AGGRESSIVE FUNCTION AND INDUCED DEVELOPMENT OF CATCH TENTACLES IN THE SEA ANEMONEMETRIDIUM SENILE(COELENTERATA, ACTINIARIA)

1. The "catch tentacles" of Metridium senile are larger and more opaque than normal feeding tentacles, are capable of great enlargement, and possess a cnidom differing strikingly from that of the regular or feeding tentacles. 2. The catch tentacles are not used in food capture. Inflated or resting catch tentacles never accepted food in animals which were feeding with regular tentacles. 3. Genetically different clones of M. senile are often separated by narrow open corridors free of anemones. Specimens of M. senile bearing catch tentacles are found along the borders of these corridors, and in most situations where individuals of different clones are adjacent and within reach of one another. 4. Catch tentacles are used in both intra- and interspecific aggression. Prolonged and repeated feeding tentacle contact between nonclonemates and clonemates where the individuals have been isolated from contact with all other anemones for a period results in the expansion of catch tentacles. Catch tentacle expansion sometimes occurs without an apparent stimulus. Nematocyst discharge of the catch tentacle tip occurs only upon contact with a genetically different individual. After contact and discharge the catch tentacle breaks, leaving about 1 cm of the tip attached to the victim; necrosis at this site follows, and occasionally the victim dies. 5. Catch tentacle formation in individuals lacking them was observed in members of two clones which were confined in close quarters for nine weeks. Feeding tentacles closest to the mouth became enlarged and more opaque, lost their feeding nematocysts and developed nematocysts characteristic of catch tentacles. The transformation was largely completed in nine weeks.