Black jellyfish or the black sea nettle, Chrysaora achlyos, Sarlacc jellyfish is a species of jellyfish that can be found in the waters of the Pacific Ocean. Its range is thought to be from Monterey Bay in the north, down to southern Baja California and Mexico, though there are reports of sightings as far north as British Columbia.
The black sea nettle can be quite massive, with a bell diameter potentially up to 1 meter and oral arms extending to 5 or 6 meters. The bell color is a distinctive opaque dark purple to nearly black, with the margin having a lighter brown reticulated pattern. No other West Coast jelly that visits nearshore waters has this dark pigmentation. Four gonads are attached to finger-like projections that extend through subumbrellar openings (the ostia). Marginal sense organs are spaced around the bell margin after every set of 3 tentacles, for a total of 8. Black sea nettles are occasionally seen in large numbers in surface waters off the coast of Baja California and southern California. Large swarms have occurred most recently in 1989 and 1999. During most years their whereabouts are unknown. Despite the distinctive nature of this species and its abundance when present, it was only recently officially described, and is actually the largest invertebrate to have been described in the 20th Century.