THE DEEP SEA is the largest but also the least studied habitat on earth. At depths below 1800 m, deep sea is totally dark, extremely cold and under extreme pressures. It was therefore previously assumed that life would be sparse in the deep sea; on the contrary, life is abundant. To cope with these harsh living conditions, DEEP SEA ORGANISMS HAVE DEVELOPED SPECIAL ADAPTATIONS. For example, many deep-sea fish have large eyes to capture what little light exists or are equipped with a powerful sense of smell to find potential mates in total darkness. Other adaptations consist of expandable stomachs to hold large quantities of scarce food, large mouths and long teeth to prevent prey from escaping. Little energy is spent for swimming in search of food; predators remain in one place and ambush their prey. But more than the absence of light, the near-freezing temperatures or the lack of oxygen and food, hydrostatic pressure is the most important environmental factor affecting deep-sea life. Pressure in the deep sea therefore ranges from 180 atm to more than 1,000 atm! How deep sea creatures have adapted to such pressures? With limited body cavities that would collapse under intense pressure, with muscle tissue with a reduced protein content… Little is known about the adaptation of fish bone to high hydrostatic pressure. The literature regarding bone biology in deep-sea environment is scarce and available information is weak, mostly speculative and goes back to several decades.  What are the mechanical properties of these bones? Are internal structures and micro-anatomy altered? Is the mineral content reduced or is there also a change in mineral composition? Finally, are changes directly related to increasing hydrostatic pressure?

The DEEP SEA is one of the least explored areas on Earth and the ELEVATED HYDROSTATIC PRESSURE that occurs at these depths is largely responsible for the current lack of knowledge on deep-sea environment. Hydrostatic pressure increases by approximately 1 atm for every 10·m of depth in the ocean (Saunders and Fofonoff 1976) and physiological and biochemical processes happening in DEEP-LIVING ORGANISMS are certainly affected (Siebenaller and Somero 1989). Evolutionary adaptations have however permit success in these harsh living conditions (Somero 1992). For example, proteins of deep-living organisms show adaptations (e.g. modulation of catalytic activity, change in protein structure) that allow them to operate optimally at elevated pressure (Somero 1992). Membranes of deep-sea fish also adapt their lipid composition to sustain extreme pressure (Hazel and Williams 1990). On the contrary, relatively little is known about bone adaptation to hydrostatic pressure. It has been proposed that deep-living teleost fish would exhibit a reduced skeletal ossification (Alexander 1974; Tont et al 1977; Merrett and Haedrich 1997) but no number, no image, no publication have ever established these affirmations. Interestingly, a number of human beings exposed to a high pressure environment such as diving or working in compressed air develop aseptic necrosis of bone, also named dysbaric osteonecrosis (Davidson 1981). Bone tissue dies and causes the bone to collapse. WHAT CAN WE LEARN FROM DEEP-LIVING TELEOST FISH? Shallow-living teleost fish have already been found suitable to model several human skeletal diseases, e.g. osteogenesis imperfecta (Fisher et al 2003), craniofacial syndromes (Belloni et al 1996; Nissen et al 2006) and idiopathic scoliosis (Gorman et al 2007). Deep-living teleost fish could model dysbaric osteonecrosis and help to understand how bone tissue can withstand high hydrostatic pressure... providing that basic data exists.

UNDERSTAND BONE COMPLEXITY OF DEEP-LIVING TELEOST FISH. We proposed, within the scope DEEPBONE project, to study the mechanisms underlying fish bone adaptation to high hydrostatic pressure. The working hypothesis of DEEPBONE project was that changes in bone mineral content, composition, micro-anatomy, density, strength and/or in bone marker gene expression could explain how fish bone has adapted to the extreme pressure and harsh environmental conditions. The comparative analysis of bone features in deep-living teleost fish and shallow-living relatives could provide insights into the mechanisms involved in bone adaptation to high pressure. The first and most challenging task in this project was to collect the primary material, bones of deep-sea teleost fish. Accessibility to deep-sea environment is difficult and costly and was provided by the Ifremer, THE FRENCH RESEARCH INSTITUTE FOR EXPLOITATION OF THE SEA, through access to oceanographic ships and deep underwater equipment (Sarrazin et al 2006 and 2007). Data related to bone MINERAL CONTENT AND COMPOSITION by X-ray diffraction and mass spectrophotometry (Fjelldal et al 2007a; Yamada et al 2001), bone MICRO-ANATOMY by histomorphometry (Gavaia et al 2000; Witten et al 1997 and 2003), bone STRUCTURE AND DENSITY by tomography and absorptiometry (Marie 2005; Yousfi et al 2001), bone STRENGTH by mechanical tests (Fjelldal et al 2007b; Fjelldal et al 2008) and finally bone MARKER GENE EXPRESSION by real-time PCR (Laizé et al 2005 and 2006; Stefanni et al 2007; Fonseca et al 2007) was proposed to be collected within the scope of the remaining 5 tasks of the project. INTEGRATED AND COMPARATIVE ANALYSES of these data was performed to identify factors involved in the adaptation of fish bone to hydrostatic pressure.