INTRASPECIFIC AGGRESSION AND POPULATION DISTRIBUTIONS OF THE SEA ANEMONE METRIDIUM SENILE

¹ Department of Biological Sciences and the Marine Science Institute, University of California, Santa Barbara, California 93106
² Hopkins Marine Station of Stanford University, Pacific Grove, California 93950

Aggregations of the sea anemone Metridium senile in Monterey Harbor sometimes contained only one clone (genetically identical individuals) but often contained two or more intermingled clones. The frequent occurrence of mixed clonal aggregations was perplexing, because M. senile uses fighting ("catch") tentacles in intraspecific agonistic interactions. A photographic survey of 19 quadrats showed that mixed clonal aggregations of anemones persisted throughout the 3.5 year study. Locomotion by the anemones, low frequency of nonclonemate contact, low population density, and infrequent occurrence of fighting tentacles were eliminated as possible explanations for the persistence of mixed clonal aggregations. Laboratory studies revealed differences among clones in the frequencies of fighting tentacle inflation and of injury to nonclonemates. We believe that low expression of these aggressive traits might permit intermingling of clones. However, high expression of aggressive traits was not found consistently in clones which were not intermingled, and therefore may not cause the segregation of such clones. Apparently, the most important factor contributing to the intermingling of clones was habituation of anemones to nonclonemate contact. The decrease in fighting tentacle inflation observed during two, six-day laboratory experiments was not due to fatigue, because contact with unfamiliar nonclonemates renewed aggression. We believe that habituation to nonclonemate contact occurs in situ, and that loss of habituation must occur occasionally to account for the occurrence of fighting tentacles.

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