When Is Jellyfish Season In Florida Keys?

The Florida Keys sit in warm, shallow tropical waters between the Atlantic Ocean and the Gulf of Mexico — ideal conditions for several species of jellyfish and their relatives. If you are planning to swim, snorkel, or dive in the Keys, understanding which species are present, when they peak, and what to do if you are stung will help you enjoy the water safely.

When Is Jellyfish Season?

There is no single “jellyfish season” in the Keys because different species peak at different times of year — and some are present year-round. The practical answer for visitors is that August through October tends to be the worst period for moon jellyfish encounters, particularly for snorkelers at the coral reefs around Key Largo, Islamorada, and Key West. Large blooms of moon jellies can make snorkeling nearly impossible during these months, especially after periods of rain. November through April is when Portuguese man-of-war are most commonly blown into Keys waters by winter winds, with February typically being the peak month. The spring months (March through May) can bring thimble jellyfish larvae (commonly called “sea lice”) that cause an itchy rash. In short, something is present in the water at almost any time of year — but the risk and severity varies.

Species You May Encounter

Moon Jellyfish

The most common jellyfish in the Florida Keys. Moon jellies are translucent, saucer-shaped, and typically 6 to 8 inches in diameter, with four horseshoe-shaped reproductive organs visible through the bell. They have short, fine tentacles along the bell’s rim. Moon jellies are present in Keys waters year-round, with the largest blooms occurring in late summer and early fall (August through October). Their sting is very mild — most people who accidentally touch a moon jelly do not feel anything at all. People with sensitive skin may notice a slight irritation. Moon jellies are not dangerous to healthy humans.

Portuguese Man-of-War

The Portuguese man-of-war is not technically a jellyfish — it is a siphonophore, a colony of specialized organisms functioning together as one. It floats on the surface with a blue or purple gas-filled bladder that acts as a sail, while its tentacles trail 30 to 100 feet or more beneath the surface. This is critical: you can be stung by a man-of-war whose visible float is very far away from you. Man-of-war are venomous (not poisonous — they inject venom through nematocysts in their tentacles). The sting is intensely painful and causes welts, and in rare cases can trigger severe allergic reactions, difficulty breathing, or cardiac issues. Man-of-war are most common in the Keys during winter months (November through April), driven toward shore by strong easterly and southerly winds. They are easy to spot on the surface — look for a bluish, translucent, balloon-like float. If you see one, leave the water. Dead man-of-war washed up on beaches can still sting, so do not touch them.

Upside-Down Jellyfish (Cassiopea)

A unique species found in the shallow, warm waters of mangrove areas, seagrass beds, and tidal flats throughout the Keys. Unlike other jellyfish, Cassiopea rests on the bottom with its bell facing down and its branching oral arms facing upward, looking somewhat like a flower or a head of lettuce. It is brownish-green due to symbiotic algae living in its tissues. Upside-down jellies are commonly encountered by waders and kayakers in shallow backcountry areas. Their sting is mild but can cause irritation, especially if you step on one or sit on the bottom near them. They can also release mucus containing stinging cells into the surrounding water.

Cannonball Jellyfish

A dome-shaped jellyfish about 5 to 7 inches in diameter with a smooth, firm bell and a brownish border. Cannonball jellies are good swimmers compared to most jellyfish and wash ashore in large numbers at times. Their tentacles are very short, and their sting is mild — generally not a concern for swimmers. They are sometimes called cabbagehead jellies. In Asia, cannonball jellyfish are harvested and eaten as a delicacy.

Comb Jellies

Comb jellies (ctenophores) are sometimes confused with true jellyfish but are a completely different group of animals. They have a transparent, walnut-shaped body and no tentacles in the stinging sense. Rows of tiny hair-like cilia along their body refract light, creating a shimmering rainbow effect. Comb jellies are bioluminescent — they flash with light when disturbed. Comb jellies do not sting humans at all. They are harmless and are often seen in Keys waters at night.

Pink Meanie

A relatively recently described species (identified as a new family of jellyfish in 2000), the pink meanie can grow very large — bells up to 3 feet in diameter with tentacles stretching dozens of feet. As the name suggests, it is pinkish in color. The pink meanie is notable for being a predator of other jellyfish, particularly moon jellies. Its sting is painful to humans. Pink meanies appear in the Keys primarily during winter and early spring.

Sea Lice

“Sea lice” in Florida waters are the larvae of thimble jellyfish (Linuche unguiculata). They are nearly invisible to the naked eye and become a problem when they get trapped between your skin and your swimsuit, rash guard, or hair. When compressed against skin, they fire their tiny stinging cells, causing a condition called seabather’s eruption — an itchy, red, bumpy rash that typically appears hours after you leave the water and can last several days to two weeks. Sea lice are most common in Keys and South Florida waters during spring and early summer (March through June). If you swim through an area with sea lice, shower immediately after leaving the water and remove your swimsuit as soon as possible. Machine-wash the suit with detergent before wearing it again.

What to Do If You Are Stung

For a moon jellyfish sting (mild irritation): rinse the area with seawater (not fresh water, which can cause remaining nematocysts to fire). Apply vinegar to neutralize stinging cells. The discomfort typically passes within 30 to 40 minutes.

For a Portuguese man-of-war sting (severe pain, welts): carefully remove any visible tentacle fragments using tweezers or a gloved hand — do not touch with bare skin. Rinse with seawater. Apply vinegar or hot water (as hot as you can tolerate without burning, approximately 110°F/45°C) to the affected area for 20 to 45 minutes. Over-the-counter pain relievers and antihistamines can help with pain and swelling. Seek medical attention immediately if the person experiences difficulty breathing, chest pain, severe swelling, nausea, or signs of an allergic reaction.

Do not apply urine (a persistent myth with no medical support), fresh water, ice directly on the sting, or alcohol. These can worsen the sting by triggering unfired nematocysts.

Practical Tips for Visitors

Wear a rash guard or thin wetsuit when snorkeling or diving — this is the single most effective protection against accidental stings. Check for purple warning flags on lifeguarded beaches, which indicate dangerous marine life has been spotted. Before booking a snorkel tour, ask the operator about current jellyfish conditions — reputable operators in Key West, Key Largo, and Marathon monitor conditions daily and will advise you honestly. Look at the waterline on the beach before entering — if you see man-of-war or other jellyfish washed up on the sand, they are likely in the water too. Avoid swimming near floating seaweed, which can harbor sea lice and juvenile jellyfish. If you are snorkeling in August or September, be prepared for the possibility of moon jelly blooms that may cut your snorkel time short — some visitors find wearing a full-body dive skin makes all the difference.