Introduction

Ecosystems known as cold seeps are found where reduced sulphur and methane emerge from seafloor sediments without an appreciable temperature rise. Cold seep environments are among the most recently discovered marine habitats; the first such system was found just 20 yr ago, on the Florida Escarpment in the Gulf of Mexico (Paull et al. 1984). Initial exploration of this seep and others in the Gulf of Mexico revealed communities dominated by symbiont-bearing tube worms, mussels and clams, often belonging to genera found earlier at hydrothermal vents. Since that discovery, large numbers of cold seeps have been identified in a broad range of tectonic settings, on both passive and active continental margins (Sibuet & Olu 1998, Kojima 2002). Many fossil seeps have been discovered (or reinterpreted) as well (Figure 1) (Campbell et al. 2002).

Most biological studies of cold seeps have focused on large, symbiont-bearing megafauna (vestimentiferan tube worms, mytilid mussels, vesicomyid clams), or on microbiological processes. Major reviews of megafaunal community structure at methane seeps have been prepared by Sibuet & Olu (1998), Sibuet & Olu-LeRoy (2002) and Tunnicliffe et al. (2003), and by Kojima (2002) for western Pacific seeps. Seep microbiology is reviewed in Valentine & Reeburgh (2000), Hinrichs & Boetius (2002) and Valentine (2002). Detailed understanding of the sediment-animal-microbe interactions at seeps has only just begun to emerge, along with new discoveries related to anaerobic methane oxidation.

The present review addresses the communities of organisms that inhabit cold seep sediments, focusing on soft-bodied, mid-size organisms (e.g., macrofauna and meiofauna) and on the nature of their interaction with biogeochemical processes. To fully understand the ecology of cold seep sediment-dwellers it is necessary to understand the environmental conditions at a scale that is

Figure 1 Distribution of modern and fossil cold seeps. (Modified from Campbell et al. 2002)

relevant to the organisms. To this end the review briefly considers the different types of cold seeps, patterns of fluid flow and aspects of their sediment geochemistry that are most likely to influence animals. The role of microbial activity in shaping the geochemical environment is discussed as is how this environment regulates the distribution and lifestyles of animals on different spatial scales. In this context the review describes the geochemical links to faunal abundance, composition, nutrition and behaviour, focusing on organisms and processes that occur within seep sediments. Because the large (megafaunal) seep organisms influence the sediment environment, providing physical structure and modulating geochemistry through oxygenation (pumping) and ion uptake activities, relevant features of the epibenthic megafauna are also included. The study of animal-sediment interactions at cold seeps is unquestionably still in its infancy. Where appropriate, those classes of organism-sediment interactions that are relatively unknown, but could yield interesting insights if researched further, are highlighted.

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