The earliest occurrence of venom within the animal tree of life is in Cnidaria, the group that includes corals, sea anemones, and jellyfish. In cnidarians, venom is delivered via microscopic intracellular structures called nematocysts, and is implicated a broad array of physiological functions. The venom of sea anemones is more biochemically stable than that of other cnidarians, and broad-scale studies of its biochemistry and toxicology have shown remarkable diversity in venom proteins. Because the functional specialization of the sea anemone body depends largely on the distribution of nematocysts, the diversification of nematocysts and its venom have likely contributed to the diversification of these animals. Venom itself has been the target of evolutionary change in lineages in which former prey or predators like fish, crabs, or snail have evolved a symbiotic relationship.
Venom mediates interactions between sea anemones and the rest of their communities: it defends them from predators, helps them gain prey, and is modified to support symbioses. Understanding the evolution of venom will impact our ability to predict the reactivity and function of venoms within the groups, but will also help us understand how interactions between organisms become codified in genes and how these genes change in response to changes in the organismal interactions.
