Little is known about venom in young developmental stages of animals. The appearance of toxins and stinging cells during early embryonic stages in the sea anemone suggests that venom is already expressed in eggs and larvae of this species. Here， we harness transcriptomic， biochemical and transgenic tools to study venom production dynamics in . We find that venom composition and arsenal of toxin-producing cells change dramatically between developmental stages of this species. These findings can be explained by the vastly different interspecific interactions of each life stage， as individuals develop from a miniature non-feeding mobile planula to a larger sessile polyp that predates on other animals and interact differently with predators. Indeed， behavioral assays involving prey， predators and are consistent with this hypothesis. Further， the results of this work suggest a much wider and dynamic venom landscape than initially appreciated in animals with a complex life cycle.