The symbiotic relationship between the syllid polychaete, Haplosyllides floridana Augener, 1922, and the barrel-shaped sponge Xestospongia muta (Schmidt, 1870), originally documented solely in the Caribbean, is presumed to be species-specific. Recently, the host sponge has been found as a single species distributed across approximately 8000 km (from 26°N to 22°S). Herein we addressed new spatial and bathymetric population records of H. floridana to question the persistence of the association with X. muta, and whether, in the case of an association, the polychaete remains as a single species throughout this extensive distribution. Our findings change the restricted distribution of H. floridana, revealing a remarkable association with X. muta from Tropical north-western to south-western Atlantic including records in the Fernando de Noronha Archipelago and the Almirante Saldanha Seamount, at 270 m deep. The populations analysed in this study are morphologically consistent. The study underscores the need for further investigations into the connectivity of Xestospongia species, the presence of Haplosyllides in other ocean basins, and the shared evolutionary history between Xestospongia and Haplosyllides. The observed life cycle completion of H. floridana within the host sponge suggests a potential co-evolutionary relationship, offering insights into the intricate dynamics of symbiotic associations in marine ecosystems.

The family Syllidae, aside from representing the most species-rich family in Annelida, is characterized by a number of sexual and asexual reproductive strategies. With the exception of a few viviparous species, the subfamily Syllinae is characterized by schizogamous reproduction with pelagic larval stages and without parental care. Laboratory rearing of ripe specimens of Syllis rosea showed a different reproductive strategy, hitherto unknown in this subfamily. While male stolons rapidly degenerated after fertilization, female ones released large eggs in a gelatinous cluster attached to the middle-posterior chaetigers. The gel mass progressively compacted as a cocoon wrapped by the stolon body; 7 days after the deposition the larvae hatched out from the cocoon at the metatrochophore stage and the female stolon died after a few days. After hatching the larvae remained associated to the stolon, and young specimens of S. rosea survived up to the 3-chaetiger stage. Until now cocoon brooding by the stolon has only been reported for some Autolytinae. The production of gelatinous egg masses and parental care are known in basally branching clades within Syllidae, suggesting that this reproduction mode might retain some ancestral features. The scarce knowledge about reproductive cycles in Syllinae does not allow clarification whether this strategy is unique for S. rosea, or it occurs in other congeneric species. Further research is needed to understand possible relationships between sexual reproduction and phylogeny, stolon morphology and its adaptation to parental care, and ultimately between reproductive strategies and ecology.

Among the polychaetes, the Syllidae comprise numerous species whose study over many years has benefitted from valuable revisions and descriptions of new species in Europe. This abundant literature proves very useful for revisiting the taxonomy and distribution of the Syllidae in the English Channel (EC), mainly as regards existing studies on coarse sediment communities in the eastern part of the Channel. This habitat is one of the most widespread in the EC and is known as favourable for the small polychaete fauna including Syllidae. A 2-year survey (winter and summer sampling periods) covering 19 stations, associated with the Branchiostoma lanceolatum coarse sand community offshore Dieppe-Le Tréport, led to the identification of 6537 individuals from 29 taxa including 27 species. Six fine sand stations were also sampled in which only 12 individuals were collected. Among these species, seven are new for the EC polychaete fauna and six others are observed for the first time in the eastern part of the EC. All the new species for the EC are warm temperate species previously only known south of the Bay of Biscay. The Syllidae list given in Dauvin et al. (2003) has been re-analysed and amended with our list and that of the Chausey Archipelago study (Olivier et al., 2012). To date, 91 Syllidae species have been recorded in the EC and are established mostly in coarse sediments.

