Variations in deep-sea hydrothermal vent communities on the Mid-Atlantic Ridge near the Azores plateau

Near the Azores Triple Junction as the Azores Plateau is approached, the ridge axis becomes shallower; its depth decreases from ca. 2400 m in the Rainbow vent field (36°13′N) to ca. 850 m in the Menez Gwen vent field (37°35′N). In this area, extensive mussel beds of the mytilid Bathymodiolus azoricus dominate the hydrothermal vent fauna, along with populations of three shrimps (Rimicaris exoculata, Mirocaris fortunata and Chorocaris chacei). The main physical and chemical characteristics of the vent habitat were studied by discrete sampling, in situ analysis and sediment trap moorings. The vent fauna is distributed along a variable band where the vent fluids and seawater mix, with R. exoculata living in the most concentrated areas and Bathymodiolus azoricus in the most diluted zones. Various non-endemic species live at the border of the vent field. The variations observed in structure and composition of the communities along the depth gradient are most likely due to changes in vent fluid toxicity (metallic and sulphide content) and suspended mineral particles, which render the fluids harsher for species living there. The main faunal differences observed between Lucky Strike and Menez Gwen hydrothermal fields are due to an impoverishment in the hydrothermal endemic species and to the penetration of bathyal species. The comparison of the three studied vent fields suggests the existence of a succession of several biogeographic islands rather than a single province.     

Megafaunal distribution and assessment of total methane and sulfide consumption by mussel beds at Menez Gwen hydrothermal vent,based on geo-referenced photomosaics

The Menez Gwen hydrothermal vents, located on the flanks of a small young volcanic structure in the axial valley of the Menez Gwen seamount, are the shallowest known vent systems on the Mid-Atlantic Ridge that host chemosynthetic communities. Although visited several times by research cruises, very few images have been published of the active sites, and their spatial dimensions and morphologies remain difficult to comprehend. We visited the vents on the eastern flank of the small Menez Gwen volcano during cruises with RV Poseidon (POS402, 2010) and RV Meteor (M82/3, 2010), and used new bathymetry and imagery data to provide first detailed information on the extents, surface morphologies, spatial patterns of the hydrothermal discharge and the distribution of dominant megafauna of five active sites. The investigated sites were mostly covered by soft sediments and abundant white precipitates, and bordered by basaltic pillows. The hydrothermally-influenced areas of the sites ranged from 59 to 200 m². Geo-referenced photomosaics and video data revealed that the symbiotic mussel Bathymodiolus azoricus was the dominant species and present at all sites. Using literature data on average body sizes and biomasses of Menez Gwen B. azoricus, we estimated that the B. azoricus populations inhabiting the eastern flank sites of the small volcano range between 28,640 and 50,120 individuals with a total biomass of 50 to 380 kg wet weight. Based on modeled rates of chemical consumption by the symbionts, the annual methane and sulfide consumption by B. azoricus could reach 1760 mol CH₄ yr⁻¹ and 11,060 mol H₂S yr⁻¹. We propose that the chemical consumption by B. azoricus over at the Menez Gwen sites is low compared to the natural release of methane and sulfide via venting fluids.     

Hydrothermal faunal assemblages and habitat characterisation at the Eiffel Tower edifice (Lucky Strike,Mid‐Atlantic Ridge)

The Eiffel Tower edifice is situated in the Lucky Strike hydrothermal vent field at a mean depth of 1690 m on the Mid‐Atlantic Ridge (MAR). At this 11‐m‐high hydrothermal structure, different faunal assemblages, varying in visibly dominant species (mussels and shrimp), in mussel size and in density of mussel coverage, were sampled biologically and chemically. Temperature and sulphide (∑S) were measured on the different types of mussel‐based assemblages and on a shrimp‐dominated assemblage. Temperature was used as a proxy for calculating total concentrations of CH₄. Based on the physico‐chemical measurements, two microhabitats were identified, corresponding to (i) a more variable habitat featuring the greatest fluctuations in environmental variables and (ii) a second, more stable, habitat. The highest temperature variability and the highest maximum recorded temperatures were found in the assemblages visibly inhabited by alvinocaridid shrimp and dense mussel beds of large Bathymodiolus azoricus, whereas the less variable habitats were inhabited by smaller‐sized mussels with increasing bare surface in between. Larger mussels appeared to consume more ∑S compared with smaller‐sized (<1 cm) individuals and thus had a greater influence on the local chemistry. In addition, the mussel size was shown to be significantly positively correlated to temperature and negatively to the richness of the associated macrofauna. The presence of microbial mats was not linked to specific environmental conditions, but had a negative effect on the presence and abundance of macro‐fauna, notably gastropods. Whereas some taxa or species are found in only one of the two microhabitats, others, such as polychaetes and Mirocaris shrimp, cross the different microhabitats. Temperature was proposed to be a more limiting factor in species distribution than ∑S.     

