What is Pfiesteria?

Pfiesteria are a kind of dinoflagellate. In the 1980s & 1990s, they were believed to have killed huge amounts of fish off the North Carolinian coast and near the Chesapeake Bay. It was initially believed that they killed their prey by releasing a toxin, but scientists have not been able to isolate this toxin & now are unsure whether it truly exists.