Our study describes the extraordinary capability of the endosymbiont-bearing bivalve Lucinoma aequizonata to tolerate environmental anoxia. The clam survives without oxygen for 262 days (50% mortality). The total quantity of glycogen in a specimen does not decrease significantly after long-term anoxia (10.5 mo). Common glycogen-derived anaerobic products (opines, lactate, succinate, acetate, and propionate) are only produced in minor quantities. This indicates either severe metabolic depression or the utilization of alternative energy sources. We have found indications that the endosymbiotic bacteria might function as an important carbon source for the bivalve. Transmission electron microscopy studies showed that the symbionts are largely degraded after L. aequizonata was incubated anoxically for 10.5 mo. Polyphosphates detected in symbiont granules by energy dispersive X-ray spectrometry (EDX) might represent an alternative energy source for the clam's metabolism under this stress situation.