Most Common Types Of Jellyfish in Florida Keys – Are They Dangerous?

Jellyfish live in oceans all over the world, so they frequent the waters of Sunshine State too. Although most jellyfish in Florida Keys have no or mild stings, it is good to learn how to identify them so you will know which types are harmful and which are dangerous. Make your Florida Keys snorkeling experience safer by getting to know the area’s common jellies!

Are there jellyfish in Florida Keys?

Several types of jellyfish are present in the Keys waters. Their movement is driven by current and wind patterns; therefore, they are often brought into shallow lagoons, close to the shore and even get washed up on the beaches, mainly along the Atlantic coast but on the Gulf side too.

Consequently, it has a high possibility that you encounter jellyfish at Florida’s beaches and around the offshore coral reefs too.

Types of Florida Keys jellyfish

Moon jellyfish

Moon jellyfish is a common Florida Keys jellyfish. They appear around the Keys year-round but in higher volume during the late spring and summer months. These purple jellyfish are about 6-7.8 inch (15-20 cm) in diameter. Moon jellies have disk-shaped, highly transparent bodies and small, 1-5 cm long tentacles so in clear water they are nearly invisible.

Although this species is pretty big and can occur sometimes in large numbers, they are not dangerous. Even if they sting humans, their stinging power is so mild that only those who have very sensitive skin can feel it. Most people do not feel anything at all even if they touch it (which you should not be supposed to do but in case of accidental contact, there is no need to worry).

Portuguese man-o-war

An otherworldly creature, the Portuguese man-o-war has a gas-filled bladder that makes it float at the surface. Therefore, it is called blue bottle or blue balloon jellyfish too. It has long tentacles that grow up to 30-100ft/10-30m. There are venom-filled stinging capsules (nematocysts) along the tentacles.

Although the Portuguese man-o-war is called a jellyfish, in reality, they are not true jellies but a colony of polyps. This species appears around the Keys and Southern Florida when there are strong southern winds, typically in wintertime.

The Portuguese man-o-war is one of those types of jellyfish in Key West and Key Largo that you need to avoid. Although their sting is rarely fatal, it is very painful.

Cannonball jellyfish

Another very common jellyfish in the Florida Keys is the dome-shaped Cannonball jelly. It has a solid, yellow body with a brown border and is approximately a size of a cabbage; due to its size and look, it is often called Cabbagehead jelly too.

Cannonball jellies are harmless; their tentacles are short so contacting them is rare, and even if it happens, their sting is mild. However, it can cause discomfort and various symptoms if you are allergic or sensitive to its venom so avoiding bumping into one is still recommended.

These jellies are good swimmers and are found not only around the Keys but up to Pensacola in the Gulf of Mexico. They are often washed up on beaches in large numbers.

Author’s note: An interesting fact is that Cannonball Jellyfish are edible and consumed as a delicacy in Asian cuisines.

Cassiopea upside-down jellyfish

An unusual, cauliflower-like creature that frequents Florida’s waters. Instead of swimming, Cassiopea jellies spend is usually found pulsing upside-down with their tentacles facing upward in well-lit waters.

In the Keys, they usually appear around shallow seagrass meadows and mangroves. Their sting is mild and only sensitive people will actually feel it.

Sea Thimble/Sea Lice

A small jellyfish in Florida, the Sea Thimble is common during springtime. It is actually not the jellyfish but its larvae (known as “Sea Lice”) that can cause irritation and mild itch. These symptoms should not last long but those who have sensitive skin might experience redness and a burning sensation.

Luckily, swimming into sea lice is easy to avoid as their presence can be recognized; it looks like an oil patch on the surface and usually they colonize around seaweed therefore can be seen from a distance.

Comb jellyfish

Comb jellyfish are small, oval-shaped jellies with comb-like plates. Their translucent body retracts light making a rainbow effect. They can also produce greenish-blue light when disturbed; this phenomenon is called bioluminescence.

Although comb jellies are common in the Keys, they are not dangerous to humans. In fact, they are also not true jellyfish as lack the stinging cells.

Box jellyfish

Various types of box jellies can be found in the Keys and in South Florida. The first sightings were reported in 2009 and they became more widespread since then, but fortunately, encountering these jellies is still rare.

The most known box jellyfish species is the Australian Chironex fleckeri (sea wasp) which is extremely venomous. Its sting can cause paralysis and cardiac arrest or can be even fatal. Florida’s box jellyfish are not deadly but have painful stings. Of course, you should avoid contacting them as new, more poisonous types can appear at any time.

When is the Florida Keys jellyfish season?

In the Florida Keys, the jellyfish season is typically between late August through early April. Usually, they can be found in temperate coastal waters and shallow areas that’s why it is easy to encounter them while snorkeling in Key West or discovering Key Largo snorkel spots.

The season might start earlier and last longer as ocean temperatures are rising. The worst months are August and September, so if you do not feel comfortable being in the ocean when there is a high possibility of swimming into big patches of jellyfish or do not want to risk canceling your snorkel trip because of the presence of jellies, the best is avoiding visiting the area during these months.

What to do if you get stung by a jellyfish?

In most cases, you will experience a stinging-itching sensation and redness, moderate pain and maybe mild headache, but such symptoms should resolve in a few hours-days. If severe reactions occur (nausea, vomiting, muscle pain, dizziness or shortness of breath) or you know (or suspect) that you got stung by a dangerous jellyfish, seek medical help immediately.

As always, prevention is better than cure so wearing a full-body snorkeling rash guard (that provides protection not only against sunrays but against stingers too at a certain level) and monitoring the local conditions is highly recommended.

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