Adaptation of the antioxidant defence system in hydrothermal-vent mussels (Bathymodiolus azoricus) transplanted between two Mid-Atlantic Ridge sites

The vent mussel Bathymodiolus azoricus is the dominant member of the Northern Mid‐Atlantic Ridge (MAR) hydrothermal megafauna, and lives in an environment characterized by temporal and spatial variations in the levels of heavy metals, methane and hydrogen sulphide, substances which are known to increase reactive oxygen species levels in the tissues of exposed organisms. To evaluate the effects of two contrasting hydrothermal environments on the antioxidant defence system of this vent mussel species, a 2‐week transplant experiment was carried out involving mussels collected from the relatively deep (2300 m), and chemical rich, Rainbow vent field. These were transplanted to the shallower (1700 m), and relatively less toxic, Lucky Strike vent field. To achieve this objective, levels of superoxide dismutase, catalase (CAT), total glutathione peroxidase (GPx), selenium‐dependent glutathione peroxidase and lipid peroxidation (LPO) were measured in the gills and mantle tissues of resident and transplant mussels before and after the transplant experiment. With the exception of CAT, the gills of the transplanted mussels had significantly higher antioxidant enzyme activity compared with the basal levels in the donor (Rainbow) and recipient (Lucky Strike) populations; whereas the antioxidant enzyme levels in the mantle tissues of the transplants reflected the baseline levels of activity in the native Lucky Strike mussels after 2 weeks. In contrast, LPO levels were significantly higher in both tissue types in the transplants than in either the source or the recipient populations, which suggested a response to hydrostatic pressure change (note, the transplant animals were brought to the surface for transportation between the two vent fields). The fact that the Rainbow mussels survived the transplant experience indicates that B. azoricus has a very robust constitution, which enables it to cope behaviourally, physiologically and genetically with the extreme conditions found in its naturally contaminated deep‐sea environment.     

High-resolution dynamics of a deep-sea hydrothermal mussel assemblage monitored by the EMSO-Açores MoMAR observatory

Although the spatial distribution of hydrothermal vent assemblages in relation to environmental conditions has been assessed in several studies, there is little documented data on the temporal variation of the fauna and corresponding abiotic factors in a vent community. Here, we present one of the longest integrated (faunal and environmental data) time series ever obtained in a hydrothermal ecosystem. The data were acquired using the TEMPO ecological module that was deployed between 2006 and 2008 on the Mid-Atlantic Ridge, providing the first insights into the day-to-day variations in a Bathymodiolus azoricus mussel assemblage from the Lucky Strike vent field for the 48 days during which the video camera operated. The time-series yielded additional valuable information on longer-term variation in faunal distribution (comparing ~2 years), temperature (11.7 months) and iron concentrations (3.8 months).Results from daily observations showed that the vent mussel assemblage was quite stable over the 48 days of the study, reflecting the relative stability of environmental conditions during this period. B. azoricus mussels appeared to thrive in areas of very limited hydrothermal fluid input in habitats that are, as in other deep-sea ecosystems, significantly influenced by ocean tidal signals. Variation in species abundance was observed but, with the exception of Mirocaris fortunata shrimp, no links could be established with measured environmental variables. Although we did not observe any clear tidal influence on vent fauna, it is likely that physiological processes and species’ activities are influenced by these periodic variations. Longer time series are currently being acquired by different experiments deployed on the EMSO-Açores MoMAR observatory (2010–2013 and still recording). They should further improve our knowledge of the dynamics of hydrothermal systems and their associated faunal communities.     

