While conducting a survey of sea urchins off the coast of San Diego, Mike Bear and fellow scientific divers found themselves in a most unexpected situation: surrounded by a swarm of giant black sea jellies, some as long as 15 feet (4.5 m). The divers drifted among the rare creatures, capturing footage of them and their symbiotic partners. Mike Bear describes the experience and what is known about this elusive species.

A giant black sea nettle encountered by divers off the coast of San Diego, California (Credit: Mike Bear)

In July of 2012, while doing a marine life survey of sea urchins off the coast of Pt. Loma, San Diego, to assist local marine biologists who track urchin populations in local kelp forests, we were astonished to find ourselves surrounded by a ‘swarm’ of giant black Sea Nettles, large, pelagic sea jellies otherwise known as Chrysaora achlyos—some of them over twice the length of an average diver.

Looking towards the surface, we could see dozens of them, ranging in size from a mere 3 feet (91 cm) long to much larger adults up to 15 feet (4.5 m) long, their tentacles gently drifting in the current high above us.

Since some of us had video and still cameras with us, we deliberately surfaced in their midst, to allow them to drift past us, so we could photograph these remarkable and rarely seen giants of the sea.

These giant sea jellies, some as long as 15 feet, dwarfed the divers in their midst. (Credit: Mike Bear)

Zealous divers, venomous tentacles

In our zeal to get close-up video of these jellies, some of us accidentally came into contact with the venomous tentacles as they drifted by, but because the venom is comparatively mild, we noticed only mild itching and burning which faded after 40 minutes or so. Most of us were wearing dry suits and gloves which provided fairly good protection everywhere except in the small gap between our hoods and masks.

As the Wikipedia entry for this species explains, “Each nettle tentacle is coated with thousands of microscopic nematocysts; in turn, every individual nematocyst has a ‘trigger’ (cnidocil) paired with a capsule containing a coiled stinging filament. Upon contact, the cnidocil will immediately initiate a process that ejects the venom-coated filament from its capsule and into the target. This will inject toxins capable of killing smaller prey or stunning perceived predators. On humans, this will most likely cause a nonlethal, but painful stinging sensation which can last for forty minutes.”

According to the Monterey Bay Aquarium, 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.

No other United States West Coast jelly that visits nearshore waters has the same dark pigmentation, according to The JelliesZone website.  It has four gonads, 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. It is thought that these sense organs are primitive light sensors, precursors to eyes.

"Rarely seen...whereabouts unknown"

Despite the distinctive nature of this species and its abundance when present—past swarms have been known to include dozens of sea nettles—it was only recently officially described (in 1997), and is actually the largest invertebrate to have been described in the 20th century.

These large jellies are rarely seen and their whereabouts most of the year are mostly unknown, although they have made two prior appearances in Southern California: once in 1989, and then, again, ten years later in 1999.

The giant black sea nettle was only officially described in 1997.

We floated along with them in the current for over an hour, our cameras clicking away, until the air in our tanks ran low, fascinated by our encounter with these amazing animals.

We were lucky enough to be able to document the fact, via video footage, that the silvery fish inside the bell of one of these animals was a Pacific butterfish (Peprilus simillimus), which frequently associates symbiotically with large jellies, for protection during its juvenile stage.

Despite the fact that these large jellies have made two prior appearances off the coast of California, very little is known about their life cycle.  We considered ourselves fortunate to have had this rare encounter.

Video showing a nettle with its symbiotic partner, the Pacific butterfish, inside its bell.
(Courtesy of Barbara Lloyd, Stella Luna Productions)

Find out more

"Rare Black Sea Nettles", Shedd Aquarium
http://interactive.sheddaquarium.org/2012/07/rare-black-sea-nettles.html
Jellywatch, a page for recording sightings of jellyfish and other marine organisms
http://jellywatch.org/

This article was originally published on Nov. 7, 2012.