Members of the kingdom Dinoflagellata are single-celled, have a long whip-like flagellum, and are encased in a shell or test. They are mostly marine, forming an important part of the plankton. Some photosynthesize and many of these form symbiotic relationships with corals, anemones and clams. Under certain conditions, dinoflagellates can multiply prodigiously and cause "blooms". Some produce powerful toxins that can be poisonous to marine organisms, affecting humans when, for example, shellfish are consumed.