The search for the origin of cnidarian nematocysts in dinoflagellates

The phylum Cnidaria is thought to be unique among animals as it contains a nematocyst or cnidocyst, which is a stinging organelle used for prey capture, defense and movement. Questions have been raised regarding the evolution of nematocysts since nematocyst-like structures can be found in some protists. In particular, the nematocyst of Polykrikos kofoidii or Polykrikos schwartzii structurally resembles the stenotele of Hydra at the electron microscope level. Both structures not only share morphological resemblance, but also the manner of catching prey. Evidence also suggests that a part of the Polykrikos nematocyst is assembled with a protein similar to Hydra minicollagen. In this chapter, we summarize our findings to date and discuss the evolutionary processes that may underlie the similarities of the nematocysts of Polykrikos and Hydra.