Andrey V. Gebruk

Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity?

In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007–2010. The MAR, 3,704,404 km2 in area, accounts for 44.7% lower bathyal habitat (800–3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime.

Deep-sea hydrothermal vent communities of the Logatchev area (14°45′N, Mid-Atlantic Ridge): diverse biotopes and high biomass

Results from recent biological studies (including three Alvin dives in July 1997) of the southernmost deep-sea hydrothermal area known on the Mid-Atlantic Ridge (MAR), the Logatchev hydrothermal area, are described. The Logatchev area includes three vent fields, each with a different geological setting corresponding to: (1) a large sulphide mound with ‘smoking craters’; (2) an active chimney complex (known as ‘Irina-2’); and (3) a diffuse flow through soft sediment (‘Anyas Garden’). Hydrothermal communities found at these fields differ considerably: Anyas Garden is dominated by vesicomyid clams, Ectenagena sp. aff. kaikoi , Irina-2 by mytilid mussels, Bathymodiolus sp. aff. puteoserpentis , with swarms of shrimps and other rich associated fauna, whereas on the main mound the fauna as a whole is less abundant and less diverse. This is the first record of a living clam population on the MAR. Other bivalves found on the clam field included thyasirids, Thyasira ( Parathyasira ) spp., not reported previously from deep-sea hot vent communities. The biomass on the mussel bed at Irina-2 exceeds 70 kg m −2 (wet weight with shells) and is the highest known on the MAR. The large shrimp population at Irina-2 includes swarms of Rimicaris exoculata . A diverse shrimp fauna (including a new species Alvinocaris sp. aff. muricola ) on the mussel bed is dominated by Mirocaris keldyshi . Overall the Logatchev area is dominated by mussels which may be attributable to the presence of both methane-oxidizing (dominant type) and sulphur-oxidizing bacterial symbionts in their gills. The diversity of vent organisms and biotopes at Logatchev appears to be the highest on the MAR.

Food sources, behaviour, and distribution of hydrothermal vent shrimps at the Mid-Atlantic Ridge

Five species of bresilioid shrimp were investigated at seven hydrothermal sites on the Mid-Atlantic Ridge: Menez Gwen, Lucky Strike, Rainbow, Broken Spur, TAG, Snake Pit and Logatchev. Samples were prepared for analysis of stable isotopes, elemental composition and lipids. Shrimp behaviour was observed from the submersible ‘Alvin’ and in the laboratory aboard RV ‘Atlantis’. The distribution and zonation of the shrimp species was recorded. Juvenile shrimp of all species arrive at the vents carrying reserves of photosynthetic origin, built-up in the pelagic larval stages. These reserves are used while the shrimp metamorphose to the adult form and, in Rimicaris exoculata and Chorocaris chacei , while they develop epibiotic bacteria supporting structures, the modified mouthparts and the inside of the carapace. The main food of adult R. exoculata is filamentous bacteria that grow on these structures. The intermediate sizes of C. chacei also feed on such bacteria, but the final stage gets some food by scavenging or predation. Mirocaris species scavenge diverse sources; they are not trophically dependent on either R. exoculata or mussels. Adults of Alvinocaris markensis are predators of other vent animals, including R. exoculata . The dense swarms of R. exoculata , with their exosymbionts, can be compared to endosymbiont-containing animals such as Bathymodiolus and the vestimentiferan tube-worms of the Pacific vents. Such associations, whether endo- or ectosymbiotic, may be necessary for the development of flourishing communities at hydrothermal vents.

Methane seep community of the Håkon Mosby mud volcano (the Norwegian Sea): composition and trophic aspects

The Håkon Mosby mud volcano (the Norwegian Sea, depth c. 1250 m) was studied in July 1998 by a joint Russian–German–USA–Norwegian expedition on the 40th cruise of the RV Akademik Mstislav Keldysh using the two Mir submersibles. The benthic community is dominated by two species of symbiotrophic pogonophores, Sclerolinum contortum (more abundant) and Oligobrachia haakonmosbiensis. The biomass of S. contortum reaches at least 435 g m ; for O. haakonmosbiensis the value is 350 g m . Different benthic organisms form associations with each species of pogonophore. The total list of known benthic fauna includes 46 species. A zoarcid fish, Lycodes squamiventer, is a common member of the benthic community. Bacterial mats are found over an extensive part of the crater. The background benthic community is much poorer and is dominated by ophiuroids (Ophiocten gracilis, Ophiopleura borealis). Pycnogonids (Collossendeis proboscidea), buccinid gastropods and asteroids are also present. Stable carbon isotope data showed significant depletion of C in both species of pogonophores: in S. contortum δ C was up to −48.3‰, in O. haakonmosbiensis the value varied from −51.1 to −56.1‰. It can be suggested that the methane carbon contributes to the nutrition of the pogonophoran O. haakonmosbisensis. Carbon isotopes also revealed incorporation of non‐photosynthetic carbon into local trophic webs: δ C in Metacaprella horrida (amphipod) showed −44.9‰, in the tube‐dwelling polychaete (Amphinomidae) −40.6‰. In the bacterial mat δ C varied from −17.6 to −53.0.

