Three new species of Strongylacidon from north-east Brazil are described: Strongylacidon oxychaetum sp. nov. and Strongylacidon solangeae sp. nov. from Bahia State, and Strongylacidon chelospinata sp. nov. from Fernando de Noronha Archipelago, Pernambuco State. Strongylacidon oxychaetum sp. nov. has thinly encrusting shape with lobate projections, strongyles, oxychaetes and arcuate chelae; S. solangeae sp. nov. is characterized by having massive shape with hispid surface, strongyles and unguiferate chelae; and S. chelospinata sp. nov. present thinly encrusting shape, strongyles, and arcuate isochelae with large spines on the outer edge of the axis. An identification key for the nine valid species of Strongylacidon from the Tropical Western Atlantic is provided. New morphological characters, viz., oxychaetes and spined arcuate isochelae (here termed acanthochelae), are added to the definition of the genus Strongylacidon and of the family Chondropsidae.

A new Brazilian endemic Monanchora is described, M. brasiliensis sp. nov. It is clearly distinct from any other species in the genus by the monodentate shape of its anchorate isochelae, and its smooth, centrotylote and strongyloid microrhabds. Monodentate anchorate isochelae are relatively common, normally abundant, and represent the main diagnostic character for recognizing the new species, but microrhabds are usually rare and may be even absent. Monanchora brasiliensis sp. nov. can be further differentiated from other Atlantic species of the genus in eventually showing tetralobate branches and a skeleton formed by well-defined honey-coloured spongin fibres. The affinities of the new species with other crambeids and the nature of anchorate isochelae, microrhabds as well as the megascleres in defining Monanchora are discussed. An amended diagnosis for the genus is proposed.

The new species of Zanclea sango sp. nov. is described from Okinawajima Island, Ryukyu Archipelago, south-western Japan. The new hydrozoan species is associated with at least three scleractinian corals (Pavona divaricata, P. venosa and Psammocora contigua). Zanclea sango sp. nov. is a polymorphic hydroid and the hydrorhiza grows between the coral skeleton and calicoblastic ectoderm. The hydrocaulus and hydrorhiza are surrounded by perisarc. Newly released medusae are almost spherical, with four perradial exumbrellar nematocyst pouches including stenoteles, and two long marginal tentacles with cnidophores containing macrobasic euryteles. Zanclea sango sp. nov. is allied to Zanclea gilii Boero et al., 2000 and Zanclea margaritae Pantos & Bythell, 2010, but it is distinguished by its cnidome, the presence of a perisarc around hydrorhiza, and lower host-specificity. According to a hypothetical Zanclea phylogeny, the ancestral species of Zanclea had an opportunistic association with some benthic organisms, such as algae or bivalves, and its hydrorhiza was covered by a perisarc. Later, some species established specific associations with benthic animals, after which the hydrorhiza lost the perisarc and became directly covered with host tissue. Among Zanclea inhabiting corals, the present species, with multiple coral host species and a perisarc around the hydrorhiza, seems to retain more ancestral character states than Z. gilii and Z. margaritae, which have specific coral host species and no perisarc around the hydrorhiza.

In this work we describe Actinia ebhayiensis sp. nov. from South Africa. The species is externally similar to the type species of the genus, Actinia equina, from which it can be distinguished by nematocyst and allozyme data. Actinia ebhayiensis has a smooth, red column, with well-delimited parapet leading to a deep fosse where conspicuous blue acrorhagi can be found. Tentacles and oral disc are crimson red, and the pedal disc has a lighter pink colour. The microbasic p-mastigophores and b-mastigophores of the mesenterial filaments of the new species are significantly smaller than those of A. equina. Actinia ebhayiensis can also be clearly distinguished from other species of the genus in nematocyst measurements, genetic data and muscle morphology.

Phymactis papillosa is a rocky shore sea anemone that is commonly found in the Pacific Ocean, from the Gulf of California to Tierra del Fuego, and in the Mar del Plata region, Argentina. The genus Phymactis is closely related to Bunodosoma and, due to character plasticity, a number of misidentifications have occurred. Therefore, the presence of P. papillosa in Argentina has been doubted but the matter had not been investigated in detail. Here we analyse P. papillosa specimens from Argentina and compare them, using molecular and morphological markers, to specimens from the species' type locality. In a phylogenetic analysis using 19 allozyme markers and ribosomal internal transcribed spacers sequences of different sea anemone genera, including all West Atlantic Bunodosoma species, we have found that the specimens from Argentina were genetically divergent from P. papillosa from Chile and closely related to West Atlantic Bunodosoma species. The genetic and morphological analyses indicate that those specimens belong to a new species of the genus Bunodosoma, described here as B. zamponii sp. nov.

