Neutral and polar lipids of the cold-seep clam Calyptogena phaseoliformis were determined to assess the trophic relationship at hydrothermal vents. The clam obtained many of its lipids from the chemosynthetic microorganisms within the bacteriocytes on its gill filaments. The major fatty acids in the triacylglycerols and phospholipids in the clam consisted only of the n-4 family polyunsaturated fatty acids (PUFA) plus saturated and n-7 monounsaturated fatty acids and were completely lacking n-3 PUFA, such as docosahexaenoic acid (22:6n-3) and icosapentaenoic acid (20:5n-3). Such unique fatty acid composition markedly differs from those reported for other marine animals, which depend on organic matter derived from surface-layer phytoplankton. The relationship between C. phaseoliformis and its symbiotic bacteria is documented and revealed a closed novel food chain, independent of the photosynthetic food chain. The fluidity of the fatty acids in the animal's membrane are maintained by adaptation of the exogenous n-4 family PUFA. These unusual but dominant fatty acids are assimilated from the symbiotic chemosynthetic bacteria, which use only geothermal energy and minerals from the cold-seep vents.All marine animals contain n-3 polyunsaturated fatty acids (PUFA), which originate from both phytoplankton and any symbiotic microorganisms depending on solar energy (Ackman 1989). In particular, marine animals contain high levels of long-chain n-3 PUFA, such as docosahexaenoic acid ([DHA] 22:6n-3) and icosapentaenoic acid ([EPA] 20:5n-3), in their depot and tissue lipids (Ackman 1989;Tocher 2003). Similarly, n-6 PUFA, such as linoleic acid (18:2n-6) and arachidonic acid (20:4n-6), are generally important for terrestrial animals. Either n-3 or n-6 PUFA usually dominate in marine or terrestrial animals, in particular in their cell membrane phospholipids.In the deep sea, all nutritional constituents are provided by the fall of organic matter from the surface (the detrital food chain) and vertical migrations of zooplankton and micronekton (the grazing food chain) (Raymont 1983). Not only do all marine animals in the upper water layers gain nutrition through the marine grazing food chain, but also benthic animals involved in the detrital food chain on or near the ocean bottom in some communities are indirectly supported by surface phytoplankton (Ackman 1989).The deep-sea Vesicomyidae clams of the genus Calyptogena, which have degenerate alimentary canals and house symbiotic chemosynthetic bacteria within the bacteriocytes on their gill filaments, have been observed on many coldseep sites of the bathyal to hadal zones (Sibuet and Olu 1998). The cold-seep bivalves, including Vesicomyids, thrive in cold seawater (2-5uC) in the deep sea and are conspicuous and abundant members of the deep-sea biological communities supported by sulfide-oxidizing chemoautotrophic bacteria (Felbeck et al. 1981;Rau 1981). They assimilate the products and energy from these endosymbionts and completely depend on the chemosynthetic symbionts and t...