Beachgoers in a South Florida town found their terrirory invaded by little purple jellyfish this past week. Hallandale Beach outside Miami was covered in thousands of sea creatures called Velella velella, which have little sails that normally allow them to steer clear of the shore. However, the jellyfish, which are also known as “purple sailors” or “by-the-wind sailors” can sometimes find themselves blown toward beaches en masse in a phenomenon that locals say happens every three years or so.

“We are flying our Purple flag for dangerous marine life,” the city wrote on its Facebook page Thursday. While not dangerous to humans themselves, velella are often accompanied by the Portuguese Man-o-War, a species that can serious harm humans. FULL REPORT