Chrysaora achlyos, the black sea nettle (family Pelagiidae), is a large scyphozoan jellyfish with a bell exceeding 90 cm in diameter—one of the largest jellyfish in the world—periodically appearing in enormous aggregations off the coast of California and Baja California. The species feeds on zooplankton and small fish and is capable of causing moderate skin irritation in swimmers. Its episodic and poorly understood bloom dynamics make it an intriguing subject for marine ecology research. Genomic data have been generated for this species and are available in public databases, enabling comparative studies of cnidarian evolution and ecology.