The deepest recorded depth for trematodes currently stands at approximately 6200 m. This depth record was achieved solely through sequence datasets of Lepidapedon sp. obtained from a gastropod. Given that trematodes of this genus typically use fish as definitive hosts, the origin of the trematode sequence was thought to be larval stages. However, the specific species remained unclear owing to the absence of reported adult-stage sequences. In the present study, we definitively identified the deepest trematode as Lepidapedon oregonense by comparing 28S ribosomal DNA sequences from adult worms from the macrourid fish Coelorinchus gilberti with data from the gastropod in the previous study.

Deep-sea hydrothermal vents host chemosynthesis-based ecosystems inhabited chiefly by specially adapted animals that do not live anywhere else, and depth has been shown to be a major driver of species composition at vents around Japan. Though the Ryukyu region in southern Japan is home to many hot vents, only two – Minami-Ensei Knoll and Yoron Hole – have been found shallower than 1000 m. Here, we report the discovery of a new vent field on the Amami Rift northwest off Amami Ōshima at 630 m deep. A total of 29 macrofaunal species were recorded from Amami Rift, including 19 vent specialists. Comparison of species composition across the three shallow Ryukyu vents revealed only three shared species, highlighting that all three display distinct community structure. Amami Rift exhibits distinct zonation patterns and is generally more similar to Minami-Ensei than Yoron Hole, but the presence of key taxa such as the sulphide worm Paralvinella and the mussel ‘Bathymodiolus’ platifrons as well as the absence of the symbiotic squat lobster Shinkaia and the limpet Lepetodrilus exemplify its difference with Minami-Ensei. Furthermore, the non-vent specific predators seen in these two sites were completely different. Overall, the Amami Rift vent field can be considered a shallow vent with a unique set of fauna, warranting future research on the mechanisms shaping disparate macrofaunal diversity between nearby shallow vents such as Amami Rift and Minami-Ensei. The unusual geological setting of Amami Rift at the converging point of Okinawa Trough and Ryukyu Arc may influence fluid chemistry to drive such differences.

Smectite close to the pure Fe end member of the nontronite-beidellite series was found in the fine clay separated from a 354-cm deep sediment core in the southwestern Pacific Basin. The mineral has a b-axis of 9.09 Å and an unusually low dehydroxylation temperature of 454°C and is composed of sheaves of fibers less than 50 Å wide. Its charge density is 5.09 × 10~4 esu/cm2. The charge originates mainly from the presence of 18% of the total Fe in tetrahedral positions, as determined by Mössbauer analysis. Slight deviations of the infrared spectra from those reported for nontronites are probably due to the presence of more octahedral Mg. The presence of authigenic quartz in the same sample permits some speculation on the concentration of dissolved silicon during nontronite genesis. A δ18O value of 26 ± 0.3‰ indicates a temperature of formation of about 22°C. The Sr isotope ratio suggests that the nontronite formed at least 12 million years ago.

A eulimid gastropod of the genus Ophieulima, parasitic on the disc of the ophiactid brittle star Ophiactis dyscrita, was collected from 256–343 m deep off Kanagawa, central Japan. This represents the first record of the genus from Japanese waters and even from the North Pacific Ocean. Here we describe it as Ophieulima yoshiharai n. sp. The generic assignment is justified by its conchological characters including (1) a small, conical shape with convex teleoconch whorls, (2) many fine growth lines and some strong growth pause scars on the teleoconch, (3) a well-developed, twisted columella, and (4) a multispiral, brownish transparent protoconch. Ophieulima yoshiharai n. sp. is distinguishable from the three (two named and one undescribed) extant, as well as two fossil congeners by its slender shell with the length of 2.1 times larger than width, a small aperture and a protoconch of 3.5 whorls. The new species is also characterized by its bathymetric distribution, which is shallower than the extant species of Ophieulima. On the other hand, the hosts of Ophieulima species are so far restricted to ophiuroids of the genus Ophiactis and the three named species have mostly been found attached to the lateral and/or dorsal sides of the host's disc, suggesting their similar parasitic ecology.

This paper reports the first record of the Atlantic soft pout Melanostigma atlanticum Koefoed, 1952, from the eastern Mediterranean Sea. The specimen was caught in a pelagic trawl during a research trip carried out north of the island of Crete (Greece) in December 2019. The fish measured 66.1 mm in total length and weighed 0.810 g. The presence of M. atlanticum in the eastern Mediterranean Sea indicates that the geographical distribution of the species is not restricted only to the western part of it but spreads throughout the basin. Melanostigma atlanticum is typically found in the deep sea, where limited surveys and research have resulted in major data gaps. The lack of recording of the species until recently in the eastern Mediterranean Sea is attributed to the species' small size and slender body shape that allow it to escape traditional fishing gears, as well as the low trawling activity in depths greater than 400 m and the lack of surveys targeting the mesopelagic fish communities.

