Karenia mikimotoi is one of the most common HAB causing species of dinoflagellate in the North Atlantic region and it is also commonly found around Japan. Blooms of this species are often considerable, reaching over a million cells per litre, and are associated with large marine fauna mortalities. The exact cause of the harmful effect is not yet fully understood, but measurements of cytotoxic polyethers have been extracted from cultures of this species (Silke et al. 2005).