Hawaiʻi beach goers are most likely to encounter the painful and sometimes toxic sting of box jellies 8–10 days after the full moon. Now the University of Hawaiʻi’s Waikīkī Aquarium has made it easier to calculate just when that is with a box jelly calendar on the aquarium website.
Box jellies are named for the squarish shape of their transparent, bell-shaped bodies. Four thin, pinkish tentacles that trail from the “corners” of the transparent body, loaded with barbed stingers. The jellies are active predators, capturing small fishes and crustaceans with their potent sting.
According to the aquarium education department, box jellies are usually found in quiet, shallow waters of protected bays and estuaries and over sandy-bottomed shorelines. Box jellies apparently descend to deeper water during daylight hours, but during summer months, adults are often reported at the surface.
While the sting of Hawaiʻi’s box jellies (scientifically Carybdea alata, C. rastoni and C. sivickisi) is not usually lethal, it is reported to be more painful than that of the more common Portuguese man-of-war. However, sensitivity varies from individual to individual; some people may be highly allergic and require a physician’s care.
The largest of the locally occurring species, C. alata is identified in the recurring “invasions” reported on Oʻahu shores since late l988. Local water safety officials teamed with a marine scientist to find the pattern of occurrence.
More than a year of data revealed a lunar and tidal pattern. The present theory is that when certain high tides occur at night, the jellies are carried over the reef and into the shallows along the beach. In most cases, the jellyfish “invasions” occur about 8–10 days after a full moon.
Select Waikīkī Aquarium profiles of marine creatures, including box jellies, to download.