The crescent shaped Mascarene Plateau (south-western Indian Ocean), some 2200 km in length, forms a partial barrier to the (predominantly westward) flow of the South Equatorial Current. Shallow areas of the Mascarene Plateau effectively form a large shelf sea without an associated coastline. Zooplankton sampling transects were made across the plateau and also the basin to the west, to investigate the role the partial interruption of flow has on zooplankton biomass and community structure over the region. Biomass data from optical plankton counter (OPC) analysis, and variability in community structure from taxonomic analysis, appear to indicate that the obstruction by the plateau causes upwelling, nutrient enrichment and enhanced chlorophyll and secondary production levels downstream.  The Mascarene Basin is clearly distinguishable from the ridge itself, and from the waters to the south and north, both in terms of size-distributed zooplankton biomass and community structure. Satellite remote sensing data, particularly remotely-sensed ocean colour imagery and the sea surface height anomaly (SSHA), indicate support for this effect. A correlation was found between OPC biovolume and SSHA and sea surface temperature (SST), which may indicate the physical processes driving mesozooplankton variability in this area. Biomass values away from the influence of the ridge averaged 24 mg m−3, but downstream of the ridge biomass averaged 263 mg m−3. Copepods comprised 60% of the mean total organisms. Calanoid copepod numbers varied considerably between regions, being lowest away from the influence of the plateau, where higher numbers of the cyclopoid copepods Oithona spp., Corycaeus spp. and Oncaea spp., and the harpacticoid Microsetella spp. were found.

The morphology and infraciliature of the marine hypotrichous ciliate Holostichanagasakiensisi sp. nov. from the coastal waters of Nagasaki, Japan are described based on living and protargol-impregnated specimens. The new species is characterized by: medium-sized marine Holosticha,in vivo 100–150×30–40 μm with elongated body shape and slightly greyish to reddish cell colour; cortical granules arranged in conspicuously longitudinal rows; 24–32 adoral membranelles; ∼20 midventral cirral pairs and 7–10 transverse cirri; 28–42 left and 25–38 right marginal cirri; constantly 4 frontal cirri and 3 dorsal kineties; 23–63 macronuclear nodules.

Two new species of lithistid sponges, Gastrophanellaphoeniciensis sp. nov. (Siphonidiidae) and Microscleroderma lamina sp. nov. (Scleritodermidae) are described from a submarine cave, depth 2–3 m, in Lebanon. They are the first Mediterranean records of two genera with a mainly tropical distribution, Gastrophanella being known from the western Atlantic (Caribbean, Brazil), central eastern Pacific (Mexico) and South Africa, and Microscleroderma from the Indo-Pacific and the eastern Atlantic (Senegal). They are interpreted as remnants of a warmer period in the Mediterranean that survived the subsequent cooling only in the warmer eastern basin. Gastrophanella phoeniciensis sp. nov. is closely related to G. cavernicola from Brazil and Belize, from which it differs essentially in its shape. Microscleroderma lamina sp. nov. is characterized by a lamellar shape, short oxeas and having hair-like oxeas arranged around the aquiferous openings.

Two new Mycalina (Porifera: Poecilosclerida) are described from the south-western Atlantic, collected during Programme REVIZEE (1996–2002)—South Score, off the São Paulo State coast, Brazil. Esperiopsis bathyalis sp. nov. is characterized by the possession of two well differentiated categories of sigmas, the smaller ones being sigmancistras. Hamacantha microxifera sp. nov. is set apart by its possession of abundant small microxea-like raphides, diancistras 100–150 μm long, and absence of sigmas.

Two species of the genus Schizotricha new to science (Schizotricha crassa sp. nov. and S. southgeorgiae sp. nov.) have been studied. Both species are described and figured and their systematic position amongst the remaining species of the genus is discussed. The material was collected during the United States Antarctic Research Programme (USARP) with RV ‘Eltanin’ and RV ‘Islas Orcadas’. A comparative table listing the main features and distribution of the known species of Schizotricha, together with an identification key, are presented.

This paper describes a new species, genus, and subfamily within the Scyphomedusae. Stellamedusa ventana is a relatively large scyphomedusa that reaches up to 9·4 cm in diameter. The medusa is rare, with only seven observations in over a decade of extensive surveys of the Monterey Bay and Sea of Cortez using remotely operated vehicles. Stellamedusa ventana continues the trend found in several other deep-sea semaeostome medusae in lacking marginal tentacles. The exumbrella and oral arms are covered with large nematocyst laden projections which are able to capture food items of various sizes in the laboratory. A large cydippid ctenophore was found in the gut of one captured specimen.

Aricidea (Allia) pisanoi sp. nov. (Annelida: Polychaeta), is described from southern Chile. The new species was recorded from the Strait of Magellan (52°S) south to the continental slope of the Drake Passage (56°S). This new species is distinguishable from other species of the subgenus by only having capillary setae on postbranchial segments.

