Moon Jellies: mass stranding near San Francisco

Just this past September I reported on a sea jelly invasion in Monterey, California. For a good portion of the late summer, the popular central California ocean community was beset with west coast sea nettles - beautiful to look at but potentially painful, given the numbers that were accumulating from the bay right up to the shoreline.

Another high concentration of sea jellies recently occurred at Ocean Beach, located near San Francisco. This time it was thousands upon thousands of moon jellies that had washed up on the beach overnight this past weekend. But in this instance, it was not so much an invasion as it was a freak occurrence linked to a cyclical event.

Moon jellies are one of the more prolific of all jellies. A popular attraction at many aquariums, they are relatively easier to breed, compared to other species, and do not pose a threat to humans. In the fall, they can have large spikes in their populations along the eastern Pacific coast. These sea jelly "blooms" can go unnoticed for the most part, unless the right tidal and wave conditions push the floating moon jellies ashore. Which is apparently what happened this past weekend.

Gary Williams, a researcher of invertebrates at the California Academy of Sciences described it as a "regular event," “Jellyfish cluster in massive blooms well offshore that we rarely see. But sometimes, with just the right combination of wind and currents, those blooms wash ashore.”

As relatively common as the moon jelly is, there is surprisingly little we know about them. The when, where, and how of their reproduction habits is still predominantly a mystery. Because of that, it makes it difficult for scientists to attribute any apparent rise in population to outside factors such as climate change.

However, further south from San Francisco in San Diego, California, a rare black sea nettle has been making more frequent appearances along that Southern California coast. Actually deep purple in color, the black sea nettle is much larger than the west coast sea nettle, reaching 3 feet across and with up to 30 feet of trailing tentacles.

Though not yet seen in invasion-size numbers, the black sea nettle, according to some oceanographers, could be increasing in numbers due to warming temperatures or a consistent increase in the primary food source, plankton - which also "blooms" with warmer water temperatures.

Beautiful, fascinating, and in many ways a mystery, sea jellies are basic predatory invertebrates (and, technically, by definition not a fish - as in "jellyfish"), simple in structure with no brain or heart but exhibiting behaviors that have yet to be fully explained.

Read about San Francisco's moon jelly stranding.
Read about San Diego's black sea nettle.