What Are the Most Common Jellies in the Keys?


Cassiopeas frequent mangroves throughout the Keys. Credit: Dauphin Island Sea Lab / Wikipedia Commons

Fortunately, the most common jellyfish in the Florida Keys, the Cassiopeas (Cassiopea fronosa or Cassiopea xamachana) tend to give only a mild sting. Their tentacles are upward facing, giving them a characteristic upside-down appearance. As they are common among mangroves, they are sometimes called the ‘mangrove upside down jelly.’

The cannonball jelly (Stromolophus melegris), another species found in the Keys’ waters, also tends to have little or no sting. Some attest that they don’t feel a sting at all but it’s best to be cautious if you’ve never encountered one. This is the same jelly that is often found washed up on shorelines along Florida's Atlantic coast.

A common springtime jelly is the sea thimble (Linuche unguiculata). In some years, swarms of them appear in the Keys around April and May, and they can resemble an oil slick from a distance. It is the larvae of this species that is thought to cause “sea lice” which is common from Mother’s Day to Father’s Day.


Swarms of moon jellies occur from August to October.

From late August until October, the translucent moon jelly (Aurelia aurita) is a common site. Like Cassiopea and cannonball jellies, its sting tends to be on the mild side for most people.

Though not a true jelly, the Portuguese man o’ war (also called a bluebottle, Physalia physalis) can drift into the Keys and South Florida during the winter, when there are strong winds from the south.

The small, transparent comb jelly – which is both delicate and completely harmless to humans – is a common site in the Keys. Their rows of short, wispy hairs that resemble combs and can emit a stunning greenish-blue glow at night.

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