Biogeosciences, 7, 2851–2899, 2010 www.biogeosciences.net/7/2851/2010/ Biogeosciences doi:10.5194/bg-7-2851-2010 © Author(s) 2010. CC Attribution 3.0 License. Deep, diverse and definitely different: unique attributes of the world’s largest ecosystem E. Ramirez-Llodra1, A. Brandt2, R. Danovaro3, B. De Mol4, E. Escobar5, C. R. German6, L. A. Levin7, P. Martinez Arbizu8, L. Menot9, P. Buhl-Mortensen10, B. E. Narayanaswamy11, C. R. Smith12, D. P. Tittensor13, P. A. Tyler14, A. Vanreusel15, and M. Vecchione16 1Institut de Ciencies` del Mar, CSIC. Passeig Mar´ıtim de la Barceloneta 37-49, 08003 Barcelona, Spain 2Biocentrum Grindel and Zoological Museum, Martin-Luther-King-Platz 3, 20146 Hamburg, Germany 3Department of Marine Sciences, Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy 4GRC Geociencies` Marines, Parc Cient´ıfic de Barcelona, Universitat de Barcelona, Adolf Florensa 8, 08028 Barcelona, Spain 5Universidad Nacional Autonoma´ de Mexico,´ Instituto de Ciencias del Mar y Limnolog´ıa, A.P. 70-305 Ciudad Universitaria, 04510 Mexico,` Mexico´ 6Woods Hole Oceanographic Institution, MS #24, Woods Hole, MA 02543, USA 7Integrative Oceanography Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0218, USA 8Deutsches Zentrum fur¨ Marine Biodiversitatsforschung,¨ Sudstrand¨ 44, 26382 Wilhelmshaven, Germany 9Ifremer Brest, DEEP/LEP, BP 70, 29280 Plouzane, France 10Institute of Marine Research, P.O. Box 1870, Nordnes, 5817 Bergen, Norway 11Scottish Association for Marine Science, Scottish Marine Institute, Oban, Argyll, PA37 1QA, UK 12Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 97822, USA 13Department of Biology, Dalhousie University, Halifax, NS, Canada 14National Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, UK 15University Ghent, Marine Biology, Krijgslaan 281, 9000 Ghent, Belgium 16National Museum of Natural History, NMFS National Systematics Laboratory, Washington, DC, 20013-7012, USA Received: 22 December 2009 – Published in Biogeosciences Discuss.: 7 April 2010 Revised: 13 September 2010 – Accepted: 15 September 2010 – Published: 22 September 2010 Abstract. The deep sea, the largest biome on Earth, has a geochemical settings of the deep-sea floor and the water col- series of characteristics that make this environment both dis- umn form a series of different habitats with unique char- tinct from other marine and land ecosystems and unique for acteristics that support specific faunal communities. Since the entire planet. This review describes these patterns and 1840, 28 new habitats/ecosystems have been discovered from processes, from geological settings to biological processes, the shelf break to the deep trenches and discoveries of new biodiversity and biogeographical patterns. It concludes with habitats are still happening in the early 21st century. How- a brief discussion of current threats from anthropogenic ac- ever, for most of these habitats the global area covered is tivities to deep-sea habitats and their fauna. unknown or has been only very roughly estimated; an even Investigations of deep-sea habitats and their fauna began smaller – indeed, minimal – proportion has actually been in the late 19th century. In the intervening years, techno- sampled and investigated. We currently perceive most of the logical developments and stimulating discoveries have pro- deep-sea ecosystems as heterotrophic, depending ultimately moted deep-sea research and changed our way of understand- on the flux on organic matter produced in the overlying sur- ing life on the planet. Nevertheless, the deep sea is still face ocean through photosynthesis. The resulting strong food mostly unknown and current discovery rates of both habi- limitation thus shapes deep-sea biota and communities, with tats and species remain high. The geological, physical and exceptions only in reducing ecosystems such as inter alia hy- drothermal vents or cold seeps. Here, chemoautolithotrophic bacteria play the role of primary producers fuelled by chemi- Correspondence to: E. Ramirez-Llodra cal energy sources rather than sunlight. Other ecosystems, ([email protected]) such as seamounts, canyons or cold-water corals have an Published by Copernicus Publications on behalf of the European Geosciences Union. 