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A new report from University of Hawaii researchers sheds light on the behavior of box jellyfish near Waikiki, where the gelatinous creatures have flourished in recent decades and now sting thousands of ocean-goers each year.

It’s actually a lack of light that triggers the jellies’ “clockwork” migration into the shallow waters right off that popular visitor destination, according to the , which appears in the June issue of .

Researchers found that the creatures swim toward shore on the nights in the lunar cycle when there’s an especially long period between twilight and moonrise – and that they do so in order to spawn. The box jellyfish reliably appear near shore eight to 10 days after the full moon. The study, however, looked at why that is and what causes them to do so.

“These events have been called ‘strandings’ or ‘beachings’ which connote a passive event perhaps out of the organism’s control,” the UH researchers write. “However, animal behavioral observations … consistently demonstrate that the animals spend a certain amount of time actively swimming in close proximity to each other close to shore.”

The study was led by longtime local jellyfish researcher Angel Yanagihara, an associate research professor at the university’s School of Ocean and Earth Science and Technology and the John A. Burns School of Medicine.

“The period of the lunar cycle with a key number of hours of darkness or the ‘absence of light’ cues mature animals to specifically swim to the shore line to spawn,” Yanagihara said in a statement Wednesday.

Further, Yanagihara and the other researchers pinpointed where the box jellyfish congregate off south Oahu when they’re not swimming and moving toward shore. They generally live in an area about a mile straight out from the Kapahulu groin, where a circular current “corrals” the box jellyfish and the creatures they feed on, Yanagihara said in a follow-up interview.

The decade-long study involved overnight boat trips and scuba dives off Waikiki to monitor the box jellyfish, as well as computer modeling and other methods, according to a UH release.

The study specifically examined the Alatina alata species of box jellyfish, which is found in Hawaii and other tropical waters in the Pacific Ocean and Caribbean Sea.

Of more than 40 species of box jellyfish, it’s the only one whose migration and spawning patterns follow this clockwork lunar cycle, Yanagihara said.

The report also said that since the 1980s the “number and frequency” of the box jellyfish migrations have ramped up in and around Waikiki. While their appearance used to be sporadic, now it occurs regularly each month thanks to a notable growth in the local creatures’ population.

This could have been caused at least in part by a series of intentional sinkings of old, metal ships off Waikiki, as well as a nearby plane wreck in those waters, whose presence there benefited the local scuba-diving industry, Yanagihara said.

Those wrecks created artificial reefs that allowed the existing box jellyfish population to explode, she added. “You suddenly have all this new real estate to attach to,” Yanagihara said.

“Unanticipated blooms” in local jellyfish populations have been linked to similar underwater wrecks in other parts of the world, she said.

Now, there are “hundreds of thousands” of stings in those shallow waters around Waikiki each year, many causing serious injury, according to the study.

The prevalence of those stings makes the research into the box jellies especially important, Yanagihara said.

Human activity and profound changes to the environment in Waikiki have also helped drive the explosive growth in box jellyfish there, Yanagihara said. Such changes include a decrease in the turtle and fish species that usually keep the box jellyfish numbers in check.

Those factors are also helping the jellies’ population to grow in other areas, such as Kailua, where they’ve become more of a problem, she added.

The study notes that there’s a “significant challenge” in the coastal management of Waikiki, which is the busiest tourist hub in Hawaii, to help reduce the public health and safety threats posed by the box jellyfish.