We deployed jaws of the common thresher shark (Alopias vulpinus) on the seafloor at ~1000 m depth off Monterey California for 8 months. The jaws disintegrated, with all the hyaline cartilage disappearing, leaving some fragments of tessellated cartilage and the teeth. Two different Osedax species, O. packardorum and O. talkovici were found to have bored into the roots of some of the teeth, and were using the dentin pulp, which is rich in collagen, as a food source. The enameloid crowns of the shark teeth and the tessellated cartilage showed no signs of Osedax activity. This is the first demonstration of Osedax exploiting a source of food that is not bone. This raises questions as to the original food source of Osedax ‘bone worms'. Examination for the presence of Osedax in the skeletons and teeth of Mesozoic and possibly even Palaeozoic fossil sharks, bony fish and reptiles is warranted.

Lacydonia Marion & Bobretsky, 1875 is the sole genus in the family Lacydoniidae Bergström, 1914. We herein describe the new species of Lacydonia shohoensis sp. nov. from 2042-m deep bottoms at Shoho Seamount of the Nishi-Shichito Ridge, the Northwest Pacific Ocean. It is most similar to L. anapaulae Rizzo et al., 2016 in having a depression on the median anterior region and lacking lateral lobes on the posterior margin of prostomium whereas it is distinguished by possessing pygidium dorsally pigmented with three reddish spots and non-pigmented pygidial lateral cirri equally elongated.

Dirivultid copepods (Siphonostomatoida), one of the most successful meiobenthic organisms found at deep-sea hydrothermal vents, have been the focus of most previous ecological studies among meiofauna in these habitats. The ecology of Harpacticoida, a major benthic copepod group in typical deep-sea floor, however, is not well understood in terms of variations in community structure and controlling factors at venting sites. The spatial heterogeneities in benthic harpacticoid composition and their association with environmental parameters were investigated at hydrothermal vent chimney structures in the calderas of three neighbouring sea knolls (Bayonnaise Knoll, Myojin Knoll and Myojin-sho Caldera) in the western North Pacific. While a previous study had reported the distribution of dirivultids was strongly associated with spatial differences in stable carbon isotopic signatures (δ13C) of organic matter in the detritus on active chimneys in the field, multivariate analyses detected no significant corelation between the parameter and harpacticoid composition in this study. Instead, high associations of the harpacticoid composition with differences in water depth and total organic carbon (TOC) concentration were detected. Ectinosomatidae dominated at vent sites with lower TOC values in the shallowest Bayonnaise Knoll, while they were less prevalent at deeper vent fields in the other knolls, where Miraciidae was the most abundant family. This study indicated the availability of vent chemoautotrophic carbon is not a primary factor controlling the composition of harpacticoids even in the habitats on the hydrothermal vents, but instead by the food amount, regardless of its resources (including marine snow from the sea surface), in the study area.

The red-blooded limpet ‘Shinkailepas’ briandi (Neritimorpha: Phenacolepadidae) is one of the commonest gastropod species at deep-sea hydrothermal vents on the Mid-Atlantic Ridge (MAR). We investigated its population connectivity along MAR as the first such study for gastropods and explored the importance of larval migration for the distribution of vent-endemic animals. Our analyses, based on 1.3-kbp DNA sequences from the mitochondrial COI gene, showed a panmictic population throughout its geographic and bathymetric ranges that span from the northernmost and shallowest Menez Gwen vent field (38°N; 814–831 m depth) to the southernmost and deepest Ashadze field (13°N; 4090 m). Early development of this species is presumed to have a long pelagic duration as a planktotrophic larva; the hatchling with a shell diameter of 170–180 μm attains a constant settlement size of 706 ± 8 μm (mean ± SD). Retention of eye pigmentation in newly settled juveniles, along with the genetic panmixia, suggests that the hatched larva of ‘S.’ briandi migrates vertically to the surface water, presumably to take advantage of richer food supplies and stronger currents for dispersal, as has been shown for confamilial species at hydrothermal vents and cold methane seeps.

