The black sea nettle is considered a giant jelly; its distinctive purplish bell can reach over three feet (91 cm) in diameter; its lacy, pinkish oral arms can reach nearly 20 feet (6 m) in length and its stinging tentacles 25 feet (7.6 m) or more. It probably lives in deeper, calmer waters but has appeared in large blooms in California coastal waters, most recently in 2010.
Giant black sea nettles appeared in droves along the San Diego shoreline in the summer of 1989. Then they mysteriously disappeared. The giant drifters reappeared again ten years later, in the summer of 1999. Increased numbers of sea nettles may be an indication that human activities have changed the condition of the ocean. Increased organic material means more nutrients. More nutrients, plus fertilizers from farms, enrich the plankton, providing more food for jellies and allowing them to increase in number. It is likely that the appearance of black sea nettles in coastal California waters is also related to El Nino/La Nina events.
The black sea nettle provides the Pacific butterfish with food and protection. The silvery butterfish feeds on the plankton gathered by the jelly, and when danger approaches, the butterfish actually hides inside the jelly's bell.
The black sea nettle is a mysterious creature; during most years its whereabouts are unknown. Scientists just recently named this jelly in 1997, although pictures of the species were taken as early as 1926. Much about its behavior, distribution and life cycle remain a puzzle.