So-called sea lice — actually jellyfish larvae — reported at Jersey Shore

Some cases of seabather's eruption—a sometimes painful skin rash that emerges after recreating in the ocean surf—have been reported in folks spending time at the Jersey Shore, from Cape May on up to Monmouth County, according to the Asbury Park Press.

So what causes this rash? Sea lice. Though the term was coined in Florida in the 1950s, sea lice are not lice (as in insects that hide in your hair) at all but rather the tiny larvae of certain kinds of jellyfish, often the thimble jellyfish, or the larvae of sea anemones. True sea lice are small crustaceans, which are entirely different organisms.

Thimble jellyfish, also known as sea thimbles, are common in the Gulf of Mexico and the Caribbean Sea. But strong currents and storms bring them to the shores of the Southeast and now increasingly along the coast in the Northeast as well.

"In some years, swarms of them appear in the [Florida] Keys around April and May, and they can resemble an oil slick from a distance," according to NOAA National Marine Sanctuaries. "Nearly microscopic, they start as a tingling sensation when they first brush the skin. Later, this develops into an itchy red rash that can last for weeks."

A stretch of beach in Atlantic City, N.J. (FOX 5 NY file photo)

In the case of this summer's Jersey Shore invasion, you can blame Tropical Storm Isaias, which likely carried the microscopic translucent jellies north last week, The Press of Atlantic City reported.

Here is what happens. You go for a dip in the ocean—you body surf, jump around the waves, ride a surfboard, whatever—and the sea lice can get caught in your hair and under your bathing suit. 

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"When you get out, the water drains off of you, but the larvae stay behind," WebMD states in an article. "Your bathing suit rubs against your skin, causing the larvae to sting you and inject their toxin."

Bam—rash.

The treatment for the rash varies by how serious it is. You should take off your swimsuit as soon as possible and then rinse your body, WebMD recommends. After that depends on your personal medical history but various over-the-counter remedies, such as hydrocortisone cream and calamine lotion, may help.