Changes of cnida composition during planula development of a reef-building coral Acropora nasuta

Abstract

Changes of cnida composition during larval development are described for a scleractinian coral Acropora nasuta. Two types of cnidae, a microbasic b-mastigophore nematocyst and a spirocyst, were observed in planulae; whereas, four types were identified in polyps: microbasic p-mastigophore, holotrichous isorhiza, and the two types of cnidae found in planulae. The cnidae of planulae appeared 3-4 days after fertilization and gradually increased in number until the 8th day. The appearance of cnidae 3-4 days after fertilization and the maximum number of cnidae on the 8th day after fertilization coincided with the period of first settlement and maximum settlement, respectively. On the 15th day of in vitro culture, the number of nematocysts remained, but the spirocysts decreased until they were virtually absent in the larvae. The spirocysts might be used up during the repeated trial of settlement. We supposed that the nematocysts act as a defense against predators, whereas the spirocysts were used for initial attachment to the substrata. The number of cnidae will be a useful indicator to find larval maturity.