After a year's absence, the jellyfish have reappeared in the waters off Italy's southern Tyrrhenian coast just as swimmers were eagerly anticipating the start of the beach season, according to ANSA. What was formerly a 12-year cycle of jellyfish infestations now has happened nearly every year since 2003, leading some to believe climate change is playing a part, the Italian news agency said.

Silvio Greco, scientific director at the Institute for Applied Marine Research, told ANSA he's concerned the creatures' vast numbers are upsetting the ocean's balance.

"This is confirmation that something is going wrong in the marine ecosystem," he said. "The jellyfish are feeding on fish larvae, and that's also going to have serious consequences for the fish."

The pinkish Pelagia noctiluca jellyfish gives off a greenish glow in the dark and can grow as wide as 6 inches, while its tentacles can stretch out to more than 13 feet. Its sting can be very painful.