Nereididae is among the most familiar of marine annelid families, common and well-studied in most marine environments but paradoxically no recent key or identification guide exists to the world’s genera. Here updated generic descriptions, a list of characters, a linear key to genera, and minimal diagnoses that distinguish each genus from all others in the family are provided. This information is generated from a Delta database of 186 morphological characters and a link is provided to downloadable software allowing the same data to be interrogated using the open-source Delta program Intkey – a nonlinear multiple entry point computerised interactive key. For each genus the recent literature is also summarised, comments on taxonomic status provided, and published keys to species cited. Nexus format matrices are provided for all 45 genera and 158 Nereididae species, representing all genera, scored for 146 multistate characters from the same character list to facilitate future phylogenetic studies.

The large polychaete genus Polycirrus (Terebellida: Polycirridae) is revised based on a morphological study of the type material of most of the approximately 74 nominal species; 59 species, including a new species, P. coibensis n.sp., from the Pacific coast of Panama, are accepted as valid. All characters are reanalysed and discussed in the light of recent phylogenetic studies of terebellomorph taxa, and a standard set of characters is developed. Most of the 59 accepted species are redescribed and illustrations provided in order to facilitate future identifications of members of the genus. We designate a neotype for the type species of the genus, Polycirrus medusa Grube, 1850, and a lectotype for P. elisabethae McIntosh, 1915, in order to stabilize the concept and type locality of these taxa. A replacement name is proposed for Polycirrus kerguelensis McIntosh, 1885, which is a secondary junior homonym of Ereutho kergulenesis McIntosh, 1885, viz. Polycirrus macintoshi new name. Two species are newly synonymised: Polycirrus insignis Gravier, 1907 (senior synonym P. antarcticus (Willey, 1902)) and Polycirrus habitats Carrerette & Nogueira, 2013 (senior synonym, P. clavatus (Kinberg, 1867)). In addition, several previously synonymised species are reinstated. All available names in the genus are tabulated with type species and type localities and their taxonomic status is assessed. A key to all valid species is provided.

We investigate the presence of cryptic species among three highly variable nereidid polychaetes commonly found in Australian coral reefs - Nereis denhamensis Augener, 1913, Perinereis suluana (Horst, 1924) and Pseudonereis anomala Gravier, 1901 - based on morphological and molecular data (mitochondrial cytochrome c oxidase subunit I, COI, and nuclear histone H3). DNA extracted and sequenced from 70 specimens from northern Australia and the Philippines indicated the existence of eight species: three matched the types of existing species; four are newly described (Nereis heronensis, sp. nov., N. lizardensis, sp. nov., Perinereis pictilis, sp. nov. and Pseudonereis anomalopsis, sp. nov.) from NE Australia, and one species is described but not named due to lack of material. Nereis denhamensis, N. heronensis, sp. nov. and N. lizardensis, sp. nov. are distinguished from each other by proboscidial paragnath number and morphology of the metamorphosed female. Perinereis suluana can only be separated from P. pictilis, sp. nov. by colour pattern, while Pseudonereis anomala, P. anomalopsis, sp. nov. and P. sp. differ in colour pattern and the number and arrangement of paragnaths. Nereis (Lycoris) tydemani from Maluku, Indonesia, is newly synonymised with P. anomala. Divergence times estimated using COI indicated that speciation in all three groups occurred in the mid Miocene (20–17 ± 7 mya), which corresponds to a period of restricted east–west dispersal as Australia collided with the Indo-Malay archipelago, followed by range expansion opportunities in NE Australia as a result of flourishing coral reefs responding to warming seas and rising sea levels.

Phylum Annelida are ubiquitous metazoans found in almost every terrestrial and aquatic habitat on Earth. Historically, taxonomic studies on the phylum have been focused largely on its majorgroups, polychaetes, oligochaetes and leeches, so that while family-level keys for each group are available, no single-source identification guide exists to the world’s annelid families. Here, the first illustrated linear key to annelid families is provided and family-level descriptions and diagnoses that distinguish individuals of each family from those of other families in the phylum are updated. This information is generated from an annelid DELTA database of 334 characters and 166 mostly family-level taxa. A link is provided to downloadable software (ANNiKEY Interactive) allowing the same data to be interrogated using the open-source DELTA program Intkey, which enables both interactive identification and taxonomic query functionality. For each family-level taxon, a diagnosis, full description, links to taxonomic data at the World Register of Marine Species, illustrations of diagnostic features, and a summary of the recent literature, including a list of published keys to genera and species are provided.

We review the variety of morphological, physiological and behavioral modifications that annelids have acquired to cope with environments either unsuitable for, or on the limits of, survival for most animals. We focus on polychaetes (excluding sipunculans and echiurans) and clitellates (oligochaetes and leeches) and source information mostly from the primary literature. We identified many modifications common to both polychaetes and clitellates, and others that are specific to one or the other group. For example, certain land-adapted polychaetes show reduction in nuchal organs, epidermal ciliation and receptor cells, and other coastal polychaetes use adhesive glands and glue-reinforced tubes to maintain position in surf zones, while oligochaetes, with their simple body plans, appear to be ‘pre-adapted’ to life underground. Modifications common to both groups include the ability to construct protective cocoons, make cryoprotective substances such as antifreeze and heat shock proteins, develop gills, transform their bodies into a home for symbiotic chemoautotrophic bacteria, metabolize contaminants, and display avoidance behaviors. Convergent evolution in both directions has enabled annelids to transition from salt water to freshwater, sea to land via beaches, freshwater to soil, and surface water to subterranean water. A superficially simple worm-like body and a mostly benthic/burrowing lifestyle has facilitated radiation into every conceivable environment, making annelids among the most common and diverse animal groups on the planet.