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Introduction
The term 'extremophile' usually evokes images of prokaryotes living in acidic hot springs or in the Antarctic Dry Valleys, environments very different from the human environment. However, the taxonomic range of extremophilic organisms spans all three domains, including multicellular organisms and vertebrates (Rothschild & Mancinelli 2001; Clegg & Trotman 2002). For example, hydrothermally formed chimneys harbour dense and diverse microbial and multicellular communities, including luxuriant populations of clams (e.g. Calyptogena magnifica ), mussels (e.g. Bathymodiolus thermophilus ), tube worms (e.g. Riftia pachyptila , Tevnia jerichonana ) and a variety of grazers, which are supported by the high chemosynthetic primary production of the hydrothermal vents (Turner 1886). These systems are usually hosted in peridotites, an environment which is not only high in temperature and in pressure, but also very reducing and associated with acidic pH conditions. Other extreme environments useful to understand the range of environments in which terrestrial life can thrive include studies of winter sea-ice (Krembs et al. 2002), active sulphide chimneys (Kelley et al. 2002) and the acidic, iron-rich river, Rio Tinto, in southern Spain (Zettler et al. 2002). The study of these types of environments and an associated survey of the full biodiversity will provide an essential learning tool for the exploration of extraterrestrial life on other planetary bodies and shed light on evolutionary trajectories towards multicellular complexity (Schulze-Makuch & Irwin 2004). In addition, the study of extremophiles will contribute to our understandings of mechanisms of organismic survival, the origin of life and the limits for life in the universe (Rothschild 2006).
Adaptation strategies to environmental stresses by multicellular organisms
Adaptation in a biological sense is a genetically controlled characteristic that increases an organism's ability to survive and reproduce in its environment. Stresses to which organisms have evolved adaptations include dehydration (anhydrobiosis), extreme temperature (thermobiosis) including freezing temperatures (cryobiosis) and lack of oxygen (anoxia) (Hochachka & Somero 1984, 2002). In 1702 van Leeuwenhoek observed inactive 'animalcules' (tardigrades and rotifers) in dry sediments from the gutters of roofs of houses (Keilin 1959; Wright 2001). Shortly after the organisms came into contact with water they started to move. A similar ability of organisms to tolerate dehydration was reported by Needham (1743), Spallanzani (1776) and Doyère (1842). This phenomenon is referred to here as...