They have no brains, no blood and are made up of nearly 99 percent water, but they still pack quite a sting.
That's what ocean swimmers, particularly off the Florida coast, are prone to discover this summer until about July. Lifeguards stationed at beaches across the Sunshine State have been hoisting blue flags as a warning in a season that marine safety officials describe as particularly "itchy."
"It's been pretty bad this year," says Rob Caldwell of the marine safety department at Florida's Lantana Beach, describing this season's incidences of what many call sea bather's eruption or sea lice. "People are getting stung all over the place." Unsuspecting swimmers who pass through an invisible swarm of thimble jellyfish larvae often end up with a nasty, itchy, acne-like rash, usually in the most awkward spots where their bathing suits cling to their bodies.
Scientists believe the stinging larvae can infest warm coastal waters such as in Florida or the Caribbean, particularly when water temperatures read between 76 and 82 degrees Fahrenheit (24 and 28 degrees Celsius).
Just last week Ray McAllister, a retired marine biologist with Florida Atlantic University, narrowly missed an unpleasant brush with the larvae while diving near Palm Beach County.
"A lady I was with got an awful rash around the straps of her bikini," he says. "I wasn't stung at allmust be they don't like 78-year-old skin."
Experts say the best way to avoid the thimble jellyfish's sting is to listen to reports and don't go in the water if they're expected. Others suggest wearing a waterproof moisturizer such as zinc oxide or thick layers of Vaseline to block the stings. Israeli researchers have also developed a lotion that, they say, borrows from mechanisms found in the clown fish and can prevent jellyfish stings.
Another option (if it is an option) is to swim naked. As the rule goes: No suit, no sting.
"They usually only sting when they get caught between you and your swimsuit and they get squished," explains Doug Allen, a jellyfish expert at the National Aquarium in Baltimore. "That's when their matecysts, the cells that act as stingers, fire off."
History books have long recorded the uncomfortable results of encounters with so-called sea lice.