Spatial variation of metal bioaccumulation in the hydrothermal vent mussel Bathymodiolus azoricus

The variability of the bioaccumulation of metals (Ag, Cd, Cu, Fe, Mn and Zn) was extensively studied in the mussel Bathymodiolus azoricus from five hydrothermal vent sites inside three main vent fields of increasing depth along the Mid-Atlantic Ridge: Menez Gwen, Lucky Strike and Rainbow. Metal bioaccumulation varied greatly between vent fields and even between sites inside a vent field with B. azoricus showing a great capacity to accumulate metals. The bioaccumulation of these metals also varied significantly among tissues. The main target was the gills where metals were mainly associated with soluble compounds whereas in the digestive gland they were mainly associated with insoluble compounds. Storage of metals under insoluble forms in B. azoricus seems to be a major pathway for the detoxification of both essential and non-essential metals. Mussels from the studied fields can be discriminated following their metallic load but the segregation relies partially on the composition of the metal-enriched fluids.     

Community structure in mussel beds at Logatchev hydrothermal vents and a comparison of macrofaunal species richness on slow- and fast-spreading mid-ocean ridges

Species lists for vent fields on the Mid‐Atlantic Ridge (MAR) from 14°N to 38°N suggest that there is a northern (>27°N), shallow (<2000 m) fauna and a southern (<27°N), deeper (>3000 m) endemic vent fauna, but little is known about how community structure varies along the ridge axis. In this study, quantitative samples of macrofaunal invertebrates associated with mussels (Bathymodiolus puteoserpentis) were collected at Logatchev (14°45′N), the southern‐most explored vent field on the MAR. Community structure (including species composition, species richness, diversity, and relative species abundances) in mussel beds at Logatchev was compared with that of Snake Pit (23°22′N) and Lucky Strike (37°17′N) mussel beds. The most striking feature of the Logatchev mussel‐bed macrofaunal invertebrate community was the tremendous abundance (up to 2390 individuals per liter of mussel‐volume sampled) and biomass of the ophiuroid, Ophioctenella acies. Logatchev and Snake Pit mussel beds share >50% of their associated macrofaunal species; these two sites share only 20–25% of their macrofaunal species with Lucky Strike. Species–effort curves and univariate measures of diversity (H′, J′) do not support the claim that diversity of vent organisms on the MAR is highest at Logatchev, at least when one assesses this within a habitat type. Multivariate analysis readily differentiates the species‐abundance characteristics of Logatchev, Snake Pit, and Lucky Strike mussel‐bed macrofaunas. The relationship between sea‐floor spreading rate and diversity was explored through comparison of species richness in mussel‐bed habitats on slow‐spreading (MAR), fast‐spreading [northern East Pacific Rise (EPR)], and ultra‐fast‐spreading (southern EPR) mid‐ocean ridges. Species richness was greater in samples from the faster‐spreading ridge axes, where vents are more closely spaced but shorter lived, than on slow‐spreading centers, where vents are further apart but longer lived.     

Distribution and spatial variation of hydrothermal faunal assemblages at Lucky Strike (Mid-Atlantic Ridge) revealed by high-resolution video image analysis

Whilst the fauna inhabiting hydrothermal vent structures in the Atlantic Ocean is reasonably well known, less is understood about the spatial distributions of the fauna in relation to abiotic and biotic factors. In this study, a major active hydrothermal edifice (Eiffel Tower, at 1690 m depth) on the Lucky Strike vent field (Mid-Atlantic Ridge (MAR)) was investigated. Video transects were carried out by ROV Victor 6000 and complete image coverage was acquired. Four distinct assemblages, ranging from dense larger-sized Bathymodiolus mussel beds to smaller-sized mussel clumps and alvinocaridid shrimps, and two types of substrata were defined based on high definition photographs and video imagery. To evaluate spatial variation, faunal distribution was mapped in three dimensions. A high degree of patchiness characterizes this 11 m high sulfide structure. The differences observed in assemblage and substratum distribution were related to habitat characteristics (fluid exits, depth and structure orientation). Gradients in community structure were observed, which coincided with an increasing distance from the fluid exits. A biological zonation model for the Eiffel Tower edifice was created in which faunal composition and distribution can be visually explained by the presence/absence of fluid exits.     

