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Hydrothermal vent community dynamics over a ten year span was studied in the Logatchev area, 14 degrees 45'N on the Mid-Atlantic Ridge. On the SERPENTINE cruise in March 2007 with the RN Pourquoi pas? and the ROV Victor 6000 (IFREMER), composition, abundance and distribution of dominant megafauna at Logatchev were examined and compared to published data from 1997. Our results revealed notable changes in several community characteristics. The most significant changes evident in March 2007 compared to July 1997 were the increase up to by order of magnitude in the population density of predatory gastropods Phymorchynchus spp. at the site Irina-2 and disappearance of a live population of vesicomyids at the site Anya's Garden. Other notable differences between the two periods of observation were the overall increase in the abundance in 2007 of the mussel Bathymodiolus puteoserpentis and a high abundance of gravid females of the shrimp Rimicaris exoculata at Irina-2. It is suggested that the extinction of vesicomyids occurred owing to either a sediment slide at a steep slope of the Anya's Garden locality or instability of a biotope required for vesicomyids. Live vesicomyids were not found over a large area explored at Logatchev in 2007. Other changes observed in 2007 apparently were related to seasonal fluctuations in the community at Logatchev or fluctuations in the hydrothermal activity. Any successional stages over a decadal scale at Logatchev were not observed.

A new species of scale-worm belonging to the sub-family Macellicephalinae, Levensteiniella iris sp. nov., is described. It was collected on the Mid-Atlantic Ridge, on the Lucky Strike and Rainbow vent sites, associated to the mussel Bathymodiolus azoricus on very low flow hydrothermal vents., Une nouvelle espèce de Polynoidae appartenant à la sous-famille des Macellicephalinae, Levensteiniella iris sp. nov., est décrite. Elle a été récoltée sur la dorsale médio-Atlantique, sur les sites hydrothermaux Lucky Strike et Rainbow, associée à des moulières à Bathymodiolus azoricus sur des évents tièdes à très faible flux.

Two new species of polychaetous annelids Prionospio unilamellata sp. nov. and Laonice asaccata sp. nov. (Spionidae) are described from samples collected at Mid-Atlantic hydrothermal vent fields close to the Azores Triple Junction and in the Snake Pit vent field. These two species are important components of the communities associated with active venting. Prionospio unilamellata differs from all other Prionospio species in the form of anterior notopodial lamellae. Laonice asaccata differs from other Laonice species by lacking interparapodial pouches., Deux nouvelles espèces d'annélides polychètes Prionospio unilamellata sp. nov. et Laonice asaccata sp. nov. (Spionidae) sont décrites à partir des collections recueillies par les submersibles sur les sources hydrothermales actives de la ride médio-atlantique, à proximité du point triple des Açores et dans le champ hydrothermal Snake Pit. Ces deux espèces sont fréquentes dans les communautés associées aux fluides hydrothermaux. Prionospio unilamellata sp. nov. diffère de toutes les espèces de ce genre par la forme de la lamelle notopodiale antérieure. Laonice asaccata sp. nov. diffère des autres espèces de ce genre par l'absence de poche inter-parapodiale.

Introduction : The evolutionary age of some of the hydrothermal vent taxa have recently become a highly debated issue. Vestimentiferan worms (= siboglinid polychaetes) in particular, have been claimed to be as old as 430 million years based on the occurrence of fossilized tubes resembling those of currently known Vestimentifera (Little et al., 1997), while other authors, based on molecular data, have claimed they must be younger than 100 million years old (Black et al, 1997; Halanych et al., 1998). One explanation for this apparent discrepancy might be that tubes, such as those found in the Silurian fossil vent communities, did not belong to vestimentiferans, but rather to other tube-dwelling polychaetes. However, it is also possible that the choice of the molecular clock used to estimate the age of the vestimentiferan radiation from the molecular data, was simply not appropriate. Opportunities to accurately calibrate molecular clocks are relatively rare and cannot be generalized to all genes nor to all taxa. Such an opportunity appeared during a phylogeographic study of the hydrothermal-vent ampharetid polychaete Amphisamytha galapagensis Zottoli, 1983, which is widely distributed in the Eastern Pacific (Chevaldonne et al., in prep.). Originally described from the Galapagos Rift, this worm was later found at most vent sites of the East Pacific Rise (EPR), but also on the northeastern Pacific ridge systems (Desbruyeres, 1997). However, our ongoing molecular studies suggest that specimens from the northeastern ridges (Gorda, Juan de Fuca and Explorer ridges) should be assigned to a new cryptic sister-species that is distinct from A. galapagensis but very closely related (Chevaldonne et al., in prep.). These related worms occupy distinct ridge systems that formerly were part of the Farallon-Pacific Ridge, which was disrupted by subduction under the North-American Plate about 28.5 MYA (Atwater, 1989; Severinghaus & Atwater, 1990). Thus, they provide a unique opportunity to investigate the processes that lead to allopatric speciation events between vent assemblages of both ridge systems (Tunnicliffe, 1988), and to calibrate the rate of molecular evolution in deep-sea annelids.

