Alexandrium is one of those dinoflagellates you don't want to run into. This harmful alga poisons shellfish and fish that consume the plankton and can kill or paralyze animals as well as people that eat the (shell)fish. When it blossoms in large amounts, the seawater turns red or brown, causing a so-called red tide. However, not all red tides are necessarily Alexandrium or even harmful algae.