A taxonomic revision of the Nystiellidae from Brazil, including samples from the Rio Grande Rise, South Atlantic, was performed based on shell morphology. Five genera and 17 species were recognized. For the richest genus, Eccliseogyra, the three species previously recorded from Brazil were revised: E. brasiliensis and E. maracatu, previously known only from their respective type series, were re-examined. Newly available material of E. maracatu expanded the known geographic range of this species to off south-east Brazil. Eccliseogyra nitida is now recorded from north-eastern to south-eastern Brazil, as well as from the Rio Grande Rise. Three species of Eccliseogyra are newly recorded from the South Atlantic: E. monnioti, previously known from the north-eastern Atlantic, occurs off eastern Brazil and on the Rio Grande Rise; its protoconch is described for the first time, confirming its family allocation. Eccliseogyra pyrrhias occurs off eastern Brazil and on the Rio Grande Rise, and E. folini off eastern Brazil. The genus Iphitus is newly recorded from the South Atlantic. Iphitus robertsi was found off northern Brazil, although the shells show some differences from the type material, with less-pronounced spiral keels. Additional new finds showed that Iphitus cancellatus ranges from eastern Brazil to the Rio Grande Rise, and Iphitus notios sp. nov. is restricted to the Rio Grande Rise. Narrimania, previously recorded from Brazil based on dubious records, is confirmed, including the only two living species described for the genus: N. azelotes, previously only known from the type locality in Florida, and N. concinna, previously known from the Mediterranean. A third species, Narrimania raquelae sp. nov. is described from eastern Brazil, diagnosed by its numerous and thinner cancellate sculpture. To the three species of Opaliopsis previously known from Brazil, a fourth species, O. arnaldoi sp. nov., is added from eastern Brazil, and diagnosed by its very thin spiral sculpture, absence of a varix, and thinner microscopic parallel axial striae. Papuliscala nordestina, originally described from north-east Brazil, is recorded off eastern Brazil and synonymized with P. elongata, a species previously known only from the North Atlantic.

Two new species of free-living marine nematodes, Litinium dispariseta sp. nov. and Wieseria minor sp. nov. are described from an abyssal plain with a water depth of 4117–5035 m near the Southern Kyushu-Palau Ridge in the tropical Western Pacific Ocean. Litinium dispariseta sp. nov. is characterized by having distinct inner labial setae two times longer than the outer labial setae, wrench-like amphideal foveas, a single preanal midventral supplementary seta and a short cylindrical tail with bluntly rounded tip. It differs from congeners by its distinctly long inner labial setae relative to the outer labial setae (two times longer vs shorter or equal in length) and peculiar wrench-like amphideal foveas (vs horseshoe shaped or ovoid with round to oval anterior aperture). Wieseria minor sp. nov. has a clavate tail, a character found only in the four congeners W. glandulosa (Kreis, 1929), W. longiseta (Allgén, 1947), W. clavata Gerlach, 1956 and W. inaequalis Gerlach, 1956. However, Wieseria minor sp. nov. differs from these congeners by its much smaller body size (1045 µm vs 2120–3125 µm) and oblong amphideal foveas with double contour (vs a single oblong or ovoid loop). An emended diagnosis for Wieseria and pictorial keys for Litinium and Wieseria are provided.

Existing population studies of deep-sea amphipods have focused on species that inhabit deep-sea vent or trench environments but few cosmopolitan species have been studied. Here we provide new insight into the life history and population ecology of the pan-oceanic scavenging amphipod Paralicella caperesca and discuss the influence of nutrient-rich submarine canyon environments on the growth and reproduction of this species. Data were collected through the dissection and measurement of 2997 P. caperesca from 14 samples taken from abyssal plains, continental slopes and submarine canyons in the North-East Atlantic. Sexual dimorphism was less pronounced than observed for other scavenging amphipod species but females were significantly larger and had shorter antennae than males. The size of oostegites in female P. caperesca varied considerably within size classes, ovaries contained a relatively large number of oocytes, and no empty ovaries were observed. These factors, in combination with absence of mature females, suggest that P. caperesca practices semelparity, a reproductive strategy that complements the feeding strategy of this obligate necrophage. Five male and seven female size-grouped cohorts were identified for P. caperesca. Cohorts from deep-sea submarine canyon populations showed consistently larger mean total body lengths than non-canyon cohorts. Individuals from canyon samples also expressed sexual characteristics at smaller sizes than non-canyon individuals. We hypothesize that these trends are indicative of nutrient-mediated growth, maturation, and reproduction in P. caperesca. The species is able to grow and reproduce relatively quickly in response to increased nutrient input in canyon environments and therefore dominates scavenging amphipod assemblages.

A novel species of nematode belonging to the rare genus Psammonema Verschelde & Vincx, 1995 is described from the continental margin of the north-eastern Arabian Sea (214 m). Psammonema kuriani sp. nov. shows a different position of the lateral alae compared with the original genus diagnosis, hence an emended genus diagnosis is given. The genus Psammonema was previously described from the estuarine sediments of Indian Ocean region and the present study reports the genus from the deep sea for the first time.

Until now only two species of Cladorhiza have been reported from the SW Atlantic, namely C. diminuta and C. inversa, despite a total of 39 species reported from various parts of the globe. Here we describe a new species, C. nicoleae sp. nov., dredged from 750 m depth on the continental slope off SE Brazil during the French RV ‘Marion Dufresne’ expedition in 1987. It is an erect, pedunculated and club-shaped sponge, 26 mm high and with 12 radially arranged whip-like projections (each up to 3 mm long). The new species differs from its closest relative, C. inversa (redescribed here), by its possession of sigmas and sigmancistras. The holotype of Cladorhiza inversa is also a pedunculated sponge, 1.9 cm tall, with a cup-shaped body with an apical spur-like continuation of the stem and a crown of 16 projections (up to 8 mm long) radiating from the rim of the body.

