The Science of Sea Lice

May 29, 2001 -- 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 through 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.

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 all — must be they don't like 78-year-old skin."

Swim Naked

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.

In 1903, one Miami resident recalled, "[W]e were all poisoned … with some kind of rash which set up an intense itching. It was not so bad for us as we could stay home and doctor ourselves with lotions but the poor men having to work in the fields or hot packing houses were the ones who really suffered," as recorded by sea lice researcher Mary Russell in a Florida Atlantic University study.

Although people have experienced unpleasant encounters with sea lice for years, it's only recently that scientists have begun to trace the cause to species of the thimble jellyfish.

Like most jellyfish, thimble jellyfish exist in three different stages: as larvae, polyps and adult, floating medusas. The form that causes most trouble to swimmers is the larval stage of the animal, which is released by the millions into waters by spawning adult female jellyfish.

A Speck-Sized Weapon

A thimble jellyfish larva, writes Russell, "appears like a speck of finely ground pepper. Its size is approximately that of a pinhead floating on the surface layers of the water."

Once swimmers inadvertently traps the young jellyfish in their swim caps or bathing suits, the larva fire off stinging cells.

The stinging cell is made up of a capsule with a sensory hair, a lid containing a nematocyst, which is designed to sting and subdue prey. When the sensory hair brushes against another animal, the nematocyst fires from the capsule like a harpoon within milliseconds.

An itchy, stinging rash from the nematocysts usually appears about a day later.

Russell reports it's not uncommon to see evidence of 200 or more stings under a person's bathing suit.

Apply Oatmeal

So what to do if you end up a target of these tiny stinging cells?

First, says Russell, after swimming in the ocean always take a freshwater shower with your bathing suit off (leaving it on can actually lead to more stings).

If a sea lice rash appears, Russell suggests smearing cooked oatmeal or a calamine lotion on the outbreak to ease the itchy pain. Over-the-counter treatments are also available.

As Allen points out, despite its simplicity, the thimble jellyfish should never be underestimated.

"These critters are ancient and very low on the evolutionary scale," he says, "but simplicity can be very effective."