rhodoquinone and complex ii of the electron transport chain in anaerobically functioning eukaryotes many anaerobically functioning eukaryotes have an anaerobic energy metabolism in which fumarate is reduced to succinate this reduction of fumarate is the opposite reaction to succinate oxidation catalyzed by succinateubiquinone oxidoreductase complex ii of the aerobic respiratory chain prokaryotes are known to contain two distinct enzyme complexes and distinct quinones menaquinone and ubiquinone q for the reduction of fumarate and the oxidation of succinate respectively parasitic helminths are also known to contain two different quinones q and rhodoquinone rq this report demonstrates that rq was present in all examined eukaryotes that reduce fumarate during anoxia not only in parasitic helminths but also in freshwater snails mussels lugworms and oysters it was shown that the measured rqq ratio correlated with the importance of fumarate reduction in vivo this is the first demonstration of the role of rq in eukaryotes other than parasitic helminths furthermore throughout the development of the liver fluke fasciola hepatica a strong correlation was found between the quinone composition and the type of metabolism the amount of q was correlated with the use of the aerobic respiratory chain and the amount of rq with the use of fumarate reduction it can be concluded that rq is an essential component for fumarate reduction in eukaryotes in contrast to prokaryotes which use menaquinone in this process analyses of enzyme kinetics as well as the known differences in primary structures of prokaryotic and eukaryotic complexes that reduce fumarate support the idea that fumaratereducing eukaryotes possess an enzyme complex for the reduction of fumarate structurally related to the succinate dehydrogenasetype complex ii but with the functional characteristics of the prokaryotic fumarate reductases