Ecology and Biogeography of the Hydrothermal Vent Fauna of the Mid-Atlantic Ridge

Abstract Six sites of hydrothermal activity with associated specialized fauna have been studied, lying between 14°45′ and 37°51′N along the Mid-Atlantic Ridge (MAR). To date about 100 species have been recorded, but the number increases with each expedition. There are common patterns in the distribution of the animals. The community organization usually changes with distance from the centre of venting, and two to three zones can be distinguished, related to environmental factors and trophic relations. The Atlantic vent communities differ considerably from those in the Pacific, notably in the absence of vestimentiferan tube worms and in the abundance of shrimp, but there is a common three-level trophic structure. The dominant animals are bresilioid shrimps; six species belonging to five genera from two families have been described so far. Rimicaris exoculata is one of the most abundant shrimps and the adult stage appears to depend on exosymbiosis with suplhur bacteria. It is not clear at what stage exosymbiosis is established and how the specifc nature of the relationship is maintained. The other species show a graded series of morphological and feeding adaptations, indicating a less specialized diet. More studies are needed on the life history, dispersal and microscale distribution of the shrimps, and of other vent-adapted animals at MAR. There are some similarities at the genus level between the hydrothermal vent and the cold seep faunas, indicating historic links, but this topic reqires further study. There may also be historic links between vent faunas of the West and East Pacific and the Atlantic. Further studies are needed to assess the importance of prokaryotic/eukaryotic symbiosis, including the role of exo-versus endosymbiotic bacteria, in structuring the vent communities in the different oceans.

The fauna of hydrothermal vents on the Mohn Ridge (North Atlantic)

Abstract The macrofauna of the newly discovered hydrothermal vent field on the Mohn Ridge at 71°N was investigated. Samples were collected during the cruise BIODEEP 2006 using the ROV ‘Bathysaurus’. A total of 180 species-level taxa were identified. The region contains very few vent-endemic species, but some species of Porifera, Crustacea and Mollusca may be vent-associated. Dense aggregations of motile non-vent species such as Heliometra glacialis and Gorgonocephalus eucnemis surrounded the vent area, but the area in general only held small numbers of sedentary animals. Calcareous sponges comprised an unusually high portion of the sponge species found and they constitute one of the first pioneers among the sessile invertebrates settling on these vents. Possible explanations for the structure of the fauna in the region are discussed.

'Lophenteropneust' hypothesis refuted by collection and photos of new deep-sea hemichordates.

The deep ocean is home to a group of broad-collared hemichordates—the so-called ‘lophenteropneusts’—that have been photographed gliding on the sea floor but have not previously been collected. It has been claimed that these worms have collar tentacles and blend morphological features of the two main hemichordate body plans, namely the tentacle-less enteropneusts and the tentacle-bearing pterobranchs. Consequently, lophenteropneusts have been invoked as missing links to suggest that the former evolved into the latter. The most significant aspect of the lophenteropneust hypothesis is its prediction that the fundamental body plan within a basal phylum of deuterostomes was enteropneust-like. The assumption of such an ancestral state influences ideas about the evolution of the vertebrates from the invertebrates. Here we report on the first collected specimen of a broad-collared, deep-sea enteropneust and describe it as a new family, genus and species. The collar, although disproportionately broad, lacks tentacles. In addition, we find no evidence of tentacles in the available deep-sea photographs (published and unpublished) of broad-collared enteropneusts, including those formerly designated as lophenteropneusts. Thus, the lophenteropneust hypothesis was based on misinterpretation of deep-sea photographs of low quality and should no longer be used to support the idea that the enteropneust body plan is basal within the phylum Hemichordata.

Diversification of acorn worms (Hemichordata, Enteropneusta) revealed in the deep sea

Enteropneusts (phylum Hemichordata), although studied extensively because of their close relationship to chordates, have long been considered shallow-water, burrowing animals. The present paper more than doubles the number of enteropneust species recorded in the deep sea based on high-resolution imaging and sampling with remotely operated vehicles. We provide direct evidence that some enteropneusts are highly mobile—using changes in posture and currents to drift between feeding sites—and are prominent members of deep, epibenthic communities. In addition, we provide ecological information for each species. We also show that despite their great morphological diversity, most deep-living enteropneusts form a single clade (the rediagnosed family Torquaratoridae) on the basis of rDNA sequences and morphology of the proboscis skeleton and stomochord. The phylogenetic position of the torquaratorids indicates that the group, after evolving from near-shore ancestors, radiated extensively in the deep sea.

Different energy sources for three symbiont-dependent bivalve molluscs at the Logatchev hydrothermal site (Mid-Atlantic Ridge)

The vent mussel Bathymodiolus puteoserpentis , a large vesicomyid clam and a smaller thyasirid were collected from an area of sediment subject to diffuse hydrothermal flow. The mussels live on the surface, the vesicomyids are partly buried and the thyasirids burrow in the sediment. The fine structure of the gills differs in the three bivalves. Bathymodiolus puteoserpentis hosts two types of bacterial symbiont, one methanotrophic, and another probably thiotrophic. The other two bivalves have single types of symbiont of different shapes. Stable isotope ratios of carbon and nitrogen indicate thiotrophy in the vesicomyid and thyasirid, but a predominance of methanotrophy in the mussel. This is the first time that such an assemblage has been found at a hydrothermal site on the Mid-Atlantic Ridge (MAR), with the different faunistic elements exploiting different energy resources.

New species of the genus Isorropodon (Bivalvia: Vesicomyidae: Pliocardiinae) from cold methane seeps at Nyegga (Norwegian Sea, Vøring Plateau, Storrega Slide)

A new species of vesicomyid bivalve (Isorropodon nyeggaensis sp. nov.) is described based on shell morphology, from the Nyegga cold methane seep area on the Norwegian continental margin. This is the first description of vesicomyids from the Norwegian Sea and the northernmost record of recent representatives of the family Vesicomyidae. A dispersion of the genus into the Norwegian Sea basin from the north-eastern Atlantic is suggested. A brief description of other macrofauna from methane seep sites at Nyegga is also given.