A collection of more than 1500 specimens of Syllidae (Annelida, Polychaeta) from sub-Arctic and Boreal regions along the coast of Norway has been studied. We report 32 species, two of them new to science, belonging to the genera Sphaerosyllis Claparède, 1863 and Syllis Savigny in Lamarck, 1818, and 16 new to Norway. Sphaerosyllis tarquei sp. nov. is characterized by having long blades on its compound chaetae and the presence of glands inside the dorsal cirri. Syllis kas sp. nov. was reported, figured and described by Uschakov (1955) as Syllis oerstedi (Malmgren, 1867); however, revision of the types of Chaetosyllis oerstedi reveals that they are sexual stolons of Syllis cornuta Rathke, 1843; this species is characterized by the shape of the spiniger-like chaetae, which are relatively short, distally blunt, with a thin distal spine, and unidentate falcigers some of which have similar spines. We also include descriptions and figures of Parexogone longicirris (Webster & Benedict, 1887) and Syllides longocirrata Ørsted, 1845. Finally we include a key of all reported species in the area.

We present some additional information to Acritagasyllis longichaetosus Lucas, San Martín & Sikorski 2010, the only representative of an enigmatic genus of Syllidae from subpolar areas off Norway, based on two newly found specimens and one male and another female stolons. The armature of the pharynx was incorrectly interpreted in the original description, having in fact a trepan instead of a single pharyngeal tooth as originally described. The reproductive stolons are herein described for the first time. With this new information we performed a maximum parsimony phylogenetic analysis that placed this genus within the subfamily Autolytinae, despite its peculiar morphological characteristics. For this reason, we propose a new diagnosis to the genus Acritagasyllis as well as a modified diagnosis of the subfamily Autolytinae.

A new species, Myrianida paredesi, and new records of Myrianida pentadentata (Imajima, 1966), and Procerarea micropedata (Hartmann-Schöder, 1962), are described from the central coast of Peru. Myrianida paredesi is characterized by the presence of an additional mid-dorsal, digitiform structure, quite similar to a dorsal cirrus (unique among all syllids), a subtriangular prostomium with a posterior region extended to middle peristomium, pharynx with a single elongated sinuation and a trepan with 14–15 equal teeth in a single ring. Descriptions of male and female stolons are also provided. The phylogenetic position of the new species within Myrianida is determined using molecular data, suggesting it is closely related to the species that share the presence of one ciliary troch per segment (except M. pachycera; unknown in M. australiensis), moderate number of equal teeth on trepan (except M. pachycera), and the mode of reproduction.

Descriptions of the collected specimens of Myrianida pentadentata (Imajima, 1966), and Proceraea micropedata (Hartmann-Schröder, 1962), both new reports to Peru, are also included.

This paper deals with a new species of Myrianida collected from Rhodes Island at 1 m depth among algae in April 2014. This new species is mainly characterized by having flattened dorsal cirri; high number of falcigers in parapodia; a trepan with 16 unequal teeth; and a brownish patch (in fixed specimen) on dorsum of each segment. Individuals are at the stage of reproduction, with developing stolons.

We have found a new species of Autolytinae (Annelida, Syllidae), Proceraea janetae, feeding on the scleractinian coral Montastrea cavernosa in coral reefs surrounding the Grand Cayman Island (Cayman Islands, British West Indies). The new species has a characteristic combination of transversal brown markings on the segmental margins and diffuse white mid-dorsal transverse bars, together with a diffuse white mid-dorsal longitudinal band. Antennae are brown, tentacular cirri are pale, the first dorsal cirri are white with pale base; the second dorsal cirri are pale, and the remaining dorsal cirri are alternately long, bright yellow-orange with brownish tips and short, entirely brown. The trepan has 18 tricuspid teeth in one ring. There are 9 teeth with all cuspids equally long and 9 with a longer median cuspid, arranged in an alternating pattern. We describe and illustrate the feeding behaviour of the new species, which appears to be closer to parasitism rather than to specialized predation. Proceraea janetae sp. nov. is the second polychaete, and the first syllid, known to feed on scleractinian corals.