A new species of scale-worm, Lepidonotopodium jouinae, belonging to the sub-family Lepidonotopodinae (Polychaeta: Polynoidae) has been found in samples collected on deep-sea hydrothermal vent fields at the Azores Triple Junction on the Mid-Atlantic Ridge. It is the first species of Polynoidae described from the Mid-Atlantic Ridge hydrothermal vents. Other species of the same genus have been described from hydrothermal vents of the Pacific Ocean., L. jouinae sp. nov., appartenant à la sous-famille des Lepidonotopodinae, a été trouvée dans des échantillons récoltés sur les sites hydrothermaux profonds du Point Triple des Açores sur la Dorsale Médio-Atlantique. C'est la première espèce de Polynoidae décrite provenant des sites actifs de la Dorsale Médio-Atlantique. D'autres espèces du même genre ont été décrites dans la faune des sources hydrothermales de l'Océan Pacifique.

A new species of scale-worm, Lepidonotopodium atalantae, belonging to the sub-family Lepidonotopodinae (Polychaeta: Polynoidae) has been found in samples collected on the deep-sea hydrothermal vent sites of 9 degrees N and 13 degrees N on the East Pacific Rise. It is mainly characterized by elytra, regularly ornamented on their posterior border, and very long anal cirri. This is the fifth species of Lepidonotopodium, all belonging to the deep-sea hydrothermal vent fauna., Lepidonotopodium atalantae sp. nov., appartenant à la sous-famille des Lepidonotopodinae, a été trouvée dans des échantillons récoltés sur les sites hydrothermaux profonds de 9°N et 13°N sur la dorsale du Pacifique oriental. Elle se distingue principalement par ses élytres portant des ornementations régulières sur leur bord postérieur et par de très longs cirres anaux. C'est la cinquième espèce de Lepidonotopodium, toutes appartenant à la faune des sources hydrothermales profondes.

The objective of this study is to describe the chemical and physical environment surrounding the vent organisms at the Genesis site (EPR, 2640 m). The main chimney is colonized by Riftia pachyptila, fishes Zoarcidae and crabs Bythograeidae. The top of the smoker is covered with tubes of polychaetes Alvinellidae, the frontier zone by limpet gastropods. Temperature measurements and water sampling were made on an axis along the chimney. The environment was characterized using relationships between chemical concentrations and temperature to provide a chemo-thermal model of the site. Discrete temperature ranges were 1-1.6 degrees C in sea water, 1.6-10 degrees C among Riftia plumes (up to 25 degrees C at the tube base), 7-91 degrees C close to the alvinellid tubes, fluid emission was 262-289 degrees C. This study describes the habitat of Alvinellidae emphasizing its chemical (Sigma S [H2S + HS- + S-2-] 3-300 mu mol l(-1), CO2 3.5-6 mmol l(-1), pH 5.7-7.5) and thermal specificities compared to the Riftia ones (Sigma S 4-12 mu mol l(-1); CO2 2-3.5 mmol l(-1;) pH 5.8-7.7). The size of Riftia allowed us to define its environment at the organism (temperature gradient along the tube 0.5-1 degrees C cm(-1)) and population scales (temperature difference between organisms from the same clump: 10-20 degrees C). Such spatial heterogeneity may play a significant role in the animal physiology and calls for further investigations., Le but de ce travail est de décrire l’environnement physico-chimique des organismes hydrothermaux sur le site Genesis (EPR, 2 640 m). La cheminée centrale est colonisée par iftia pachyptila, des poissons Zoarcidae et des crabes Bythograeidae. Le sommet du fumeur est couvert de polychètes Alvinellidae, la zone frontière par des gastéropodes patelliformes. Un modèle reliant la température et la concentration en ΣS et CO2 est proposé. Les valeurs discrètes de températures s’échelonnent depuis 1-1,6°C dans l’eau de mer, 1,6-10°C autour de l’organe branchial de Riftia et jusqu’à 25°C à la base des tubes. Elles atteignent 7-91°C à proximité des alvinellidés et 262-289°C pour le fluide pur. Un environnement spécifique pour chaque communauté d’organismes est présenté : autour des Alvinellidae ΣS 3-300 µmol 1-1, CO2 3,5-6 mmol 1-1, pH 5,7-7,5 ; autour de Riftia pachyptila ΣS 4-12 µmol 1-1, CO2 2-3,5 mmol 1-1, pH 5,8-7,7. La taille de R. pachyptila a permis de préciser cet environnement à l’échelle de l’organisme (gradient de 0,5-1°C cm-1 le long du tube) et de la communauté (différence de 10-20°C au sein d’un même bouquet d’individus). Les travaux ultérieurs devront porter sur le rôle physiologique de cette hérérogénéité spatiale.