Species of the genera Progoniada and Goniadella are recorded from off the Brazilian coast, from depths of 93 to 808 m. The description of Progoniada regularis is complemented with new data on the number of chevrons and micrognaths. Goniadella revizee sp. nov. is described; it differs from the five known species of the genus mainly in having acicular chaetae above the dorsal cirrus, together with 20–21 uniramous chaetigers. The proboscideal papillae of both species are illustrated by scanning electron micrographs.

A new species, Longibrachium falcigerum, is described from seamounts in the North Atlantic Ocean, from depths of 495–695 m. Longibrachium falcigerum sp. nov. is unique among its congeners for possessing falcigers and spinigers in the lower chaetal bundle. Another member of the Rhamphobrachium complex, Rhamphobrachium (Spinigerium) brevibrachiatum, occurring with the new species, is briefly characterized.

During a study devoted to the characterization of the polychaete fauna associated with dead coral (Pocillopora spp.) in the Coiba National Park (Pacific coast of Panamá), nine species belonging to seven genera of sabellids (Annelida: Polychaeta: Sabellidae) were found. Three species new to science are described. Amphicorina bichaeta sp. nov. is characterized by having three pairs of radioles, smooth collar, broad thoracic notochaetae of two kinds (plus bayonet chaetae), and abdominal uncini with a large anterior tooth. Amphicorina perkinsi sp. nov. is characterized by having four pairs of radioles, six to eight abdominal chaetigers, smooth and oblique collar, broad thoracic notochaetae and thoracic uncini with main fang surmounted by a distinct subdistal tooth and a curved row of little teeth. Amphiglena jimenezi sp. nov. is characterized by having long radiolar appendages, medium-sized handles in thoracic uncini, and gametes in middle abdominal segments. Another six species are recorded: Branchiomma cf. bairdi and Notaulax occidentalis, which are new for the Pacific Ocean, while Notaulax nudicollis and Megalomma circumspectum, are new for the Pacific coast of Panamá. The presence of Bispira melanostigma and Pseudopotamilla intermedia is verified.

Four species of Pisionidae (Polychaeta) are reported from Coiba National Park (Pacific, Panamá), one is a well known species of the area Pisionidens indica, one is a new species of Pisione and there are two new reports: Pisione galapagoensis and Pisione cf. papuensis. Pisione longispinulata sp. nov. possesses buccal aciculae distally elongated with a conspicuous notch, three compound chaetae per parapodium, with dense and long spines on margins, and a single pair of copulatory organs, each with modified ventral cirrus, distal spherical enlargement and distal papillae. The closest species is Pisione garciavaldecasasi, also from the Coiba National Park, although it lives in freshwater streams in the northern part of Coiba Island.

Lumbricalus is redefined and revised. Its diagnostic features are chaetae of four types: composite spinigers, composite multidentate hooded hooks, simple multidentate hooded hooks, and limbate chaetae; the maxillary apparatus is labidognath with five pairs of maxillae, maxillae II are shorter than maxillae I with a wide connecting plate along the base of maxillae II. Lumbricalus, as redefined herein, includes L. januarii (type species), L. adriatica, L. composita, L. aotearoae, L. foresti stat. nov., L. harrisae sp. nov., L. vivianneae sp. nov., and L. vossae sp. nov. A key to all species is included.

Four new species belonging to the families Eunicidae and Onuphidae are described on the basis of specimens collected from El Salvador, Central America. Additionally ten species belonging to Onuphidae are mentioned, nine of these are newly reported from El Salvador. Eunice chicasi sp. nov., belong to the group of species with acicula distally hammer head, peristomial cirri smooth only reaching middle peristomium, branchiae from chaetiger 5 to the end of body, with up to six filaments, falcigers with lateral tooth triangular, well developed. Eunice salvadorensis sp. nov. has moniliform and cylindrical articles in the antennae, peristomial cirri with 17 small moniliform articles. Kinbergonuphis kristiani sp. nov. presents up to ten branchial filaments, ventral cirri digitiform on the first seven chaetigers, pseudocompound hooks tridentate appear on chaetigers 1–5, large tridentate hooks on chaetigers 5–7, and subacicular hooks from chaetiger 19. Paradiopatra barrazai sp. nov. belongs to the group of species with inner lateral antennae longer than the median antennae, ceratophores with four rings, ventral cirri of chaetigers 1–2 cirriform, postchaetal lobe digitate visible to the end of the body, tridentate pseudocompound hooks on chaetigers 1–4, large tridentate hook on chaetigers 5–28, subacicular hooks from chaetiger 30.

