Biotic interactions

Abstract

In sea anemones, intraspecific competition for space involving specialized catch tentacles and acrorhagi occurs between members of different clonal aggregations in the intertidal zone and in other restricted habitats where space is limiting. In aclonal actiniids (which tend to be large, solitary and sublittoral in distribution), acrorhagial conflict is rare, and it seems to provide information about availability of space rather than be an agonistic competition for that resource. The complete acrorhagial response typically requires physical contact between allogeneic individuals (i.e. conspecifics bearing different genotypes) and involves different (chemo)receptors on the tentacles and acrorhagi. Most allogeneic individuals discharge acrorhagial nematocysts (predominantly holotrichs) when brought into contact, but that not all will do so may indicate the existence of special histocompatibility or allorecognition loci; allogeneic individuals may occasionally bear the same recognition markers (allotypes) and not recognize each other as non-self. The (1) evocation of agonistic behaviour involving catch tentacles and acrorhagi and (2) specificity of induced morphogenesis of catch tentacles meet at least two of the three criteria for an immunological response. An instance of ‘inducible memory’ (the third criterion) has not been proved to be immunological in nature. Interspecific competition is less well documented in sea anemones, but it occurs between species of anemones, between anemones and other aggregating organisms, particularly in the sublitorial.