brooding individuals. In the latter, eggs in advanced cleavage phases were also observed inside the coelom suggesting that fertilization was internal. Eggs in late vitellogenesis presented micronucleoli in the periphery of the nucleus, a phenomenon reported previously for few polychaete species. Pure males and females were never found. It is here suggested that in D. marocensis population size may influence sex allocation and that under conditions of low population densities simultaneous hermaphrodites may be favoured. The study of the larval development confirmed the direct development reported previously for populations of Morocco and Portugal. Further results suggest consumption of nurse eggs and adelphophagy by developing larvae.

Sabellidae is the second most diverse polychaete family in the Adriatic Sea, after Syllidae. Herein we report the updated list of the Sabellidae and Fabriciidae families in the Adriatic Sea deriving both from literature data and presently conducted researches in the northern Adriatic. During these researches polychaetes were sampled from three offshore soft-bottom and three coastal rocky shore stations. Among the analysed material three Sabellidae species are new records for the Adriatic fauna, while four Sabellidae and one Fabriciidae species were recorded for the first time in the northern part. Two new species were found and described: Amphicorina rovignensis spec. nov. and Megalomma pseudogesae spec. nov. These findings increase the number of Sabellidae taxa recorded in the northern Adriatic from 29 to 38. Our results point out the lack of the recent taxonomic studies in the area, but might also indicate the widening of the species areal. Taking into consideration that studies on only six stations raised the number of known sabellid taxa from the area by 31%, our results support the need for the regular taxonomic updating of the polychaete fauna in the northern Adriatic.