Benthos of the sub-polar front area on the Mid-Atlantic Ridge: Results of the ECOMAR project

This thematic issue is the second in Marine Biology Research devoted to benthic fauna of the Mid-Atlantic Ridge (MAR). The earlier thematic issue published five years ago presented results of the MAR-ECO expedition to the northern MAR between the Azores and Iceland (Gebruk 2008). This present compilation features the taxonomy, distribution, and biology of the Charlie Gibbs Fracture Zone bottom macro-invertebrate fauna based on a successful collaboration between the UK consortium programme ECOMAR (a Census of Marine Life (CoML) daughter project of the MAR-ECO initiative) and a group of deep-sea benthos diversity experts from Russia, UK, Norway, Germany and Brazil.

The importance of mid-ocean ridges as major topographic features of the earth's crust came to be recognized in the 1950s as new bathymetric charts of the oceans revealed the continuous chain of ridges circumscribing the planet. Heezen & Tharp (1957) produced their first map of the North Atlantic Ocean showing the extent of the ridge system diverging on either side of a median rift valley (Doel et al. 2006). Although Heezen (1969) stated that the global nature of the mid-ocean ridge system was unknown until the 1950s, the importance of the Mid-Atlantic Ridge was already apparent half a century before. Murray & Hjort (1912) wrote ‘The most striking feature of the Atlantic Ocean is certainly the low central ridge (dividing the ocean into eastern and western deep basins)’, which they show as stretching from Iceland to at least 53°S in Map II of that volume. They used the term ‘Mid-Atlantic Ridge’ and described elevated abundance and diversity of fish and invertebrates discovered on these shallow mid-ocean areas during the voyage of the RV Michael Sars in 1910.

Heezen (1969) depicted the mid-oceanic ridges and rift valleys as the single largest tectonic feature of the earth's crust, the importance of which was by then understood in the context of continental drift and formation of the ocean basins. New earth's crust is formed at the axis of the ridge associated with volcanic activity and resulting in sea floor spreading. Hydrothermal venting from the sea floor was discovered by Weiss et al. (1977) in the Galapagos rift region of the East Pacific Rise and the revelation of abundant life surrounding these vents (Corliss & Ballard 1977) stimulated decades of discovery of vent sites along all mid-ocean ridge systems and research on chemosynthetically supported communities living there (Van Dover 2000). Further attention was directed to studies of life on the summits of seamounts (Rogers 1994; Mironov et al. 2006) and associated target commercial fish species (Hareide & Garnes 2001; Kukuev 2004). In the Pacific Ocean, Hubbs (1959) first drew attention to elevated fish abundance on seamounts and offshore banks.

In the second half of the twentieth century, apart from some surveys of commercial fish stocks, studies of the fauna of the vast area of the Mid-Ocean ridge away from vents and summits suffered comparative neglect. Research on deep-sea biodiversity focussed on trends along depth gradients down the ocean margins and ignored fauna occurring at equivalent depths in mid ocean. For example, in their atlas of deep-living fishes of the North Atlantic, Haedrich & Merrett (1988) do not show any data for the Mid-Atlantic ridge. Allen & Sanders (1996) show bathymetric trends of species distribution of deep-sea protobranch bivalve molluscs for the North East Atlantic and North West Atlantic but with no information on the Mid-Atlantic Ridge.

Heezen (1969) estimated that Mid-Ocean ridges comprise about 33% of total sea floor area and have now been recognized as important habitats for deep-sea fauna (UNESCO 2009). By the year 2000 when the Census of Marine Life programme began it was clear that a study of the ridge-associated fauna sustained by photosynthetic production was long overdue building on the pioneering work of Murray & Hjort (1912).

The Census of Marine Life MAR-ECO (Patterns and Processes of the Ecosystems of the Northern Mid Atlantic) programme (2000–2010) undertook an extensive study of the northern segment of the Mid-Atlantic Ridge between the Azores and Iceland (Bergstad et al. 2008a; Vecchione et al. 2010). A major effort was undertaken during the voyage of RV G.O. Sars in 2004 (Wenneck et al 2008) which covered most of the MAR between the Azores and the Reykjanes Ridge. MAR-ECO incorporated data from other cruises but it was decided that special attention should be directed to the area approximately mid-way between Iceland and the Azores in the vicinity of the Charlie Gibbs Fracture Zone. Owing to its remoteness in the middle of North Atlantic Ocean this area is rarely sampled; special cruises are necessary in contrast to the routine fishery surveys undertaken on the Reykjanes Ridge and around the Azores. Furthermore, ongoing research was indicating that for numerous taxa, both benthic and pelagic, this region is the location of a latitudinal faunal divide with distinctly different species assemblages to the north and south of this region. The UK Natural Environment Research Council therefore funded a consortium project contributing to MAR-ECO known as ECOMAR – Ecosystem of the Mid-Atlantic Ridge at the Sub-Polar Front and Charlie Gibbs Fracture Zone. The Charlie Gibbs Fracture Zone is a major discontinuity in the structure of the MAR and is the point at which the North Atlantic Current, an extension of the Gulf Stream, crosses the MAR (Read et al. 2010) defining the location of Sub-Polar Front between the Arctic and Sub-Arctic oceanic provinces to the north and North Atlantic Drift province to the south (Longhurst 1998). The aim of ECOMAR was to examine this faunal and environmental divide by undertaking strictly comparable sampling at four stations, two north of the Charlie Gibbs Fracture Zone and two to the south (Bergstad et al. 2008b) and to compare stations at the same depth either side of the MAR, and some evidence of east–west differences in fish species composition and occurrence was found, but without sufficient replication the data set was too limited for rigorous analysis. To address this problem in ECOMAR, paired stations to the north and south were placed symmetrically on either side of the ridge axis.

