The Most Common Types of Jellyfish
In Key West, you’re most likely to see moon jellies at the beach and offshore at the coral reef. This type of jellyfish has a translucent white, saucer-shaped bell with a blue-gray transparent disk at the center through which the horseshoe-shaped gonads are visible. Short, delicate, fringe-like tentacles hang from the bell margins. When deprived of food, they can shrink to one tenth of their original size to save energy; they redevelop to normal size when food is available. While they’re not poisonous, you’ll definitely know if you brush up against one because they’ll produce a sharp, uncomfortable stinging sensation. Moon jellies are found in Key West’s waters year round, however the seasons of late spring and summertime see them in the highest volume.

The Portuguese man-o-war is what you really want to avoid. These bad boys are venomous, leaving extremely painful stings that in rare cases can be lethal. With a translucent, sail-shaped bell, man-o-wars float on the ocean’s surface, resembling a plastic bag. Their deep blue and purple tentacles trail beneath them up to six feet long. The good news is that if you’re alert, they’re easy to spot on the surface and avoid. The bad news is, those tentacles can sneak up on you when you least suspect them. If man-o-wars are known to be in the water, it’s a good idea to just stay out. They’re truly unpleasant creatures to come into contact with. Fortunately, we only see them in Key West during certain tides in the wintertime.

The comb jelly looks different from other jellies because it’s not made up of a bell and tentacles. Instead, it is a translucent walnut-shaped body with wart-like bumps. For this reason, it’s sometimes called a sea walnut. Comb jellies are translucent but refract light, appearing to have rainbow colors running down their bodies on the track of internal moving cilia. They can also make their own light (bioluminescence), flashing when disturbed.

The upside-down jelly does not look like the typical jelly, appearing as a flower on the seafloor. The bell is flat and shaped like a saucer. Color can vary, but is typically greenish to gray-blue. It has four pairs of elaborately branched but unfused oral arms. Instead of swimming, this jelly spends its life pulsing upside-down in shallow, sunlit water.

The cannonball jelly is a mostly harmless variety that sometimes washes up on beaches in large numbers. It is shaped like half an egg and may be up to 7 inches in diameter and either bluish or yellowish with a brown border. It is a good swimmer.

What are Sea Lice?
Related to the jellyfish, sea lice are actually microscopic larvae of jellyfish and other types of stinging organisms in the ocean. There are many varieties and, frustratingly, they’re basically impossible to see with the naked eye. In Key West, they often colonize on seaweed floating in the ocean, so try to avoid this while swimming. If you get hit by sea lice, it’s usually no more than a temporary mild itch or burning sensation that goes away as quickly as you feel it. On rare occasions, the sting of sea lice is delayed and can be rather painful and uncomfortable for days.

Jellyfish Don’t Have Brains
Yes, you can outsmart jellyfish. While they have a complex nerve system, they don’t actually have brains. They’re slaves to the ocean’s current and sea breeze, so if you see a jellyfish in the water, you can swim around it without fear of it following you. They’re not predatory by nature.

Jellyfish Are Related to Coral
Jellyfish and coral are actually members of the same Phylum, Cnidaria. Manypeople think of coral as an inanimate rock in the water, but that couldn’t be farther from the truth—coral is a member of the Animal Kingdom. Jellyfish and coral share nematocyst stinging cells. That’s why if you brush up against coral, it also causes a sting, rash, cut or burn. As a rule of thumb, you should stay at least six feet away from coral while snorkeling to avoid coming into contact.

Jellyfish Feed on Zooplankton
While jellyfish aren’t exactly predatory, they are carnivorous. They feed on microscopic zooplankton, as well as fish eggs and even some juvenile fish.

Sea Turtles Feed on Jellyfish
Sea turtles and sharks are amongst the jellyfish’s only predators. This is why plastic waste is so dangerous to our world’s oceans. Plastic bags and cups often look like jellyfish to these animals, and when consumed they prove deadly.

(https://www.keywestaquarium.com/florida-keys-jellyfish)

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