Effects of social organization on inter-clonal dominance relationships in the sea anemone Anthopleura elegantissima

In the sea anemone Anthopleura elegantissima contact between non-clonemates often elicits inter-clonal fighting, involving the application of nematocyst-laden fighting tentacles (acrorhagi) and the locomotory retreat of losing polyps. This study confirms that isoclonal aggregations of A. elegantissima display an inducible polymorphism, such that polyps at the inter-clonal boundaries (warriors) are typically smaller and carry more fighting tentacles (acrorhagi) than more central (reproductive) polyps. Based on the outcomes of eight separate contests between each combination of like and unlike morphs for each of seven pairs of neighbouring clones (i.e. N=224 contests), this study provides the first direct evidence that warriors are aptly named. All sets of neighbouring warriors and reproductives were mutually aggressive, and warriors, despite their overall size disadvantage, won 41 out of 81 contests with the reproductive polyps of their neighbouring clone. In addition, warrior and reproductive polyps were almost equally likely to initiate an attack and to retaliate. Thus, the success of warriors against the typically far larger reproductive polyps primarily reflects the proportionately greater allocation by warriors of resources to acrorhagi. The outcomes of the inter-clonal contests further indicate that pairs of neighbouring clones are unevenly matched in terms of their fighting ability, and that inter-clonal dominance is again largely determined by relative allocation of resources to acrorhagi. In five of seven inter-clonal combinations the warriors of the apparently dominant clone defeated the majority of opposing warriors (in three cases winning all eight contests). This result represents an apparent paradox, as earlier work showed that inter-clonal borders may persist for several years. Further work is needed to determine the extent to which agonistic behaviour determines the outcomes of inter-clonal competition for space and whether the outcomes of contests between pairs of individuals reflect the true dominance relationships of entire clones. ? 1996 The Association for the Study of Animal Behaviour Many colonial invertebrates display inducible polymorphism, such that their component modules exhibit discrete morphological or behavioural variation (reviewed in Harvell 1994). In functionally integrated colonies, this polymorphism presumably increases the absolute fitness of a colony by allowing different individuals to perform a limited set of complementary tasks. The clearest examples of such polymorphisms occur in invertebrates that form colonies of asexually generated, hence genotypically identical modules, in which phenotypic variation among members of the same clone must be environmentally produced (Hughes 1989). For instance, in clonal insects such as encyrtid wasps (Cruz 1981) and several species of aphid (Aoki 1979; Dixon 1985), as well as numerous species of benthic cnidarian (reviewed in Buss et al. 1984), polymorphism often includes the production of specialized morphs that appear to play a defensive role. The functions of such specialized morphs are often inferred from several kinds of circumstantial evidence. In the most Both authors contributed equally to this research. Correspondence: D. J. Ayre, Department of Biological Sciences, University of Wollongong, Wollongong, N.S.W. 2522 Australia (email: [email protected]). R. K. Grosberg is at the Division of Biological Sciences, Section of Evolution and Ecology, University of California, Davis, CA 95616, U.S.A. (email: RKG [email protected]). 0003–3472/96/061233+13 $18.00/0 ? 1996 The Association for the Study of Animal Behaviour