Papers included in the present volume address various aspects of benthos taxonomy, distribution and ecology. Benthos samples were obtained on three voyages of RRS James Cook in 2007, 2009 and 2010. Sampling was done mainly at four stations at 2500 m depth on the flanks of the MAR; a pair of northern stations at 54°N and southern stations at 49°N (Priede et al. 2013).

Taxonomic groups considered in this volume include shell-bearing protists or ‘forams’ (Foraminifera), glass sponges (Hexactinellida), mushroom soft corals (Alcyonacea), bristleworms (Polychaeta), crustaceans (Amphipoda) and echinoderms (Asteroidea and Holothuroidea, Figures 1, 2). Taxa that are not featured in the MAR-ECO issue (Gebruk 2008) are foraminiferans, amphipods and polychaetes. Overall in the present issue one genus (benthic foraminiferans) and 11 species are described as new to science and a number of taxa are redescribed or re-evaluated.

Figure 1.  Starfish Hymenaster ecomari sp. nov., collected using the ROV Isis on the RRS James Cook Cruise 048 (in 2010) at St. 48/40, Dive 173 (Dilman, 2013). Photographer: David Shale (www.deepseaimages.co.uk).

Figure 2.  Holothurian ?Peniagone sp., collected using the ROV Isis on the RRS James Cook Cruise 048 (in 2010) at St. 48/38, Dive 172 and photographed swimming in the tank in the temperature-controlled room onboard. This elpidiid holothurian lacks calcareous ossicles and could therefore not be properly identified. Photographer: David Shale (www.deepseaimages.co.uk).

The paper by Gooday et al. (2013) deals with organic-walled benthic foraminiferans that inhabit the empty shells of planktonic foraminiferans. One genus and species of ‘inhabitant’ forams are described as new to science (Incola arantius gen. et sp. nov.); one more species, Hospitella fulva Rhumbler 1911, described 100 years ago and not reported since, is redescribed. These and other inhabitants of planktonic foraminiferan shells make an important contribution to benthic foraminiferan assemblages on the bathyal MAR. Two papers in this issue are focussed on glass sponges: Lopes & Tabachnik (2013) describe four unusual specimens of Farrea herdendorfi Duplessis & Reiswig, 2004 that are slightly different from specimens in the type series, thus adding to the knowledge of morphological variation in this species. Tabachnik & Menshenina (2013) describe two new species in the genus Sympagella (Rossellidae) and suggest a new name, Bathydorus levis pseudospinosus for Bathydorus levis spinosus Wilson, 1904, to avoid homonymy with Bathydorus spinosus Schulze, 1886. Two new species of mushroom soft corals are described in the paper by Molodtsova (2013): Anthomastus gyratus sp. nov. and Heteropolypus sol sp. nov. Diagnoses of the genera Heteropolypus and Pseudoanthomastus are re-evaluated and comments on some other species of mushroom soft corals from the North Atlantic are given. Kongsrud et al. (2013) consider all records of polychaetes between the Azores (42°N) and the southern tip of the Reykjanes Ridge (54°N) in the two large projects MAR-ECO and ECOMAR, with a total of 32 species recognized. Lumbriclymenella nasuta comb. nov. is formally redescribed and the taxonomic status of Laetmonice britannica is raised from the subspecies to the species level. In the second paper on polychaetes in the present issue, Shields et al. (2013) describe a new association between the polynoid Harmothoe bathydomus and the holothurian Deima validum. For the first time in the deep sea more than two polynoid specimens have been found on a single holothurian host. Additional taxonomic records of polynoid species recovered during remotely operated vehicle (ROV) Isis sampling in summer of 2010 are also presented. A new species of the deep-sea scavenging amphipod, Hirondellea namarensis sp. nov., is described by Horton & Thurston (2008) and an updated key to the genus Hirondellea is provided. The last two papers in this issue deal with echinoderms. Dilman (2013) sums up all records of starfish species in the ECOMAR project: 30 species were identified, 11 of them recorded for the first time on the northern Mid-Atlantic Ridge; 2 new species of Hymenaster are described and an annotated checklist of asteroids collected on the ECOMAR cruises is provided. Rogacheva et al. (2008) present data on 32 species of holothurians collected on cruises as part of ECOMAR. Three species of elasipodid holothurians are described as new to science. An annotated checklist of all species of deep-sea holothurians collected in the Charlie Gibbs Fracture Zone area is given.

This thematic issue further extends knowledge of bathyal benthos of the northern Mid-Atlantic Ridge, at the same time demonstrating that we still know very little about deep-sea fauna of the open ocean (Figure 2). One innovative advantage of benthos sampling in the ECOMAR project was targeted collection of specimens using the ROV Isis on the RRS James Cook cruise in 2010 (Priede et al. 2013). Many papers in this volume benefited from the quality of the samples obtained in this way, especially the contributions on minute foraminifera, soft and delicate pterastrid starfish and elasipodid holothurians and fragile glass sponges.

The ECOMAR consortium and the voyages of RRS James Cook were funded by the UK Natural Environment Research Council grant NE/C512961/1 with additional support from the Census of Marine Life and the Sloan Foundation. We thank the ship's company, the operators of the ROV Isis and numerous colleagues who made this work possible. We are grateful to David Shale for photographs and to numerous reviewers and editors who helped to produce this volume.

Finally, the editors would like to cordially thank Nianzhi Jiao and Martin Solan for having served our journal as subject editors over many years. And, we warmly welcome our new subject editor for Microbial Ecology, Hongyue Dang!