Seasonal variations in lipid composition of the hydrothermal vent mussel Bathymodiolus azoricus from the Menez Gwen vent field

Specimens of the hydrothermal mussel Bathymodiolus azoricus collected in Menez Gwen hydrothermal vent field (NE Atlantic) during 2002–2003 were examined for feeding patterns variations through three seasons. The fatty acid profile and lipid classes of the mussels were studied, together with the MODIS/AQUA-derived near-surface chlorophyll a to test the hypothesis that surface productivity might be related to the feeding patterns of this species. The lipid levels showed pronounced seasonal fluctuations with the highest values occurring in January and August. Seasonal variations in lipid classes and fatty acid composition of neutral and polar lipids in the mussels are presented. Differences in the fatty acid profile of lipid classes in different seasons suggest that the higher energy requirements in summer and winter were supplied by bacterial biomarkers ω7 MUFA (monounsaturated fatty acids), whereas ω6 PUFA (polyunsaturated fatty acids) and NMI (non-methylene-interrupted) fatty acids predominated during the spring. The MODIS/AQUA data show marked seasonal variability and an anomalous peak during January of 2003, although this cannot be directly linked to lipid composition variation.     

Metal concentrations and metallothionein-like protein levels in deep-sea fishes captured near hydrothermal vents in the Mid-Atlantic Ridge off Azores

The knowledge of metal contamination in deep-sea fishes living in the surroundings of hydrothermal vents is very scarce, along with the detoxification mechanisms that allow them to live near one of the most metal contaminated marine environments. Six deep-sea fish species, although not vent endemic were collected near three Mid-Atlantic Ridge (MAR) hydrothermal vents (Menez Gwen, Lucky Strike and Rainbow) and the gills, muscle and liver were selected for this study due to their importance in metal metabolism and storage. The concentrations of seven metals (Ag, Cd, Cr, Cu, Fe, Mn, and Ni) and a metal-related biomarker (metallothionein-like proteins–MTL) were assessed. Major differences in metal accumulation among fish species are related to their feeding habits and vent site of their capture. The liver and gills are in general the most important tissues for metal accumulation compared to the muscle, but tissue partitioning is very dependent on the fish species considered. Compared to other deep-sea fishes, fish capture in the vicinity of hydrothermal vents accumulates higher amounts of metals in general. However, MTL levels are not considerably different from what is found in commercial coastal fishes, and is poorly correlated with metal concentrations in the tissues. Therefore, MTL may not constitute one major detoxification system for deep-sea species living in the vicinity of three important MAR vent sites.     

Metal concentrations in the shell of Bathymodiolus azoricus from contrasting hydrothermal vent fields on the Mid-Atlantic Ridge

Specimens of Bathymodiolus azoricus were sampled along the Mid-Atlantic Ridge at the Menez Gwen, Lucky Strike and Rainbow hydrothermal fields. Individual shells (n = 51), through the weight range 0.62 to 15.70 g, were analyzed for their magnesium, strontium, iron, manganese, copper, zinc and cadmium concentrations. Amongst the marine molluscs the shell of B. azoricus is confirmed as being particularly impoverished in strontium (mean 943 microg g(-1)). Trace metal concentrations in the shells decreased in the order Fe> Mn> Zn> Cu> Cd. Despite originating from trace metal rich environments mean concentrations were low (37.9, 13.2, 10.7, 1.1 and 0.7 microg g(-1), respectively). Irrespective of geographical origin magnesium, strontium and copper concentrations were primarily dictated by shell weight. In contrast cadmium concentrations were elevated in shells from the Rainbow field and ambient seawater chemistry imparted site specific chemical fingerprints to the shells with respect to the iron to manganese ratio.     

