Close Encounter with Giant Sea Nettles

Written by on September 12, 2012 in Marine Life

Editor’s Note–Michael Bear is the former Science Diving Editor for California Diver Magazine and an active AAUS (American Academy of Underwater Sciences) diver for the California Science Center in Los Angeles with over 1,000 cold-water dives.  He is also the author of the MST article “Are Sevengill Sharks Making a Comeback?” from June 21, 2011.  He lives and works in San Diego.


By Michael Bear

In July of 2012, while diving with some friends off the coast of Pt. Loma, San Diego, we were startled to find ourselves surrounded by a swarm of large, Black Sea Nettles, otherwise known as Chrysaora achlyos.

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.

Despite the distinctive nature of this species and its abundance when present, 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.

Giant Black Sea Nettle. Photo credit Michael Bear.

Giant Black Sea Nettle. Photo credit Michael Bear. You can see more of Bear’s sea nettle photos here.

We had been diving along the distinctive underwater geological formations of Pt. Loma which run East/West and as we began to surface, we found ourselves surrounded by these large jellies, some of which had bells approaching 3 feet across, with oral arms nearly 15 feet long, dwarfing some of the divers.  Luckily, some of us had brought cameras, both still and video and were soon busily clicking away at these awe-inspiring animals, as they drifted past us, their large, dark purple bells undulating in the current.

According to the JelliesZone, 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.

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 with large jellies for protection as juveniles.

Although the tentacles are venomous, the few among us whose exposed faces accidentally came into contact with them, reported only mild itching and burning which faded after 45 minutes or so.

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.

Check out this video of a Pacific butterfish sheltering inside the bell:
Video Credit: Barbara Lloyd and Stella Luna Productions


Copyright © 2012 by Marine Science Today, a publication of Marine Science Today LLC.

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