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Abstract:The developmental changes in cnida composition of the coral Pocillopora damicornis was studied. At least two types of microbasic p-mastigophores (type I and type II MpM), large and small holotrichous isorhizas (HI), microbasic b-mastigophores (MbM) and spirocysts were observed in P. damicornis. The nematocyst composition of planulae was similar to that of one or two day old primary polyps, but markedly different from that of adult colonies. The large HI were observed only in planulae and primary polyps. MbM were most abundant in planulae and primary polyps while type I MpM were most abundant in adult colonies. The size of MbM and type II MpM was different between planulae or primary polyps and adult colonies. While large HI were retained at least for one week after settlement, spirocysts appeared to decrease temporarily after settlement. The present observations suggest that large HI are used for defense against predators at early stages of development and spirocysts are used for attachment to the substrate during settlement. The present study showed that the cnida composition and the dimensions of certain types of nematocysts change during the course of development of P. damicornis.