2852 E. Ramirez-Llodra et al.: Unique attributes of the world’s largest ecosystem increased productivity through specific physical processes, Although often largely unknown, evidence for the effects of such as topographic modification of currents and enhanced human activities in deep-water ecosystems – such as deep- transport of particles and detrital matter. Because of its sea mining, hydrocarbon exploration and exploitation, fish- unique abiotic attributes, the deep sea hosts a specialized ing, dumping and littering – is already accumulating. Be- fauna. Although there are no phyla unique to deep waters, cause of our limited knowledge of deep-sea biodiversity at lower taxonomic levels the composition of the fauna is and ecosystem functioning and because of the specific life- distinct from that found in the upper ocean. Amongst other history adaptations of many deep-sea species (e.g. slow characteristic patterns, deep-sea species may exhibit either growth and delayed maturity), it is essential that the scien- gigantism or dwarfism, related to the decrease in food avail- tific community works closely with industry, conservation ability with depth. Food limitation on the seafloor and wa- organisations and policy makers to develop robust and ef- ter column is also reflected in the trophic structure of het- ficient conservation and management options. erotrophic deep-sea communities, which are adapted to low energy availability. In most of these heterotrophic habitats, the dominant megafauna is composed of detritivores, while 1 Introduction filter feeders are abundant in habitats with hard substrata (e.g. mid-ocean ridges, seamounts, canyon walls and coral reefs). Exploration of the last frontier on earth Chemoautotrophy through symbiotic relationships is domi- nant in reducing habitats. Although the largest ecosystem on Earth, the deep ocean is Deep-sea biodiversity is among of the highest on the also the least explored and understood. The oceans cover planet, mainly composed of macro and meiofauna, with high 71% of the planet’s surface, with 50% below 3000 m depth evenness. This is true for most of the continental margins and and a mean depth of 3800 m. Only 5% of the deep sea has abyssal plains with hot spots of diversity such as seamounts been explored with remote instruments and less than 0.01% or cold-water corals. However, in some ecosystems with of the deep sea-floor (the equivalent of a few football fields) particularly “extreme” physicochemical processes (e.g. hy- has been sampled and studied in detail. Nevertheless, what drothermal vents), biodiversity is low but abundance and little we know indicates that the deep sea supports one of the biomass are high and the communities are dominated by a highest levels of biodiversity on Earth (Hessler and Sanders, few species. Two large-scale diversity patterns have been dis- 1967; Sanders, 1968; Grassle and Macioleck, 1992; Etter cussed for deep-sea benthic communities. First, a unimodal and Mullineaux, 2001; Snelgrove and Smith, 2002; Stuart relationship between diversity and depth is observed, with et al., 2003), as well as important biological and mineral re- a peak at intermediate depths (2000–3000 m), although this sources (UNEP, 2007; Baker and German, 2009). Whereas is not universal and particular abiotic processes can modify the surface waters have played a central role in the develop- the trend. Secondly, a poleward trend of decreasing diversity ment of human civilization, being used for transport of goods has been discussed, but this remains controversial and studies and people, fishing and leisure, recently the development of with larger and more robust data sets are needed. Because of marine technologies have allowed us to enter the depths of the paucity in our knowledge of habitat coverage and species the oceans, to explore, investigate and exploit its resources. composition, biogeographic studies are mostly based on re- The first record of deep-sea fauna, the ophiuroid Gorgono- gional data or on specific taxonomic groups. Recently, global cephalus caputmedusae (as Astrophyton linckii), was col- biogeographic provinces for the pelagic and benthic deep lected by Sir John Ross in 1818, while dredging at 1600 m ocean have been described, using environmental and, where during his exploration for the Northwest Passage (Menzies data were available, taxonomic information. This classifica- et al., 1973). This discovery remained hidden and when tion described 30 pelagic provinces and 38 benthic provinces Edward Forbes, dredging in the Aegean down to 420 m divided into 4 depth ranges, as well as 10 hydrothermal vent depth (H. M. S. Beacon, 1841–1842), found fewer species provinces. One of the major issues faced by deep-sea bio- with increasing depth, he concluded that no life was present diversity and biogeographical studies is related to the high in the oceans below 600 m in what became known as the number of species new to science that are collected regu- “Azoic Theory” (Forbes, 1844). This theory stimulated larly, together with the slow description rates for these new debate and investigation. In the years that followed, evi- species. Taxonomic coordination at the global scale is par- dence of life in deep-sea systems accumulated.