Temporal variation in the antioxidant defence system and lipid peroxidation in the gills and mantle of hydrothermal vent mussel Bathymodiolus azoricus

Hydrothermal vent mussels are exposed continually to toxic compounds, including high metal concentrations and other substances like dissolved sulphide, methane and natural radioactivity. Fluctuations in these parameters appear to be common because of the characteristic instability of the hydrothermal environment. Temporal variation in the antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), total glutathione peroxidases (Total GPx), selenium dependent glutathione peroxidases (Se-GPx)), metallothioneins and lipid peroxidation (LPO) in the gills and mantle of the mussel Bathymodiolus azoricus from Menez–Gwen hydrothermal vent site was evaluated and related to the accumulated metal concentrations (Ag, Cu, Cd, Fe, Mn and Zn) in the tissues. Maximum antioxidant enzyme activities in the gills were detected in the beginning of summer, followed by a gradual decrease throughout the following months. One year after, the levels of antioxidant enzyme activities were similar to those reported one year before. LPO in this tissue exhibited a similar temporal variation trend. A different pattern of temporal variation in antioxidant enzyme activities was observed in the mantle, with a gradual increase from summer to the end of autumn (November). LPO in the mantle exhibited an almost reverse trend of temporal variation to that of antioxidant enzyme activities in this tissue. Antioxidant defences in the gills of B. azoricus were significantly enhanced with increasing concentrations of Ag, Cu and Mn, while negative relationships between antioxidant enzymes and Cd, Cu, Mn and Zn concentrations in the mantle were observed, suggesting different pathways of reactive oxygen species (ROS) production and that these tissues responded differently to the metal accumulation. However, temporal variation in biomarkers of defence and damage were in general similar to coastal bivalve species and can be associated with temporal variations of the physiological status due to reproduction. These variations might also be linked to the highly unstable nature of the hydrothermal environment.     

On the role of abiogenic factors in the bioaccumulation of heavy metals by the hydrothermal fauna of the Mid-Atlantic Ridge

The distributions of Fe, Mn, Zn, Cu, Ni, Co, Cr, Pb, As, Ag, Cd, Se, Sb, and Hg in 128 samples of tissues of the organisms that inhabit hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, and Rainbow) depending on the abiotic environmental parameters were studied. The majority of the elements studied showed direct correlations between their concentrations in the fluids released and in the tissues of hydrothermal organisms. A higher degree of bioaccumulation of metals was revealed in the Bathymodiolus mussels and Rimicaris shrimps from the Rainbow hydrothermal vent field as compared to their analogues from Menez Gwen and Snake Pit. This corresponds to the maximum concentrations of the majority of the metals studied in the Rainbow high-temperature hydrothermal fluids. The highest degree of bioaccumulation of heavy metals was found in gills of the symbiotrophic mussels Bathymodiolus and in maxillipeds of the ectosymbiotic shrimps Rimicaris, i.e., in the organs that function in dependence on chemosynthetic bacteria. Within the Rainbow vent field, the shrimps, which inhabit in biotopes with more high-temperature conditions and therefore are more strongly subjected to the influence of fluids, were found to contain higher metal contents than mollusks. The Fe-Mn hydroxide films that cover mussel shells might serve as important reservoirs of other metals related to Fe and Mn.     

Benthic fauna associated with mussel beds and shrimp swarms at hydrothermal fields on the Mid-Atlantic Ridge

Macrofaunal assemblages with the prevalence of Bresiliidae shrimp and Mytilidae mussels are abundant in the hydrothermal vents along the Mid-Atlantic Ridge. The mussels inhabit the zone of diffuse seeps of hydrothermal fluids with temperature abnormalities up to several degrees. Shrimps inhabit an extreme biotope in the mixed interface between the seawater and the hydrothermal fluid at a temperature up to 20–30°C. We studied the mussel and shrimp assemblages in three hydrothermal vent fields: the Rainbow, Broken Spur, and Snake Pit. The species richness of the mussel assemblages in at least two regions (Broken Spur and Snake Pit) is higher as compared with the shrimps of the same hydrothermal vent fields. The fauna inhibiting the shrimp swarms lack almost any taxa specific for particular assemblages: almost all the taxa are also present in the mussel beds. The structure of the shrimp assemblage is less homogeneous as compared with that of the mussel assemblage. The population prevalence of one taxon (Copepoda) in the shrimp assemblage is most likely connected with the extreme and unstable conditions of the biotope occupied by the shrimps in the hydrothermal field. The taxonomic similarity between the mussel and shrimp assemblages within one hydrothermal vent field is higher as compared with the similarity between the mussel (or shrimp) assemblages from different fields.     

