This paper reports on the genus Cobbionema Filipjev, 1922 in Sweden with the description of four species and a revision of the genus. Cobbionema acrocerca Filipjev, 1922 is relatively small in size, with a tail that has a conical proximal and a digitate distal section. Cobbionema cylindrolaimoides Schuurmans Stekhoven, 1950 is similar to C. acrocerca in most characters except having a larger body size and heavily cuticularized mandibles. Cobbionema brevispicula sp. nov. is characterised by short spicules and a conoid tail. Cobbionema acuminata sp. nov. is characterised by a long two-part spicule, a conical tail and three (one mid dorsal and two ventrosublateral) sharply pointed tines in the anterior chamber of the stoma that are located more anterior than in all the other species. We also present a molecular phylogeny of the family based on the nearly full-length 18S and the D2-D3 expansion segment of the 28S rRNA genes. Maximum Likelihood and Bayesian trees inferred from both genes strongly support a clade that included Cobbionema, Demonema Cobb, 1894 and Halichoanolaimus de Man, 1888 and another clade with Gammanema Cobb, 1920 and Latronema Wieser, 1954 nested together. None of the trees supported the monophyly of the subfamilies Choniolaiminae and Selachinematinae.

A new nematode species, Latronema dyngi sp. nov., is described from Skagerrak off the west coast of Sweden with the type locality near Dyngö island. Latronema dyngi sp. nov. is characterized by multispiral amphideal fovea with circular outline, 0.2– 0.3 corresponding body diameters wide in males and 0.1–0.2 corresponding body diameters wide in females, 12 cuticular longitudinal ridges and 18–27 precloacal supplements in males. Latronema dyngi sp. nov. most closely resembles L. orcinum in terms of body length; demanian ratios a, b, c and c′; number of amphid turns in males; and the ratio of spicule length to cloacal body diameter. The two species can be differentiated by the number longitudinal ridges on the cuticle (12 for Latronema dyngi sp. nov. vs 20–22 for L. orcinum) and spicule length (65–78 μm for L. dyngi vs 60 for L. orcinum) and shape (weakly arcuate for L. dyngi sp. nov. vs strongly arcuate for L. orcinum). We also performed a maximum likelihood phylogenetic analysis on over 250 nematodes of the subclass Chromadoria based on their nearly full-length 18S rDNA sequences. In agreement with previous studies, our analysis supported Selachinematidae as a monophyletic group and placed Richtersia Steiner, 1916 within Desmodoridae Filipjev, 1922 or just outside of the main Desmodorida clade with the latter placement not well supported.

Phylogenetic relationships within many lineages of the phylum Nematoda remainunresolved, despite numerous morphology-based and molecular analyses. Weperformed several phylogenomic analyses using 286 published genomes andtranscriptomes and 19 new transcriptomes by focusing on Trichinellida, Spirurina,Rhabditina, and Tylenchina separately, and by analyzing a selection of species fromthe whole phylum Nematoda. The phylogeny of Trichinellida supported the divisionof Trichinella into encapsulated and non-encapsulated species and placed them assister to Trichuris. The Spirurina subtree supported the clades formed by species fromAscaridomorpha and Spiruromorpha respectively, but did not support Dracunculoidea.The analysis of Tylenchina supported a clade that included all sampled species fromTylenchomorpha and placed it as sister to clades that included sampled speciesfrom Cephalobomorpha and Panagrolaimomorpha, supporting the hypothesis thatpostulates the single origin of the stomatostylet. The Rhabditina subtree placed a cladecomposed of all sampled species from Diplogastridae as sister to a lineage consistingof paraphyletic Rhabditidae, a single representative of Heterorhabditidae and a cladecomposed of sampled species belonging to Strongylida. It also strongly supportedall suborders within Strongylida. In the phylum-wide analysis, a clade composedof all sampled species belonging to Enoplia were consistently placed as sister toDorylaimia + Chromadoria. The topology of the Nematoda backbone was consistentwith previous studies, including polyphyletic placement of sampled representatives ofMonhysterida and Araeolaimida.

