AGGRESSION, HABITUATION, AND CLONAL COEXISTENCE IN THE SEA-ANEMONE ANTHOPLEURA-ELEGANTISSIMA

Interclonal encounters between sessile cnidarians, including the sea anemone Anthopleura elegantissima, often elicit the deployment of specialized structures such as acrorhagi (modified tentacles) used for aggression. Fighting ability appears to be an important determinant of the outcomes of interclonal competition for space. Consequently, all else being equal, populations should come to be dominated by a small number of the most aggressive clones. Nevertheless, this and previous studies show that clonal diversity is very high in local populations of A. elegantissima. Such high levels of diversity could persist if (1) extant clones differed little in their fighting abilities, (2) rates of resource renewal were high enough to prevent the population from reaching competitive equilibrium and there were trade-offs between investment in aggression and other traits (such as sexual reproduction) associated with the acquisition of free space, or (3) clones were not mutually aggressive. This study first characterizes agonistic interactions among replicated pairings of the polyps of seven clones of A. elegantissima. We found a transitive dominance hierarchy, underlain by clone-specific variation in both agonistic behavior and in the numbers of acrorhagi per polyp. Clones consistently differed in their propensity to attack and to retaliate, and in their responses to different clones. Repeated stimulation by an alternate clone also modified the expression of agonistic behavior. These modifications, which involved both enhancement and habituation, represented responses to the stimulating clone, rather than nonspecific modification of agonistic behavior. Taken together, the results show that the outcomes of intraspecific competition for space among A. elegantissima clones may be decided by a combination of intrinsic behavioral and morphological attributes of competing clones, as well as prior histories of interactions with other clones. Because clones differ widely in their fighting abilities, clonal coexistence in A. elegantissima likely involves an interaction among the complexities of agonistic behavior, opportunities for colonization, and trade-offs between aggression and sexual reproduction.