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a b s t r a c tThe macrofaunal assemblages from three Portuguese submarine canyons, Nazaré , Cascais and Setú bal were studied from samples collected at their upper (900-1000 m), middle (3200-3500 m) and lower sections (4200-4500 m) and at the adjacent open slopes ( $ 1000 m), during the HERMES cruises D297 (R.R.S. Discovery, 2005) CD179 (R.R.S. Charles Darwin, 2006) and 64PE252 (R.V. Pelagia, 2006). The taxonomic composition and patterns in biodiversity, abundance and community structure of the benthic macrofauna were described. Annelida (42.1% of total abundance; 137 species) and Arthropoda (20.6%; 162 species) were, respectively, the most abundant and the most species-rich Phyla among the 342 taxa identified during this study. Multivariate analyses showed significant differences between and within canyons and between canyons and open slope assemblages. At their upper section, canyons supported higher macrofauna abundance but slightly lower biodiversity than the adjacent slopes at similar depth. In all canyons abundance reached the highest value in the middle section and the lowest in the upper section, with marked fluctuations in Nazaré (474-4599 ind. m

Among chemosymbiotic metazoans found at deep-sea hydrothermal vents, cold seeps and organic falls, members of the mussel clade Bathymodiolinae (Bivalvia: Mytilidae) have evolved interactions with a higher diversity of bacterial lineages than other bivalve groups. Here, we characterized the bacteria associated with "Bathymodiolus" mauritanicus and Idas-like specimens from three sites in the Northeast Atlantic (two mud volcanoes in the Gulf of Cadiz and one seamount of the Gorringe Bank). Phylogenetic analysis of bacterial 16S rRNA-encoding gene sequences demonstrated that "B". mauritanicus has a dual symbiosis dominated by two phylotypes of methane-oxidising bacteria and a less abundant phylotype of a sulphur-oxidising bacterium. The latter was the dominant phylotype in a sympatric population of Idas-like mussels at the Darwin mud volcano. These results are the first report of a bacterial phylotype shared between two deep-sea mussels from divergent clades. This sulphur-oxidising bacterium was absent from Idas-like specimens from the other two sites (Gorringe Bank and Meknès mud volcano), in which bacterial clone libraries were dominated by other Gammaproteobacteria related to symbionts previously identified in Idas modiolaeformis from the Eastern Mediterranean. All Idas-like specimens studied herein are closely related and also related to I. modiolaeformis. However, they probably display different associations with bacteria, with the possible absence of both methane- and sulphur-oxidising symbionts at the Gorringe Bank. These results draw a very complex picture of associations between mussels and bacteria in the Northeast Atlantic, which could be highly variable depending on locale characteristics of the habitats.

Deep-sea bivalves found at hydrothermal vents, cold seeps and organic falls are sustained by chemosynthetic bacteria that ensure part or all of their carbon nutrition. These symbioses are of prime importance for the functioning of the ecosystems. Similar symbioses occur in other bivalve species living in shallow and coastal reduced habitats worldwide. In recent years, several deep-sea species have been investigated from continental margins around Europe, West Africa, eastern Americas, the Gulf of Mexico, and from hydrothermal vents on the Mid-Atlantic Ridge. In parallel, numerous, more easily accessible shallow marine species have been studied. Herein we provide a summary of the current knowledge available on chemosymbiotic bivalves in the area ranging west-to-east from the Gulf of Mexico to the Sea of Marmara, and north-to-south from the Arctic to the Gulf of Guinea. Characteristics of symbioses in 53 species from the area are summarized for each of the five bivalve families documented to harbor chemosynthetic symbionts (Mytilidae, Vesicomyidae, Solemyidae, Thyasiridae and Lucinidae). Comparisons are made between the families, with special emphasis on ecology, life cycle, and connectivity. Chemosynthetic symbioses are a major adaptation to ecosystems and habitats exposed to reducing conditions. However, relatively little is known regarding their diversity and functioning, apart from a few "model species" on which effort has focused over the last 30 yr. In the context of increasing concern about biodiversity and ecosystems, and increasing anthropogenic pressure on oceans, we advocate a better assessment of the diversity of bivalve symbioses in order to evaluate the capacities of these remarkable ecological and evolutionary units to withstand environmental change