Qualitative shift to indirect development in the parasitic sea anemone Edwardsiella lineata

Direct development lies at 1 end of a continuum that encompasses various degrees of indirect development. Indirect development exists where a larval stage is interposed between the embryo and the adult and undergoes metamorphosis, though the ecological and morphological distinctiveness of the larval stage relative to the adult stage can vary tremendously. There are numerous empirical examples where direct development has evolved from indirect development, but little empirical evidence describing a recent transition from direct to indirect development. Here, we suggest 4 criteria for defining indirect, and therefore metamorphic, life histories. We then apply these criteria to address the planula-polyp transition in cnidarians, focusing on 2 species in the anthozoan family Edwardsiidae. The lined sea anemone, Edwardsiella lineata, has made a qualitative shift towards indirect development that coincides with, and was potentially facilitated by, the evolution of endoparasitism. We compare E. lineata's development with that of a closely related sea anemone, Nematostella vectensis, where the nonfeeding planula gradually develops the morphology of the adult polyp. In E. lineata, a novel parasitic life history stage is interposed between the planula and the polyp. We discuss how the evolution of endoparasitism could facilitate the evolution metamorphic life histories.