Life-history traits of the symbiotic scale-worm Branchipolynoe seepensis and its relationships with host mussels of the genus Bathymodiolus from hydrothermal vents

Associations between scale‐worms and giant mussels are common constituents of hydrothermal vent and cold seep ecosystems, but very little is known about their nature and ecology. Here, we analyze the ecological characteristics of the associations between Branchipolynoe seepensis, an obligate symbiotic polychaete, and their host mytilid mussels Bathymodiolus puteoserpentis and B. azoricus inhabiting hydrothermal vent fields on the Mid‐Atlantic Ridge. Infested mussels generally harbored a single symbiont (<4% had two to six worms). Infestation rate varied from 7.2% to 76.5%, increasing with mussel size, and was significantly lower for B. puteoserpentis. Symbiont density at Lucky Strike ranged between 1071 and 1191 individuals m⁻². Female symbiont size was always positively correlated with host size, while only males and juveniles from small mussels showed the same trend. This suggested a relatively long‐lasting host/symbiont association for females and short‐lasting association with successive reproductive migrations for adult males. The sex ratio of symbionts was always biased in favor of females. Males were smaller and more slender than females and had one mode in their size distributions, whereas females typically had three or more modes, suggestive of a longer life span in females. Between 59.1% and 72.2% of mussels had damaged soft tissues with substantially higher incidence of trauma in infested ones, suggesting that symbionts may cause trauma. The symbionts also induce tunnel‐like structures among the ctenidia, indicating fidelity to a particular location inside the host. Based on our data, together with the fact that infested mussels became relatively wider than non‐infested ones, this association is considered parasitic (likely kleptoparasitic). Our data, together with those from previous studies, allowed us to define the main life‐history traits of B. seepensis: (i) the relationship with their host is parasitic, (ii) the association begins at the smallest mytilid size classes, (iii) there is sexual dimorphism in body size, (iv) sex ratio deviates from 1:1 in favor of females, (v) fertilization occurs through temporal pairing and pseudocopulation, (vi) sperm are stored by females, (vii) eggs are large (likely lecithotrophic or with direct development), (viii) females have a longer life span than males, (ix) adult males may be semalparous, undertaking reproductive migrations followed by a short period of pairing and then death, and (x) females have a semi‐continuous iteroparous reproductive cycle.     

Macrofaunal communities of threatened subtidal maërl seabeds on Tenerife (Canary Islands, north-east Atlantic Ocean) in summer

This study contributes with the first data on physical and taxonomical structure of macrofaunal assemblages of ma¨erl beds from the Canary Islands. Ma¨erl beds and Cymodocea nodosa meadows of the Canary Islands are considered biodiversity hot-spots in terms of taxonomic and functional biodiversity with a broad geographical and depth ranges. The authors have studied the structure of the macrofaunal assemblages on different habitat types (Cymodocea, Caulerpa, sabellid field, garden eel and ma¨erl beds). Samples were taken at a range of depths between 14 and 46 m. Correlations were performed among abiotic variables (granulometry, organic matter, nitrogen and phosphates) and the most abundant taxa. Similarity analysis was performed to explore the patchiness of seabeds at a local scale. Significant differences were found in macrofaunal assemblages among seabed types, with highest abundances and lowest biodiversity in sabellid fields, where the sabellid Bispira viola dominated. The polychaetes Aponuphis bilineata and Chone filicauda and the mollusc Turritella brochii were the most abundant taxa on ma¨erl beds. The mosaic of granulometric conditions would explain the associated macroinfaunal community structure and contribute to the creation of diversity on these relatively well preserved seabeds at a local scale.     

High-resolution seafloor mapping using the DSL-120 sonar system: Quantitative assessment of sidescan and phase-bathymetry data from the Lucky Strike segment of the Mid-Atlantic Ridge

