Numerous studies of their population genetics have reported incipient reproductive isolation among sympatric populations of the common intertidal beadlet anemone Actinia equina. This has lead to certain morphs being raised to specific status. A study of the nematocysts of the green sea anemone Actinia prasina and three genetically isolated morphs of A. equina was undertaken to establish that mean nematocyst length could act as diagnostic phenotypic characters within a morphologically variable group. The results support genetic and ecological evidence for the specific status of the three red/brown coloured morphs of A. equina. The data are discussed with respect to the ecology of Actinia and concepts of species, but more work is required before the specific status of A. prasina can be confirmed.