Recent exploration of the New England and Corner Seamounts revealed four new species of Chrysogorgia, described here using a combination of molecular and morphological data. These four species are characterized by a sinistral spiral, a character that, with one known exception, has only been reported for Pacific species. In addition, two species have a sclerite composition typical of the Pacific (‘squamosae typicae’). This faunal connection between the Atlantic and the Pacific is confirmed by analysis of the mitochondrial msh1 gene. The exceptional preservation of specimens collected with remotely operated vehicles allows us to discuss the effect of growth on some morphological characters.

This paper focuses on Antomicron, Leptolaimus and Procamacolaimus, three genera inhabiting coastal water systems of the Patagonian, Chubut and Santa Cruz provinces of Argentina, providing a review of their taxonomy for a new identification key. One new species belonging to the genus Antomicron and one new species belonging to the genus Leptolaimus of the family Leptolaimidae are described from Puerto San Julián, Santa Cruz province: Antomicron alveolatum sp. nov. has 35 alveoli and three precloacal supplements and Leptolaimus gabinoi sp. nov. is characterized by the structure and number of precloacal supplements (three) with hamate distal end with two hooks. Another species belonging to the genus Procamacolaimus of the family Camacolaimidae is described and recorded from Puerto Madryn, Chubut province of Argentina: Procamacolaimus dorylaimus.

Three new species of free-living marine nematodes Micoletzkyia longispicula sp. nov., Micoletzkyia filicaudata sp. nov. and Micoletzkyia nanhaiensis sp. nov. from the Yellow Sea and the South China Sea are described. Micoletzkyia longispicula sp. nov. is characterized by spicules elongated and straight, 7 a.b.d. long, pear-shaped proximally and pointed distally; gubernaculum conical, without apophysis, and tubular supplement headlike proximally. Micoletzkyia filicaudata sp. nov. is characterized by slender body with relatively long filiform tail (8.1 a.b.d.), spicules elongated and straight (4.4 a.b.d.), cephalate proximally and pointed distally, gubernaculum tubular, with dorsal apophysis. Micoletzkyia nanhaiensis sp. nov. is characterized by spicules slender and curved, 2.5 a.b.d. long, cephalous proximally and chapiter-shaped distally, and gubernaculum with relatively long dorsal apophysis. A key for all known species of Micoletzkyia is proposed.

Three species: Oxyonchus orientalis sp. nov., O. sakchalinensis sp. nov., and O. nicholasi sp. nov. (Thoracostomopsidae: Nematoda) are described and illustrated with the aid of light microscopy and laser-scanning microscopy pictures from sediments of Far Eastern Seas (the one former species from the Sea of Japan, Eastern Russia and the two latter species from the Sea of Okhotsk, Sakhalin Island). Generic diagnosis is emended for identification of Oxyonchus. Oxyonchus orientalis sp. nov. has short cephalic sensillae (0.6 cephalic diameters), strongly developed broad mandibular plate with irregular arrangement of numerous small denticles. Oxyonchus sakhalinensis sp. nov. is particularly characterized by the weakly short cephalic capsule, and the rounded thin mandibular plates with 10 denticles. Oxyonchus nicholasi sp. nov. can be differentiated by the structure of the cephalic armament (well developed capsule, fenestrae and incisions), numerous long cervical setae and tail shape. The distribution patterns of the various Oxyonchus species was suggested.

A new species of Paraseison (Rotifera: Seisonacea: Seisonidae) is described from the eastern Pacific Ocean off the coast of California. Female and male specimens of the new species were attached inside the carapace of the leptostracan crustacean Nebalia hessleri, collected at 19 m depth in La Jolla Canyon. This recording represents the first properly identified and described species of Seisonacea for the American coasts. It is furthermore the second known species of the genus Paraseison, which until now included the species P. annulatus only, reported for the Mediterranean Sea and European west Atlantic coast. The new species was diagnosed mostly by its species-specific trophi morphology and in particular by the shape of its fulcrum. Paraseison kisfaludyi sp. nov. is the fourth described species of the order Seisonacea, which accommodates also Seison nebaliae and the recently described S. africanus, all of which live attached to species in the leptostracan genus Nebalia. Even though the Seisonacea are placed in a crucial position within the phylum and may constitute the link between rotifers and other platyzoans, this taxon has in many respects been neglected in morphological studies. Herein we provide new information on the group's taxonomy, morphology and distribution, which will be useful for future phylogenetic and zoogeographical studies.