High-resolution, side-looking sonar data collected near the seafloor (100 m altitude) provide important structural and topographic information for defining the geological history and current tectonic framework of seafloor terrains. DSL-120 kHz sonar data collected in the rift valley of the Lucky Strike segment of the Mid-Atlantic Ridge near 37° N provide the ability to quantitatively assess the effective resolution limits of both the sidescan imagery and the computed phase-bathymetry of this sonar system. While the theoretical, vertical and horizontal pixel resolutions of the DSL-120 system are <1 m, statistical analysis of DSL-120 sonar data collected from the Lucky Strike segment indicates that the effective spatial resolution of features is 1–2 m for sidescan imagery and 4 m for phase-bathymetry in the seafloor terrain of the Mid-Atlantic Ridge rift valley. Comparison of multibeam bathymetry data collected at the sea-surface with deep-tow DSL-120 bathymetry indicates that depth differences are on the order of the resolution of the multibeam system (10–30 m). Much of this residual can be accounted for by navigational mismatches and the higher resolving ability of the DSL-120 data, which has a bathymetric footprint on the seafloor that is 20 times smaller than that of hull-mounted multibeam at these seafloor depths (2000 m). Comparison of DSL-120 bathymetry with itself on crossing lines indicates that residual depth values are ±20 m, with much of that variation being accounted for by navigational errors. A DSL-120 survey conducted in 1998 on the Juan de Fuca Ridge with better navigation and less complex seafloor terrain had residual depth values half those of the Lucky Strike survey. The quality of the bathymetry data varies as a function of position within the swath, with poorer data directly beneath the tow vehicle and also towards the swath edges.Variations in sidescan amplitude observed across the rift valley and on Lucky Strike Seamount correlate well with changes in seafloor roughness caused by transitions from sedimented seafloor to bare rock outcrops. Distinct changes in sonar backscatter amplitude were also observed between areas covered with hydrothermal pavement that grade into lava flows and the collapsed surface of the lava lake in the summit depression of Lucky Strike Seamount. Small features on the seafloor, including volcanic constructional features (e.g., small cones, haystacks, fissures and collapse features) and hydrothermal vent chimneys or mounds taller than 2 m and greater than 9 m² in surface area, can easily be resolved and mapped using this system. These features at Lucky Strike have been confirmed visually using the submersible Alvin, the remotely operated vehicle Jason, and the towed optical/acoustic mapping system Argo II.     

Diversity of faunal assemblages associated with ribbed mussel beds along the South American coast: relative roles of biogeography and bioengineering

Benthic organisms are among the most diverse and abundant in the marine realm, and some species are a key factor in studies related to bioengineering. However, their importance has not been well noted in biogeographic studies. Macrofaunal assemblages associated with subtidal beds of the ribbed mussel (Aulacomya atra) along South America were studied to assess the relationship between their diversity patterns and the proposed biogeographic provinces in the Southeastern Pacific and Southwestern Atlantic Oceans. Samples from ribbed mussel beds were obtained from 10 sites distributed from the Peruvian coast (17°S) to the Argentinean coast (41°S). The sampling included eight beds in the Pacific and two in the Atlantic and the collections were carried out using five 0.04 m² quadrants per site. Faunal assemblages were assessed through classification analyses using binary and log‐transformed abundance data. Variation in the size and density of mussels, and in the species richness, abundance and structure of their faunal assemblages were tested using a permutational multivariate analysis of variance. Faunal assemblages showed a north–south latitudinal gradient along both the Pacific and Atlantic coasts. Binary and abundance data showed a difference in the resulting clustering arrangement of Pacific sites between 40°S and 44°S, indicating a pattern of continuity in the species distribution associated with biological substrates. At a regional scale, the distribution of species along the South American coast matched the general provincial pattern shown by prior studies, which show two biogeographic units on the Pacific coast separated by an intermediate (probably transitional) zone and a single province on the Atlantic coast extending up to Northern Argentina. Biological substrates such as ribbed mussel beds play an important ecological role by making a similar habitat type available on a large scale for a variety of invertebrate species. Despite such habitat homogeneity, however, the associated fauna exhibit marked distribution breaks, suggesting strong constraints on dispersal. This therefore suggests that macrofaunal assemblages could possibly be used as biogeographic indicators.     

印度洋深海热液区贻贝足丝结构和性能特征研究

贻贝营半固着生活,依靠足丝将自己牢固地黏附在底物上.足丝的结构和性能对贻贝的正常生命活动至关重要.为研究热液区极端环境下深海贻贝足丝的结构和性能,采集了印度洋龙旂深海热液区的贻贝(Bathymodiolus marisindicus)足丝,对其力学特性、结构特征和组分进行了分析.力学性能测试发现与近海贻贝(Mytilu...