Posted on: Monday, April 4, 2005

By Jan TenBruggencate
Advertiser Science Writer

Q. Why is it that box jellyfish come to the leeward Hawaiian beaches each month 10 days or so after the full moon? Seems to me the jellyfish are too large to have been born just 10 days ago. Also, there seems to be no tidal fluctuation specific to the lunar cycle that would draw these animals toward the shore 10 days after the full moon. — John Sheppard, Honolulu

A. Lifeguards regularly warn Hawai'i beachgoers about box jellyfish, which appear on leeward shorelines monthly like clockwork, or almost like clockwork, since they can sometimes be a day or two early.

These jellyfish, which scientists say are generally about 2 months old when they come to shore, are arriving to participate in a mass spawning, after which they die.

You are right that they are not responding to the tides. They are strong swimmers and can swim against tides, but scientists don't know what puts them on their specific time schedule, said Waikiki Aquarium curator Jerry Crow. "We do think it is part of their reproductive cycle," he said. "When they come into shore they're not feeding, because some of them don't have any stomach contents."

Actually, there's a lot about these creatures that scientists don't know. Two reasons are that the jellyfish are hard to track and their consistency makes tagging them a problem.

Jamie Seymour, an Australian researcher who studies the jellyfish in Hawai'i and Australia, said he believes the same species shows up on Australian coastlines at roughly the same time, although there are others who believe the Hawaiian and Australian animals are different species. He believes the animals use the phase of the moon as a cue that it's time to spawn.

The box jellies, known in Hawai'i by the scientific name Carybdea alata, have a translucent bell 2 to 3 inches high and as much as 2 inches in diameter, with four stinging tentacles that can grow to a length of 5 to 6 feet.

In their 1997 book "All Stings Considered: First Aid and Medical Treatment of Hawai'i's Marine Injuries," authors Craig Thomas and Susan Scott said box jellyfish stings can cause different reactions in different people. They cause severe pain or a rash and can last hours to days. It also affects some people in the way that bee and wasp stings can, prompting an allergic reaction that can be life-threatening and require medical attention.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766; e-mail jant@honoluluadvertiser.com; or call (808) 245-3074.