The west coast of North America is home to one of the most mysterious stinging nettles known, the black jelly, Chrysaora achlyos. Adults are huge, a deep dark purple or black, and very rarely seen. Blooms for this species were recorded only 4 times in the 20th century, in 1926, 1965, and 1989 and 1999 . In fact, when scientists described the wandering goliath in 1997, they chose the latin name Achlys– a Greek figure representing “mist, darkness,and obscurity” . Despite its evanescent visits, the jelly itself is hard to miss: the bell can reach 1 meter (> 3 feet) in diameter, with oral arms extending up to 6 meter (~ 20 feet) in length.
The black jelly is closely related to the purple striped jelly, indicated by both DNA and husbandry data. Most stinging nettles can only be housed with other member of their species, because they will eat more distantly related jellies. Black and purple striped jellies can be kept in the same tank, which means they must see each other as being members of the same species. Their DNA sequences are also so similar for some genes it is impossible to distinguish between them [Keith Bayha, personal communication]. One might be tempted to say these animals are different colors of the same species. However, color is not the only thing to distinguish them, they also have differences numbers of tentacles– even as huge adults the purple striped jelly only has 8 tentacles, and the black jelly has 24 . And these differences aren’t likely due to possible differences in environmental conditions, they’re also heritable. Black and purple striped jellies growing under identical conditions in aquariums still develop different colors and tentacle number. More than likely, these jellies share a very recent common ancestor, but have evolved into separate species.
Little is known about the life of wild black jellies. In captivity they feed on rotifers, brine shrimp and moon jelly, but their natural dietary preferences remains a mystery. Their polyp stage has also never been observed in the wild. Scientists have no idea where these animals spend most of their lives.
While the black jelly may appear ominous, it is not considered as dangerous as its cousin the Atlantic stinging nettle, with scientists commenting:
[the black jelly's] venom is not as toxic to mice, erythrocytes or hepatocytes as that of its smaller Eastern states cousin. This lesser potency appears to be fortunate for the unsuspecting Californian swimmers who might contact this large beast 
However, it does still produce some sting, and the intensity may vary from year to year. If you’re lucky enough to see this species, watching it from a distance is probably the best way to enjoy this bizarre and beautiful jelly.
 Husbandry of the black jelly (Chrysaora achlyos), a newly discovered scyphozoan in the eastern North Pacific Ocean
 Chrysaora achlyos, a Remarkable New Species of Scyphozoan from the Eastern Pacific
 Toxinological studies on the nematocyst venom of Chrysaora achlyos
 A preliminary phylogeny of Pelagiidae (Cnidaria, Scyphozoa), with new observations of Chrysaora colorata comb. nov.