🐬 At a glance
- Scientific name
- Stomolophus meleagris
- Size
- Up to 10 in. across — about a softball or small cabbage
- Tell-tale
- Milky dome with a brown-maroon band around the rim; no long tentacles
- Sting risk
- Minimal for most people — still look, don’t handle
- Where & when
- Wrack line at Keewaydin, Sand Dollar Island & Tigertail; heaviest strandings in cooler months
- Predator
- A favorite food of migrating leatherback sea turtles
What Is a Cannonball Jellyfish?
Cannonball jellyfish (Stomolophus meleagris) are the solid, dome-shaped jellies you’ll spot pulsing just under the surface of the Gulf or piled up along the wrack line on a falling tide. Locals call them jellyballs or cabbageheads, and the nicknames fit — they’re firm, round, and roughly the size of a softball or a small cabbage, topping out around 10 inches across on a big one. The bell runs milky white to pale tan with a distinctive band of brown or maroon pigment around the rim, which is the fastest way to identify one at a glance.
Flip a cannonball jellyfish over — gently, with a stick, never bare skin — and you won’t find the long trailing tentacles that make a moon jelly or a man o’war so recognizable. Instead, short, frilly oral arms hang fused together in a compact cluster underneath the bell instead of individual stinging tentacles. That structural difference is the whole reason cannonball jellies have such a mild reputation compared to some of their cousins covered in our broader jellyfish guide.
Does a Cannonball Jellyfish Sting?
Cannonball jellyfish carry some of the mildest venom of any jelly in the Gulf, and most people who brush against one in the water never feel a thing. The stinging cells are concentrated on the oral arms underneath the bell rather than in long trailing tentacles, so a swimmer bumping the smooth top of the dome usually walks away with nothing more than a slippery bump. That said, mild isn’t the same as harmless — a small percentage of people are sensitive enough to feel a brief tingling or mild rash from direct handling, and any jelly sting is far more noticeable if it reaches your eyes, lips or another patch of thin skin.
Our rule on every trip is the same one we’d give our own kids: look, don’t grab. If you do pick one up to get a closer look, handle it by the firm top of the bell, rinse your hands with seawater afterward, and keep them away from your face until you do. Compared to the genuine stingers on this coast, cannonball jellies are about as gentle as jellies get — but they’re still not a hands-on souvenir, and the same look-don’t-take ethic that protects every live shell and sand dollar on Collier County beaches applies just as well to the sea life passing through.
Where and When You’ll See Them
You’ll run into cannonball jellies two ways out here: pulsing along just under the surface as our boats idle through Gordon Pass or the backwaters, and stranded in numbers on the wrack line after a falling tide pushes them ashore. Keewaydin Island, Sand Dollar Island and Tigertail Beach all pile up jellyballs after a stiff onshore wind, sometimes by the dozen in a single stretch of sand. They live in Gulf waters year-round, but you’ll typically see the heaviest strandings in the cooler months, when wind and current patterns shift more water — and more jellies — toward the beach.
They travel in loose schools rather than solo, so where there’s one cannonball jellyfish, there are usually several more nearby, in the water or already beached. On a shelling run out to the barrier islands, spotting a jellyball or two is a common show-and-tell moment for the kids on board, right alongside whatever beach finds turn up in the same wrack line.
What Eats a Cannonball Jellyfish?
Cannonball jellyfish sit fairly low on the Gulf food chain, and their biggest predator anywhere in these waters is the leatherback sea turtle, a species that specializes almost entirely in jellyfish. Leatherbacks pass through Gulf waters on migration and can work through a remarkable number of jellies in a single day, cannonballs included, which makes every jellyball bloom along our coast a small piece of a much bigger offshore food web.
Look closely at a cannonball jellyfish before it strands and you’ll sometimes spot a hitchhiker: juvenile spider crabs or small swimming crabs commonly ride tucked into the underside of the bell, using the jelly as a floating shelter and picking up scraps as it feeds. It’s a genuine commensal relationship — the crab gets protection and a free ride, and the jelly, as far as anyone can tell, doesn’t mind the company. It’s one of those small, easy-to-miss details that makes a slow pass along the mangroves worth a second look, right alongside the rest of the Ten Thousand Islands wildlife our captains point out on the way to open water.
| Species | Sting risk | How to tell it apart | Where you’ll see it |
|---|---|---|---|
| Cannonball jellyfish | Minimal for most people | Solid brown-rimmed dome, no trailing tentacles | Wrack line after a falling tide; pulsing just offshore |
| Moon jelly | Very mild | Clear, saucer-shaped disc with four horseshoe rings visible through the bell | Calm passes and backwaters, less often on the beach |
| Upside-down jellyfish | Mild, can irritate skin | Rests bell-down, pulsing tentacles-up on the bottom | Shallow mangrove flats and seagrass, not the open wrack line |
| Portuguese man o’war | Genuinely painful — give it room | Blue gas-filled float trailing long tentacles | Occasional visitor after strong onshore wind |
For a jelly that’s calmer to look at but genuinely irritating to touch, see how the upside-down jellyfish compares.
Beach Etiquette If You Find One
If you find a cannonball jellyfish beached on the sand, the right move is the same one we’d give for any stranded sea creature: leave it where the tide put it. A jelly stranded above the waterline is already dying and won’t survive being carried back out, but it’s still part of the beach ecosystem — ghost crabs, gulls and other scavengers make quick work of a beached jellyball, so nothing goes to waste even after the drifting is done.
Skip the urge to poke it repeatedly, and skip picking it up bare-handed if you’re not sure what you’re looking at; save the hands-on moments for whatever your captain has already identified for you on the water. It’s the same low-impact approach we use with every live shell and sand dollar in Collier County — look, admire, photograph, and let it be. You’ll get a much better look at cannonball jellies pulsing naturally through clear water on one of our dolphin & wildlife tours than you ever will squinting at one stranded in the sand.