Animals and microorganisms often establish close ecological relationships. However, much of our knowledge about animal microbiomes comes from two deeply studied groups: vertebrates and arthropods. To understand interactions on a broader scale of diversity, we characterized the bacterial microbiomes of close to 1,000 microscopic marine invertebrates from 21 phyla, spanning most of the remaining tree of metazoans. Samples were collected from five temperate and tropical locations covering three marine habitats (sediment, water column and intertidal macroalgae) and bacterial microbiomes were characterized using 16S ribosomal RNA gene sequencing. Our data show that, despite their size, these animals harbour bacterial communities that differ from those in the surrounding environment. Distantly related but coexisting invertebrates tend to share many of the same bacteria, suggesting that guilds of microorganisms preferentially associated with animals, but not tied to any specific host lineage, are the main drivers of the ecological relationship. Host identity is a minor factor shaping these microbiomes, which do not show the same correlation with host phylogeny, or ‘phylosymbiosis’, observed in many large animals. Hence, the current debate on the varying strength of phylosymbiosis within selected lineages should be reframed to account for the possibility that such a pattern might be the exception rather than the rule.

A new species of Paracrobeles, P. kelsodunensis sp. nov. is described from the Kelso Dunes area, Mojave National Preserve, southern California. Paracrobeles kelsodunensis sp. nov. is particularly characterised by a body length of 469–626 μm in females and 463–569 μm in males; lateral field with four incisures, extending almost to tail terminus; three pairs of asymmetrical lips, separated by U-shaped primary axils with two long guarding processes, each lip usually with four tines along its margin; three long labial probolae, deeply bifurcated, with slender prongs without tines; metastegostom with a strong anteriorly directed dorsal tooth; pharyngeal corpus anteriorly spindle-shaped, posteriorly elongate bulbous with dilated lumen; spermatheca 24–87 μm long; postvulval uterine sac 60–133 μm long; vulva in a sunken area; spicules 33–38 μm long; and male tail with a 5–8 μm long mucro. The generic diagnosis is emended on the basis of recently described species and a key to the species of Paracrobeles is provided.

Populations of Stegelleta are described from California, New Zealand and Senegal. An amphimictic population from California is identified as belonging to S. incisa and compared with type specimens from Utah and an amphimictic population from Italy. One population from New Zealand is close to S. incisa but considered to represent a new species, Stegelleta laterocornuta sp. nov. It is particularly characterised by a 379–512 μm long body in females and 365–476 μm in males; cuticle divided into 16 rows of blocks at midbody (excluding lateral field); lateral field with four incisures; three pairs of asymmetrical lips, U-shaped primary axils without guarding processes, each lip asymmetrically rectangular with a smooth margin, only lateral lips have slender acute tines; three labial probolae, bifurcated at half of their length; vulva without flap; spermatheca 17–31 μm long; postuterine sac 7–24 μm long; spicules 21.5–23.5 μm long. Other specimens from New Zealand are identified as belonging to S. tuarua. A parthenogenetic population from Senegal is identified as belonging to S. ophioglossa and compared with type specimens from Mongolia and records of several other populations of S. ophioglossa. The generic diagnosis is emended and a key to the species of Stegelleta is provided.

Derogenes varicus (Müller, 1784) is widely reported as a trematode with exceptionally low host specificity and a wide, bipolar distribution. However, several recent studies have suggested that D. varicus represents a species complex and based on molecular evidence, four genetic lineages (labeled as “DV1–4”) have been designated within the D. varicus species complex. This possibility requires improved (ideally molecular) characterisation of specimens from the type-host (Salmo salar) and type-locality (off Denmark). During examination of trematode parasites of fish from Scandinavian and Arctic waters (Sweden and Norway), we found specimens of D. varicus in the stomach of Merlangius merlangus off the coast of Sweden, and in Gadus morhua off the coast of Sweden and Norway; we compared them to D. varicus from the type-host, the Atlantic salmon Salmo salar from Norway, to verify their conspecificity. Newly generated sequences (28S rDNA, ITS2 and cox1) of Scandinavian and Arctic specimens consistent with D. varicus all formed a single clade, DV1. 28S sequences of D. varicus from S. salar from Norway, i.e., close to the Danish type locality, clustered within the DV1 clade along with sequences of D. varicus from various hosts including Limanda limanda, G. morhua and Myoxocephalus scorpius from the White Sea and the Barents Sea (Russia), without any host-related structuring. We thus consider that the lineage DV1 represents D. varicus sensu stricto. Additionally, specimens from M. merlangus had a similar morphology and anatomy to those of D. varicus from L. limanda, G. morhua and M. scorpius from T. Odhner’s collection, supporting the presence of a single species in the DV1 lineage designated herein as D. varicus sensu stricto. We redescribe D. varicus sensu stricto, add new morphological characters and provide morphometric data. We infer that D. varicus types DV2–4 all relate to separate species. We also revise type-specimens of Derogenes minor Looss, 1901 from the A. Looss collection in the Swedish Museum of Natural History and provide redescriptions of it and of the type-species of the genus, Derogenes ruber Lühe, 1900. In light of their morphological distinctiveness relative to D. varicus sensu stricto, we reinstate D. parvus Szidat, 1950 and D. fuhrmanni Mola, 1912.