The coral reef inhabiting tubeworms from the Marshall Islands have been studied by few authors mainly by samplings in Enewetak Atoll. We report several new records of serpuline and spirorbine tubeworms from Enewetak Atoll as well as from Kwajalein, Utirik and Rongelap Atolls. Twenty-one species of Serpulinae and seven species of Spirorbinae are described and illustrated with line drawings, colour and/or scanning electron microscopy photographs. Neodexiospira turrita nom. nov. is proposed to replace Neodexiospira preacuta. Enewetak Atoll still represents the most species-rich atoll in the Marshall Islands but this is most likely the result of a greater sampling effort in this atoll for the last few decades.

We describe Dysponetus joeli sp. nov. from shallow maerl habitats in the north-east Atlantic (English Channel and Bay of Biscay). Dysponetus joeli differs from congeneric species by a unique combination of characters, including a large syllid-like pharynx, 2–4 simple serrated neurochaetae (closely similar to notochaetae, but much smaller and more delicate), D-shaped chaetal spines and ventral cirri on the third segment. A phylogenetic parsimony analysis based on morphological traits suggests that Dysponetus is not monophyletic unless it includes the closely related genera Vigtorniella and Pseudodysponetus, which are well delineated inside the dysponetid clade. Chaetal spines seem to be secondarily derived from paleae and to have originated in infaunal dysponetid forms. They should not be considered as plesiomorphic, but as evidence to support the clade made up by Dysponetus–Vigtorniella and Pseudodysponetus as delineated by a phylogenetic analysis.

Terebellomorph polychaetes are reported from hydrothermal vents and cold seeps collected in the Pacific and Atlantic Oceans. Two new species of Terebellidae, Neoamphitrite hydrothermalis sp. nov. and Streblosoma kaia sp. nov., are described from hydrothermal vents of the western Pacific. These are the first terebellid species described from hydrothermal vents. New records from hydrothermal vents and cold seeps and new geographical records are presented for nine additional species belonging to Ampharetidae, Alvinellidae, Terebellidae and Trichobranchidae. A synoptic table with diagnostic characters for all species of the genus Streblosoma Sars, 1872 is provided. Keys for all terebellomorph species currently known from hydrothermal vents and cold seeps, respectively, are included. Additionally the new combination Neoamphitrite pachyderma (Hutchings & Glasby, 1988) comb. nov. is proposed.

Galathowenia kirkegaardi a new oweniid polychaete species is described from soft bottoms of the Tamiahua Lagoon, in the Gulf of Mexico. It is characterized by the presence of lateral eyes, a constriction in the intersection of thorax and abdomen, and two well-developed pygidial lobes. The new species is compared with a small group of species in the genus characterized by the presence of eyes.

A new species of Acrocirrus is described from shallow waters of Coconut Island in Kaneohe Bay, Oahu, Hawaii. Acrocirrus bansei sp. nov. is an Acrocirrus whose segment 13 (chaetiger 11) is modified and equipped with a heavy neuropodial acicular hook. This new species is most similar to A. frontifilis based on the presence of notopodial cirri, which have been, up to now, a unique feature of A. frontifilis. The species differ most notably by the absence of the notopodial cirri on the posterior chaetigers. A key to all recognized species of Acrocirrus is presented.

Apseudopsis latreillii is a common tanaidacean species widely-distributed in the north-east Atlantic, but its diagnostic characters remain unspecified. Furthermore, intraspecific variation in relation to its developmental stages has not been described. Knowledge about this variation is needed owing to the difficulty of identification of the significant number of juvenile specimens within samples, particularly when sympatric with other apseudid species. Specimens from three different localities of the north-east Atlantic have been examined, and the size of 766 individuals has been measured. Study of postmarsupial development shows that the manca II is followed by two juvenile stages before reaching maturity. Males present two possible morphologies. After a preparatory stage, females pass through a sequence of copulatory instars followed by intermediate stages in which the female loses the ovisac after manca release. Characters allowing the distinction of A. latreillii specimens regardless of developmental stage are the combination of a pointed and downturned rostrum, pereonites without apophyses, three ventral spines on the pereopod 1 propodus, and one dorsodistal spine on the merus.