Measurements on cultivated larvae and data from the literature demonstrate that length and width of pelagic sea lice larvae, Lepeophtheirus salmonis and Caligus elongatus, are overlapping. Therefore measurements on planktonic developmental stages are of little practical value in specific identification. A standard WP-2 net with 200-μm mesh size will, in practice, collect free living sea lice larvae satisfactorily. The naupliar and copepodid stages of sea lice are illustrated and pigment distribution described. Larvae of L. salmonis are characterized by black and brown pigment, those of C. elongatus by red. The distributions of pigment are distinctive in the species mentioned, and in pelagic larvae of Lepeophtheirus pollachius and L. pectoralis which also are measured, described and illustrated for comparison. The copepodid stages of larvae are different in shape of pro- and urosome, morphology and armature of the maxilliped, distribution of pigment and its colour. The black and brown pigmented copepodid of L. salmonis is slender and oval in outline, somewhat pointed anteriorly, broadest in the middle of prosome and more narrow posteriorly. The proximal segment of the maxilliped is robust without seta, but the subchela has a slender branching barb with three to five ‘fingers’. The red pigmented prosome of C. elongatus is elongate, broad anteriorly with two lateral notches. The extreme breadth of the copepodid is clearly before the middle of the prosome. The corpus of the maxilliped is elongate and equipped with a seta. In addition the appendage has a medial 2-divided process.

A new pedunculate barnacle, Ashinkailepas seepiophila gen. and sp. nov. has been recovered from a hydrothermal cold seep at 1175 metres of depth off Hatsushima Island, Sagami Bay, central Japan. It is the second record of a Neolepadinae, otherwise known from vents, from a seep, but the two are not closely related. Its coarsely ornamented capitular plates, and primary peduncular armament being in whorls of six plates, distinguish it from all previously known neolepadines, not only as a new genus and species, but also as a distinct clade, and therefore a new tribe is proposed to accommodate it. There is a slight bilateral asymmetry in the proportions of the capitular plates as well as a marked bilateral asymmetry in the arrangement of the peduncular scales related to bending over to one side. This apomorphic characteristic distinguishes the new species from all other Scalpellomorpha. While leaning over to one side is suggestive of verrucomorphs, it is not judged a sign of affinity. Ashinkailepas is apparently the most primitive of the vent/seep barnacles, which are considered relics largely of the Mesozoic age.

A pedunculate barnacle, Leucolepas longa, occurs in densities over 1000 individuals m−2 on the summit of a small seamount near New Ireland, Papua New Guinea. Most of the population grows on vesicomyid clams projecting from sulphide-rich sediments, or on their dead shells, but the barnacle also settles on rock and on tubes of a vestimentiferan. Collections of several hundred barnacles allowed comparison of population and reproductive characteristics. The barnacle is a suspension feeder with a lightly-armoured stalk that can grow to 40 cm above the bottom. Growth appears to be rapid and both reproduction and recruitment are continuous. The barnacles brood egg masses within the capitular chamber and 46% of one sample was brooding. Lecithotrophic nauplii released upon retrieval to the surface were cultivated for 45 days. Metamorphosis to Stage IV yielded an actively swimming larva about 1 mm long overall, which still contained lipid reserves, indicating capacity for wide dispersal.

A new barnacle, Chthamalus neglectus sp. nov. is described from Hong Kong. Light microscopy and scanning electron microscopy investigations of adult morphology show obvious differences from previously recorded Chthamalus species, and that this species belongs to the ‘C. challengeri group’ of Chthamalidae.

Four new species of the genus Leucon and subgenus Leucon (sensu Watling, 1991) are described from the Atlantic Frontier Margin and other north-east Atlantic localities. They are L. sarsi sp. nov., L. holti sp. nov., L. hanseni sp. nov. and L. afeni sp. nov. Leucon hanseni sp. nov. has previously been wrongly identified as L. serratus, the holotype of which is illustrated here. A lectotype is selected for L. nathorsti and the species is redescribed from the type series and from Atlantic Frontier Margin material. A list of other Leucon (Leucon) species found in the Atlantic Frontier Environmental Network surveys is given.

The Ampelisca aequicornis group of species in the north-east Atlantic is discussed and A. aequicornis is redescribed from material identified in the collection of G.O. Sars. From Irish material a new species, Ampelisca eclimensis sp. nov., is described and an extended description of Ampelisca sorbei given. A key is presented to the A. aequicornis group of species.

Impacts of the trematode parasite Zoögonus rubellus on individuals of its host, the infaunal sandworm Nereis virens, were explored to assess influences that parasites might have on the structure of soft-bottom infaunal communities. Parasites were common in sandworms collected from a mudflat in Maine, with prevalence reaching 100% for large, commercial-sized sandworms and intensities of infection often exceeding 100 metacercariae per host. Parasitism was determined to be costly as sandworms exposed to parasites in the laboratory had higher mortality and foraged less on a main prey item, the amphipod Corophium volutator, than unexposed controls. As Nereis and Corophium are key intermediate-level members in infaunal communities, it is argued that parasitism of sandworms might directly and indirectly influence the structure of soft-bottom communities.