In less than an hour, more than 20 box jellyfish were found yesterday on the shoreline at Hanauma Bay, prompting the city to close the popular snorkeling spot.

Two dozen more were spotted swimming in the shallows. No one reported getting stung.

This morning, lifeguards will assess whether the bay should remain closed.

Yesterday was the peak of the monthly influx, which brought more than 750 box jellyfish onto Waikiki beaches and led to 46 stings. That's three times the number of jellyfish normally found in Waikiki during an influx.

Last month, Hanauma Bay was closed for two days after 18 stings were reported. There were a total of 164 stings reported in April, two at Hanauma.

"Because it's a bay, everything that comes (into shore) gets stuck there," said lifeguard Capt. Kevin Allen. "Even if there's one jellyfish found, it could sting 20 people."

The city closes Hanauma Bay if more than 20 box jellyfish are found on the shoreline or if lifeguards report more than six stings in an hour, Allen said.

It's unknown why this month has seen such an unusually large influx of box jellyfish, Allen said.

"Some months we don't see any," he said. "Even in Waikiki we'll only see a handful. But not this month."

Last month, nearly 300 box jellyfish were collected from the beaches along Waikiki. Lifeguards reported 164 stings, including a 15-year-old girl who was taken by ambulance to Straub Clinic & Hospital after suffering a serious reaction to the toxin.

Yesterday, an 18-year-old woman was taken to Straub in serious condition with complications from multiple stings after surfing at Kewalo's, said Bryan Cheplic, spokesman for the city's Emergency Services Department. On her way home, she stopped at the Makiki fire station, complaining of trouble breathing and swallowing. She developed a slight rash and experienced lower-back spasms.

Box jellyfish wash ashore at night during their spawning cycle, eight to 12 days after each full moon.

On average, about 250 box jellyfish are found along the beaches in Waikiki during the peak of an influx.

Reach Catherine E. Toth at ctoth@honoluluadvertiser.com.