brown algae of different geographic origin proved to be secreted always as a well-defined mixture of enantiomers (34). The enantiomeric composition of hormosirene from gametes of Analipus japonicus varies depending even on the locality within Japan (31). In contrast, the e.e. of hormosirene, present as a by-product (4-6%) in the pheromone bouquet of E. siliculosus, collected at various localities from all over the world, seems to remain constant (90% ± 5% e.e.; W.B. and D. G. Muller, unpublished data). If less common configurations of the pheromones are concerned, as, for example, in C₁₁ hydrocarbons with (E)-butenyl or (E,E)-hexadienyl substituents, the optical purity is often very low. 6-(1E)-(Butenyl)cyclohepta-1,4-diene from the Australian Dictyopteris acrostichoides exhibits an e.e. of only 26% (10); the cis-disubstituted cyclohexene from the blend of C. multifida (Figure 1) is virtually racemic (19). It is tempting to assume that for marine brown algae the production of characteristic enantiomeric mixtures represents a simple means for individualization of the signal blends, although up to now there is no experimental confirmation for this hypothesis.

Given the absence of methyl branches and according to the suggestive positions of the double bonds within the two acyclic C₁₁ hydrocarbons undeca-(1,3E,5Z)-triene and undeca-(1,3E,5Z,8Z)-tetraene, their origin from fatty acids is highly probable. In the case of higher plants, the