Ocean deoxygenation driven by global warming and eutrophication is a primary concern for marine life. Resistant animals may be present in dead zone sediments, however there is lack of information on their diversity and metabolism. Here we combined geochemistry, microscopy, and RNA-seq for estimating taxonomy and functionality of micrometazoans along an oxygen gradient in the largest dead zone in the world. Nematodes are metabolically active at oxygen concentrations below 1.8μmolL−1, and their diversity and community structure are different between low oxygen areas. This is likely due to toxic hydrogen sulfide and its potential to be oxidized by oxygen or nitrate. Zooplankton resting stages dominate the metazoan community, and these populations possibly use cytochrome c oxidase as an oxygen sensor to exit dormancy. Our study sheds light on mechanisms of animal adaptation to extreme environments. These biological resources can be essential for recolonization of dead zones when oxygen conditions improve.

We present an updated list of terrestrial and freshwater nematodes from all regions of the Arctic, for which records of properly identified nematode species are available: Svalbard, Jan Mayen, Iceland, Greenland, Nunavut, Northwest territories, Alaska, Lena River estuary, Taymyr and Severnaya Zemlya and Novaya Zemlya. The list includes 391 species belonging to 146 genera, 54 families and 10 orders of the phylum Nematoda.

A new species of Setostephanolaimus, S. tchesunovi sp. n., is described from bottom sediments collected in the Skagerrak and Gullmarn Fjord off the west coast of Sweden. It is characterised by 1.0–1.3 mm long body, outer labial setae 6.5–8.5 μm long, cephalic setae 9–11 μm long, subcephalic setae 4–6 μm long, transversely-oval amphid, female with monodelphic opisthodelphic reproductive system, male with 6–9 tubular and without alveolar supplements, spicules arcuate and 54–64 μm long, gubernaculum with dorsal apophysis. A tabular compendium and dichotomous identification key to species of the genus Setostephanolaimus are provided.

Four known and one new species of Onchium are described from bottom sediments collected in Skagerrak off the west coast of Sweden. The following known species are redescribed: Onchium ocellatum Cobb, 1920, O. metocellatum Wieser, 1956, O. parocellatum (Allgén, 1940) and O. robustum Gerlach, 1965. Onchium longispiculum sp. n. is characterised by the 1.34–1.77 mm long body, anterior body end without cephalic capsule, anteriormost somatic sensilla located posterior to onchiostyle base, ocelli absent, excretory pore located at basis of lips, onchiostyle uniformly cylindrical, alveolar sup- plements indistinct, tubular supplements absent, spicules arcuate and 44–65 μm long. The new species has a unique set of characters (absence of developed ocelli and very long somewhat asymmetrical spicules) separating it from all other known species of the genus Onchium. The following nomenclatorial changes are proposed: O. conicaudatum (Allgén, 1935) is considered a junior synonym of O. metocellatum; O. conicaudatum apud Wieser, 1951 is considered a synonym of O. minutum Kito, 1981. The diagnosis of the genus Onchium is emended and a tabular compendium and dichotomous iden- tification key to species of the genus Onchium are provided.

