Upside-down jellyfish on the ocean floor release venom-filled blobs of mucus, which can sting nearby swimmers, new research reveals.

The jellyfish, formally known as Cassiopea xamachana, have a weird, plant-like appearance and they typically are stuck to the ocean’s floor.

According to the Monterey Bay Aquarium, these jellyfish are typically found living in the mangrove forests and lagoons of southern Florida, Hawaii, the Indo-Pacific Ocean and the Caribbean.

When snorkelers are in those areas, they sometimes develop a strange itching sensation.

“You start to feel this tingling … More than just itchiness, like when an itch turns into a painful discomfort,” Cheryl Ames, a museum research associate and an associate professor of applied marine biology at Tohoku University in Japan, explained to Live Science.

Ames and her colleagues believe they have figured out why this happens; their work was published today in the journal Communications Biology.

The jellyfish deploy cellular bombs armed with stinging cells called nematocytes, according to their study. Those bombs release a skin-irritating venom when they make contact with a passing swimmer.

As for how to avoid being stung, researchers are still investigating whether the jellies release the venom more at certain times of the day or in response to certain types of disturbances.