Which Jellies in the Keys Can Sting Humans?


As mesmerizing as it is to watch jellies gliding gracefully through the water, no one wants a nasty encounter with their stinging tentacles. Although jellies never intentionally attack humans, their tentacles sting when they brush against swimmers and divers in the water or when they are touched by a curious beachcomber. Their sting comes from a specialized stinging cell, which releases venom in a harpoon-like nettle. The best way to avoid stings is to wear lycra suits, wetsuits, t-shirt or other layers over your skin while in the water.

Each species differs in the extent to which its venom affects humans, and people’s sensitivities can vary. Fortunately, several of the most common jellies in the Florida Keys, such as Cassiopeas, moon jellies, and cannonball jellies, do not inflict a very powerful sting. Some people find they can handle them without feeling anything, but it’s important to be cautious, as people’s sensitivity can vary. Besides, you don’t want a case of mistaken identity to ruin your day out on the water!

Watch out for the drifting groups of tiny larval jelly known in the Keys as ‘sea lice’ or ‘sea bather’s eruption.’ They cause a rash when encountered in the water, mostly during late spring and early summer (think Mother’s Day and Father’s Day), and are usually the larvae of the sea thimble jelly. 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. Sometimes people also develop fever, nausea, or stomach pain. They can be trapped under bathing suit fabric, so you may want to remove that extra layer (T-shirt, rashguard, etc) rather than adding it, to create less surface area for trapping the larvae. Listen to radio reports for warnings of outbreaks and watch for lifeguard signs. Change out of your suit as soon as possible when exiting the water and rinse in saltwater before getting into a fresh water shower. (Freshwater can release the venom in the stinging cells.) Be sure to machine-wash any swimwear that is exposed in hot water and dry on high heat to prevent a reoccurrence next time you wear the suit.

There are other jelly-like creatures in the sanctuary that can sting, such as the Portuguese man o’ war, or blue bottle. Even though it is not a true jelly, it has the same specialized stinging cells, which form 'beads' on their tentacles. They are classified in a separate group than ‘true jellies’ because they are made up of a colony of polyps that specialize in movement, catching prey, feeding and breeding rather than being a single organism. Their name comes from their gas-filled bladder, which resembles the 18th century Portuguese warship at full sail. The bladder looks like a blue balloon and enables the man o’ war to drift on currents by floating at the surface of the water. Their tentacles can sting even after they die, so keep a safe distance if you see one at the beach. They are most likely to be seen in the Keys during a southerly winter wind.

Not all jelly-like creatures can sting. The comb jelly is completely harmless to humans. They are not true jellies, but can be recognized by the fact that their small, transparent bodies are lined with tiny hairs, forming rows that appear like ‘combs.’ These hairs help them prey on tiny plankton but they don’t sting humans, because they lack the stinging cells of true jellies. At night, they can glow with a dazzling greenish-blue bioluminescence that you can enjoy close-up, without worrying about stings.