Fatty acid compositional analyses of various isolated symbiotic bacteria showed all species to be rich in 16:0, 16:1(n-7) and 18:1(n- 7) and depleted in PUFA. A variety of symbiont-containing marine invertebrates from a range of Iocations were than analysed and the majority were rich in 16:1(n-7|. 18:1(n-7) and non-methylene interrupted dienoic fatty acids(NMIDs), and depleted in polyunsaturated fatty acids (PUFA), so implying that symbiotic bacteria were a major source of nutrition for these animals. It was also considered that the NMIDs are produced by the host animal in response to a relative deficiency of conventional PUFA and an extensive bacterial production of 18:1(n-7). Members of the Thyasirid family contained, in addition to the symbiont-type fatty acids, substantial proportions of fatty acids normally associated with phytoplankton, namely 18:3(n-3) and 18:5(n-3). It was deduced from the fatty acid profiles that the Thyassirids undergo a mixotrophic mode of nutrition. Fatty acid analysis was applied to three geographically distinct methane seep sites where the major trophic processes were unclear. The results revealed that at one site the symbiotic relationship dominated while at the other sites, photosynthetic processes were of greater importance for the animals’ nutrition. Finally, the study examined the incorporation of radioactive bicarbonate into the lipid classes of Lucinoaa borealia under a variety of environmental regime. It was concluded that both the fatty acid biomarker and the radioactive incorporation techniques together provide a powerful tool for the detailed examination of host-symbiont relationships in the marine environment.