Certain endomyarian sea anemones possess marginal spherules or acrorhagi (Stephenson, 1928) which are employed only in aggressive behaviour towards other Anthozoa. The acrorhagi can be dilated and movements of the oral disc and column bring them into contact with an adversary. Pieces of acrorhagial ectoderm adhere to the body of the adversary whilst discharging into it the large atrich nematocysts characteristic of acrorhagi. Such behaviour has been described in the European anemone Actinia equina (L.) and the Californian anemones Anthopleura elegantissima (Brandt) and Anthopleura artemisia (Dana) (Äbel, 1954; Bonnin, 1964; Francis, 1973). Aggression usually occurs only following repeated contact with a genotypically distinct anemone: mere proximity does not usually initiate any response (Francis, 1973).

I have recently seen similar behaviour, apparently not recorded before, in the European anemone Anthopleura ballii (Cocks) collected from Weymouth, England. Specimens were kept in an aerated aquarium with other anemones and fed irregularly: the behaviour observed occurred between 19.00 and 22.00 h G.M.T. during August and November 1977. Following contact between two specimens of different colour varieties and the subsequent use of acrorhagi by one of them, the light-coloured passive individual immediately moved away from the dark-coloured aggressor and remained partly contracted with a ‘withered’ appearance to the tentacles for 1 week, following which it recovered (August). The same aggressor later (November) attacked a Cereus pedunculatus (Pennant), causing it to detach immediately from the substrate and to become necrotic one day later. The Cereus remained detached with the tentacles retracted and the column swollen for 5 days before it recovered.