The purpose of our contribution is to propose a robust and practical order-level classification of the families of Oligochaeta, that is, non-leech Clitellata. The order level is mandatory in Linnaean rank-based classification and is also required in many internet-based biodiversity databases. However, it has received little attention in oligochaete systematics, and the few available order-level classifications of Oligochaeta no longer represent phylogenetic relationships adequately. Our proposal is based on corroborated molecular phylogenetic evidence and takes as benchmarks class level for Clitellata, subclass level for Oligochaeta and Hirudinea, and order level for Crassiclitellata, the monophylum that includes most of the earthworm taxa. As a result, eleven orders are proposed: Alluroidida Timm & Martin, 2015; Capilloventrida Timm, n. ordo; Crassiclitellata Jamieson, 1988; Enchytraeida Kasprzak, 1984; Haplotaxida Brinkhurst & Jamieson, 1971; Lumbriculida Brinkhurst & Jamieson, 1971; Moniligastrida Brinkhurst & Jamieson, 1971; Narapida Timm, n. ordo; Parvidrilida Timm, n. ordo; Randiellida Jamieson, 1988; Tubificida Jamieson, 1978. This order-level classification is robust and easily adaptable to future insights into phylogenetic relationships.

Two common sigalionid species from the Northeast Atlantic, Sthenelais boa (Johnston, 1833) and S. limicola (Ehlers, 1864), have never been revised in detail. Although their validity has never been questioned, a number of taxonomic problems related to Sthenelais Kinberg, 1856 and the later established Fimbriosthenelais Pettibone, 1971 remain unresolved. The validity of F. minor (Pruvot & Racovitza, 1895) has been repeatedly discussed, but no agreement reached. Also the validity of Fimbriosthenelais has been at stake, affecting the generic assignment of Fimbriosthenelais zetlandica (McIntosh, 1876), another species present in the area. Among the investigated species of Sthenelais, some where thought to be synonyms of Eusthenelais hibernica McIntosh, 1876, which led us to also include Eusthenelais McIntosh, 1876. We also re-examined Eusthenelais abyssicola McIntosh, 1879, the only other species attributed to the genus, and confirm that it is indeterminable. In total, we investigated 37 nominal taxa reported from the Northeast Atlantic and as a result we consider only five species to be valid: Sthenelais boa, S. limicola, Fimbriosthenelais zetlandica, F. longipinnis (Grube, 1869) and Eusthenelais hibernica. These genera and species are described and discussed herein and an updated identification key to all Northeast Atlantic species of Sigalioninae Gonzalez et al., 2018 is given.

The tubificid worm Branchiura sowerbyi Beddard, 1892 is one of the most successful invasive freshwater oligochaetes worldwide. In the Maghreb region, this species was known only from two localities in Morocco and from one locality in Libya. This paper presents novel information about the current distribution and presence of this invasive species in Morocco, and provides guidance on further areas of research regarding invasive oligochaetes in North African.

In October 2023 and April 2024, ecological monitoring was undertaken in brackish waters of the North Sea Canal, a major shipping route linking the Port of Amsterdam to the North Sea. Amongst the macrobenthic samples, four endemic and three non-native mysid species were recorded, two of the latter for the first time in this locality. This concerns the now globally distributed Deltamysis holmquistae, and the Mediterranean Diamysis lagunaris. Based on the spatial context, these specimens are highly likely to have been introduced through human-mediated transport, with shipping providing the most obvious entry path into the North Sea Canal. Both species were found in fully artificial and in restored near-natural habitats, highlighting their versatility as invasive species. Pictorial descriptions and complete keys for both species are included in order to facilitate future assessments of potential range expansions.

Two trumpet worms (Pectinariidae), which do not resemble the two known species: Lagis koreni Malmgren, 1866 and Amphictene auricoma (Müller, 1776), have recently been collected in the Dutch North Sea (Oyster Grounds). Their characteristics match those of Pectinaria belgica (Pallas, 1766), a species with a northern distribution. This paper summarizes the current state of knowledge of Dutch Pectinariidae and describes in detail the historical records of trumpet worms recorded in the Netherlands, along with the confusion around the species epithet belgica. Pectinaria belgica is reported here for the first time from the Dutch North Sea.

Specimens belonging to a new species in the genus Protocirrineris have been found during surveys along the southwestern coasts of the Netherlands. Protocirrineris stormae sp. nov, as described herein, were collected from the muddy bottom sediments in enclosed marine waterbodies and estuaries. This species is characterized by branchiae commencing from the first chaetiger and tentacular filaments arranged in two groups arising from chaetigers 2–3 that present one/two pairs of lobes, each bearing a single dorsal tentacle. From the current records of this species, it seems that P. stormae has an affinity for enclosed marine environments or parts of estuaries with minimal variability in salinity, higher transparency, and normal temperature regime. The discovery and study of this species contributes to a comprehensive characterization of benthic fauna in locations subjected to particular environmental stresses. Considering the extensive dataset available from the Southwestern Netherlands due to many years of sampling, it is plausible that P. stormae is a pseudo-indigenous species as there seem to be no records of similar or unknown Protocirrineris before 2013.