But these sea jellies come and go mysteriously (although we hope they’ll become permanent Shedd residents). Black sea nettles are true giants among jellies, with a maximum bell diameter of 3 feet, a quartet of pink ruffled oral arms that can trail for nearly 20 feet and two dozen white or pink stinging tentacles that can be 25 feet long. (Shedd’s young exhibit-sized nettles are still growing.) Black sea nettles have the distinction of being the largest new invertebrate species identified in the 20th century.

They inhabit the deep, calm waters off North America’s Pacific coast from southern California down through Baja California, feeding on zooplankton and other jellies that come in contact with their stinging cells. At Shedd we raise brine shrimp, mysid shrimp, krill and other natural live foods for them, as well as purchase a commercial frozen planktonic food called Cyclop-eeze, which seems appropriate for such otherworldly animals.

Like other nettles, and all true jellies, black sea nettles have a two-stage life cycle, beginning as a stationary polyp that reproduces asexually to produce tiny jelly-shaped ephyras that grow into the familiar medusas, or bell-shaped mature jellies, that then produce more polyps through sexual reproduction.

Beyond that, we don’t know much about their natural history because they are so seldom encountered in the wild. When black sea nettles do appear in surface waters, it’s in huge numbers called a bloom. Black sea nettle blooms occurred along the San Diego coastline in 1989, 1999 and 2010. Nettles that washed ashore during that first bloom gave biologists a chance to get their hands on this species, although gingerly: Stinging cells remain active even after a jelly has died or the tentacles have separated from the body.

Blooms might be caused by unusually high concentrations of zooplankton, creating a feast for the jellies and optimal conditions for a population explosion. The overabundance of zooplankton, in turn, could be from nutrients such as runoff agricultural fertilizer polluting coastal waters.

Black sea nettles are also rarely seen in aquariums because, special exhibits aquarist Maureen Koneval explains, they are difficult to raise. Only a handful of aquariums have successfully bred this species and raised them from polyps. Shedd received polyps produced at the Henry Doorly Zoo in Omaha. The special exhibit team looks forward to adding this species to the list of jellies being bred at Shedd.  

The black sea nettle is named for its distinctive deep purple bell, but those in Jellies are quite light. Maureen says, “The juveniles will darken up considerably as they mature.”

Whatever the depth of their hue, these deep-sea nettles are a stunning and seldom-seen addition to the Jellies special exhibit.

Karen Furnweger, web editor