Ceramaster grenadensis is one of the most abundant bathyal seastars in the north-western Mediterranean Sea and also presents a wide geographic distribution in the Atlantic Ocean. As with other species in this genus, little information is available on the biology and reproductive strategy of C. grenadensis. In this context, we describe for the first time the reproductive cycle of this species from bathyal depths in the north-western Mediterranean Sea. Specimens (N = 141) were collected seasonally from 194 benthic trawls (141 Otter Trawls and 53 Agassiz trawls) conducted during 10 cruises from October 2008 to April 2013. Open slope and canyon systems were sampled at depths between 900 and 2250 m. The population distribution of C. grenadensis showed a depth-related structure, with the smaller adult specimens and juveniles present at greater depths. Sex ratio was 2:1 females per male, constant among seasons and depths. Histological analyses of the gonads showed an asynchronous ovarian organization, with previtellogenic and vitellogenic oocytes throughout the year. These oogenesis patterns suggest a continuous reproduction. However, the Pyloric Caeca Index (PCI) decreased in summer while the Gonad Index (GI) increased in autumn in males, suggesting a higher spawning capacity in autumn. In both sexes, an increasing GI and PCI trend was observed with increasing depth.

Specimens of the deep-sea scale worm Neopolynoe acanellae (Verrill, 1881) (Polychaeta, Polynoidae) were collected at depths between 466 and 1405 m in Canadian waters while still attached to their host, the pennatulacean coral (sea pen) Pennatula grandis Ehrenberg, 1834. The present records extend the northern latitude of occurrence of N. acanellae in North America by 17o (~2000 km) to include the northern continental shelf of Newfoundland and Labrador and the lower Arctic, off the southern coast of Baffin Island (Canada). Analysis of the worm's intestinal content confirmed the presence of sea pen soft tissues and sclerites, suggesting that this species feeds on its host and is therefore parasitic.

The cookiecutter shark, Isistius brasiliensis, is confirmed for the first time from the eastern North Pacific Ocean. The shark, a female 432 mm in total length, was caught in a sablefish pot at a depth of 1132 m off San Nicolas Island, California, USA (32°59′05″N 120°25′99″W).

Cephalopods are common inhabitants of the deep ocean's mesopelagic zones worldwide, yet very little is known about their behaviour due to the inaccessibility of this environment. Recent studies suggest that, contrary to historical predictions, deep-sea cephalopods exhibit a wide array of visual behaviours. We used in situ footage from remotely operated vehicles, coupled with laboratory observations to assemble the first behavioural ethogram for the juvenile and subadult life stages of the mesopelagic squid, Chiroteuthis calyx. The number of behavioural components we described is comparable to or exceeds those recognized in ethograms of shallow-water teuthids. We used the ethogram to make a detailed behavioural comparison between the juvenile and subadult life stages, and found distinctly different patterns. Behavioural and morphological differences between the two life stages support the hypothesis that juvenile C. calyx mimic the abundant siphonophore Nanomia bijuga, in order to deter predation.

Benthic habitat and epibenthic megafauna are described from seabed photographs taken in or near three potential deep-water Marine Protected Areas on the Hebridean Slope, surveyed in 1988, 1995–96 and 1998. Broad bathymetric zones are defined by visible substratum characteristics, and by cluster analysis using presence/absence of identified fauna. The shelf edge and upper slope (to 350 m depth) showed current-swept coarse substrata with a very sparse fauna. A transition to finer sandy sediments was apparent at 350–470 m, with a greater abundance of mobile epifauna. The brittlestar Ophiocten gracilis occurred at high density below 600 m depth in all three areas. Southern stations indicated an abrupt lower boundary for this zone at ca 1000 m. In the north, rippled sands with xenophyophores occurred below the O. gracilis zone at 1000–1100 m. Xenophyophores were also seen in smaller numbers at this depth in the south. From ~1300–1600 m a seabed of burrowed fine mud was recorded in both northern and southern areas. Bioturbated sediments on the Rockall Trough floor at ~2060 m supported more visible epifauna, principally the brittlestar Ophiomusium lymani and the octocoral Acanella arbuscula. The bathymetric pattern differs from a scheme proposed in 2010 for the deep north-east Atlantic in the lack of any obvious zonal boundary at 750 m. No cold-water corals or sponge aggregations were observed. Trawl marks were observed in all three areas, particularly in the 1998 northern survey, with highest frequency at 1300–1400 m.