The genus Megasyllis is herein reorganized excluding the size from the diagnosis, since it is not a characteristic of all the species of the genus. We provide here a taxonomic account of all known species and a key to species identification. Seven species are new combinations, and re-descriptions of the four latter are included: Megasyllis nipponica (Imajima, 1966) and M. multiannulata (Aguado, San Martín & Nishi, 2008) from Japan; Megasyllis procera (Hartman, 1965) from the Atlantic; Megasyllis pseudoheterosetosa (Böggemann & Westheide, 2004) from the Indian Ocean. Megasyllis glandulosa (Augener, 1913), from Australia; Megasyllis marquesensis (Monro, 1939) from the Marquesas Islands, Micronesia and Megasyllis subantennata (Hartmann-Schröder, 1984) from Australia. Four new species from the Pacific Ocean namely Megasyllis tigrina sp. nov., Megasyllis mariandreworum sp. nov. (both from Australia), Megasyllis chrissyae sp. nov. (from the Philippines) and Megasyllis eduardoi sp. nov. (from New Zealand) are described.

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.

Nuchalosyllis Rullier & Amoureux, 1979 is an extremely rare syllid genus, only known from the holotype of its type species, N. lamellicornis Rullier & Amoureux, 1979. The genus is characterized by the dorso-ventrally flattened body and the unique comb-like nuchal epaulettes. We herein describe the second specimen of this genus ever found, which is also a new species, Nuchalosyllis maiteae sp. nov.; it differs from N. lamellicornis in having different morphology of nuchal epaulettes, spinose papillae on cirrophores of dorsal cirri throughout and sub-bidentate posterior body falcigers.

A collection of over one hundred specimens of Syllidae (Polychaeta) from the Arctic and sub-Arctic regions has been examined and identified. The specimens were obtained from 26 stations in the Barents Sea, some Norwegian fiords and localities of the northern North Sea. A total of 21 species were identified; three species (Myrianida langerhansi (Gidholm, 1967), Syllides longocirrata Örsted, 1845 and Sphaerosyllis taylori Perkins, 1981) are new reports for the Arctic Ocean; 2 species are new to science, Streptodonta exsulis sp. nov. and Trypanosyllis troll sp. nov. Streptodonta exsulis sp. nov. have 4 thick, distally strongly knobbed aciculae on each anterior parapodia, shifting to a single, slender acicula on posterior parapodia; falcigers and pseudospinigers distally bidentate; and pharyngeal tooth located centrally and relatively close to anterior rim of pharynx. Trypanosyllis troll sp. nov. have 2, occasionally 3 straight aciculae in parapodia protruding out from parapodial lobes; falcigers bidentate; and body surface densely covered by numerous, small papillae. Based on the description of these 2 new species, some modifications are proposed in the diagnoses of the genera Streptodonta San Martín & Hutchings, 2006 and Trypanosyllis Claparède, 1864.

Sphaerosyllis sandrae sp. nov. (Annelida: Polychaeta: Syllidae) is described as a new species from shallow sandy bottoms off Guanahacabibes Biosphere Reserve (Cuba). Thirteen more syllid species belonging to nine genera (Exogone, Myrianida, Opisthosyllis, Parexogone, Plakosyllis, Salvatoria, Syllides, Sphaerosyllis and Syllis) have been found from algal and sand bottom samples from the same area; four species that belong to the genera Myrianida, Syllides and Parexogone are only identified at the generic level.

In studies carried out off the north-eastern and south-eastern coast of Brazil, three species of Prosphaerosyllis were found: P. isabellae, which was already recorded for Brazilian waters; P. xarifae, a newly recorded species for the area; and P. brachycephala sp. nov., a new to science species. Prosphaerosyllis brachycephala sp. nov., is characterized by having swollen anterior part of the body, prostomium retractable within the peristomium and anterior segments, short antennae, short peristomial and dorsal cirri, and falcigers with short, unidentate blades throughout. All these species are herein described and compared to the most similar congeners.