Three known and two new species of Leptolaimoides are described from bottom sediments collected in Skagerrak off the west coast of Sweden. The following known species are redescribed: Leptolaimoides haploopis Jensen, 1978, L. tubulosus Vitiello, 1971 and L. hexatubulosus Hoang Lai-Phu et al., 2009. Leptolaimoides filicaudatus sp. n. is characterised by the 431–543 μm long body; cephalic sensilla papilliform; amphid 23–26 μm long, located 9–10 μm from anterior end; first body pore located 35–37 μm from anterior end; lateral field simple along most of body, areolated on tail, arising 36–40 μm from anterior end; female without supplements, vagina without pars refringens, vulva midventral; male without tubu- lar and without alveolar supplements; spicules arcuate and 16 μm long. Leptolaimoides leptomicron sp. n. is characterised by the 776–847 μm long body; cephalic sensilla papilliform; amphid 15–17 μm long, located 9–13 μm from anterior end; first body pore located 40–46 μm from anterior end; lateral field areolated, arising 26–28 μm from anterior end; female without supplements, vagina without pars refringens, vulva midventral; male with three tubular and without alveolar sup- plements, spicules arcuate and 28–29 μm long. The diagnosis of the genus Leptolaimoides is emended and a tabular com- pendium and dichotomous identification key to species of the genus Leptolaimoides are provided.

In his doctoral thesis, Christian Bussau published 110 new nematode nomina, including 97 nomina of new species, 11 of new genera and two of new subfamilies. The nomenclatural availability of these nomina has not been generally accepted and is evaluated here. It is determined that the doctoral thesis published by Bussau (1993) satisfies the criteria of publication outlined in Article 8 of the Code, and that all nomenclatural acts published in it satisfy the criteria of availability outlined in Articles 11 and 13 of the Code. The correct authority for all nomina first published in Bussau (1993) is ‘Bussau, 1993’, and not ‘Bussau, 1995’ or ‘Bussau & Vopel, 1999’. All nomenclatural acts published in Bussau (1995) and Bussau & Vopel (1999) are junior objective synonyms of acts published in Bussau (1993), except for Bathynox Bussau & Vopel, 1999. Nox Bussau, 1993 is a junior homonym of Nox Nalbant, 1986, and was provided the available new replacement nomen Bathynox Bussau & Vopel, 1999. The correct authority of Bathynox clavata is ‘(Bussau, 1993)’.

Leptolaimus timmi Vitiello, 1971 is redescribed from bottom sediments collected in the Skagerrak off the west coast of Sweden. New morphological data necessitate the transfer of this species to the genus Rhadinema Cobb, 1920. The main diagnostic characters of Rhadinema timmi (Vitiello, 1971) comb. n. include: 1.3–1.8 mm long body; rounded labial region weakly offset from body contour; cephalic setae 2–4 μm long; amphid located 12–19 μm from anterior end; first body pore located 22–30 μm from anterior end; lateral field absent; stoma tubular: cheilostom with six weakly cuticularised longitudinal rugae, gymnostom with sclerotized bar-shaped rhabdia, stegostom long, tubular; female without supple- ments, vagina without pars refringens, vulva midventral; male with 10–11 tubular and without alveolar supplements; spic- ules arcuate and 21–30 μm long.

A new genus Neocamacolaimus gen. n., with one new species N. parasiticus gen. n., sp. n. is described from the benthic polychaete Sphaerosyllis cf. hystrix collected in the Skagerrak off the west coast of Sweden. Neocamacolaimus gen. n. is placed in the family Camacolaimidae and is particularly characterised by having annulated cuticle with lateral alae; seti- form cephalic sensilla located at level with amphids; amphidial fovea ventrally spiral; buccal cavity without armament; pharynx muscular; nerve ring located at base of pharynx; male reproductive system diorchic with outstretched testes; spic- ules weakly arcuate with straight manubrium; gubernaculum absent; alveolar supplements located in the pharyngeal re- gion; tubular supplements absent; tail conoid; caudal glands and spinneret present. Juveniles of this genus are particularly characterised by their parasitic lifestyle and the following unique morphological features: lips form a dorso-ventrally elon- gated perioral disc with internal sclerotizations: one midventral and two dorsosublateral (right and left); cephalic sensilla setiform, subventral sensilla are noticeably longer than the subdorsal ones; intestine extends posterior to rectum and anal opening, forming a post-anal pouch.