A new species of the genus Dahmsopottekina is described from the Mediterranean Sea. Dahmsopottekina guilvardi sp. nov. was collected from abyssal habitats at a depth range of 2340–2850 m. Like its congeners, the new species has a vermiform habitus, a highly transformed P1 in both sexes and a plough-like rostrum in the female. Dahmsopottekina guilvardi sp. nov. can be distinguished from its congeneric species by the combination of a fused basis and endopodite in P1 of both sexes and the absence of an endopodite in P2–P4 of the female. Dahmsopottekina guilvardi sp. nov. is the second record of a harpacticoid species after its congener D. peruana in which the basis and endopodite of a leg other than the P5, namely the P1, are fused. Furthermore, the new species is the only one among Dahmsopottekina species with a 1-segmented P1 exopodite in the male. Similar to its congeners, D. guilvardi sp. nov. is strongly sexually dimorphic. This is evident through the morphology of most of the cephalic appendages and the reduction of P2–P6 in the female. The results of the present study support the observation that Dahmsopottekina species are sparsely distributed and highly endemic. Nevertheless, our results do not agree with the statement of considerably larger females as the length variability between females is greater than between the two sexes. Despite the morphological characters of the species commensurate with a burrowing mode of life, its presence in sediment traps suggests that D. guilvardi sp. nov. is an active ‘swimmer’.

Rimicaris hybisae sp. nov. is described from hydrothermal vent fields on the world's deepest seafloor spreading centre, the Mid-Cayman Spreading Centre (MCSC), Caribbean, at depths of 2300–4960 m. The new species is described and illustrated on the basis of 17 specimens. Brief notes on the distribution and habitat of the new species are provided. Molecular phylogenetic data from mitochondrial COI (460 base pair (bp)), 16S ribosomal RNA (549 bp) and nuclear 18S ribosomal RNA (576 bp) regions is used to complement the description. Morphological variation within R. hybisae sp. nov. and morphological affinities with previously described species are discussed. Based on morphological and molecular evidence, the new species is provisionally assigned to the genus Rimicaris, and differs from all known species in the genus by a distinctive pair of ‘pores' on the posterior lobes of its four-lobed dorsal organ. An emended diagnosis for Rimicaris is provided. Rimicaris hybisae sp. nov. is the first taxon to be described from MCSC vent fields. This record extends the known geographical range of Rimicaris into the Caribbean Sea and constitutes the deepest documented occurrence of alvinocaridid shrimp.

We provide a review on scientific contributions concerned with Leptocaris (Harpacticoida: Copepoda) that includes the zoogeography, ecology, biology, and morphology of this genus. A tabular key is given to aid in the identification of the ignavus-group. Leptocaris ryukyuensis sp. nov. is included on the basis of specimens collected from Akeijima sandy beach of the Ryukyu Islands in Japan. The new species can be distinguished from its congeners by an abexopodal seta and 3 exopodal setae on A2, a barbed inner seta on the proximal, endopodal segment of female P1 to P3, setae of the caudal ramus (setae I and II are very small, and the basal seta is transformed to a big spine), and modified inner setules of P2 enp-2 and P3 enp-2 in the males. The new species belongs to the ignavus-group established by Kunz (1994) and is the third species of the genus Leptocaris in East Asia.

The hippolytid genus Leontocaris includes eight species, all restricted to the deep sea (240–2182 m). Associations with deep sea coralline habitats were reported and are herein confirmed. Three Australian species were recorded at seamounts as were the specimens herein identified as L. smarensis sp. nov. These specimens were sampled at the South Mid-Atlantic Ridge (SMAR) by the Mar Eco project during 12 bottom trawls using a Sigsbee trawl. The SMAR is a seamount chain that rises from 4000 m depth, with mountains of 100–200 km wide and 14,000 km length. Leontocaris smarensis sp. nov. shows closest affinity to L. lar from the north-western Atlantic and L. yarramundi, from Australia and New Zealand. It differs from L. lar mainly in: (1) the scaphocerite distolateral tooth reaching the distal margin of the blade while clearly falling short in the latter species; and (2) the mandibular palp possessing three distal setae while setae are absent in L. lar. The new species differs from L. yarramundi in the number and disposition of dorsal teeth on rostrum and in the absence of an acute posterolateral spine on abdominal somites 4 and 5.