What the heck are these things?

A jellyfish is about 95% water. No brain, no heart, no bones, no lungs — just a bell-shaped body, a simple nerve net, and trailing tentacles. They're one of the oldest animals on the planet, drifting around long before dinosaurs showed up.

They barely swim. The bell pulses to hold position or rise and fall, but where a jelly ends up is decided by tide, current, and wind. That's why a beach can be clear in the morning and full of nettles by afternoon — they got pushed in, they didn't come hunting.

The one you'll meet in Ocean City is the Atlantic sea nettle. It's a plankton eater, drifting with tentacles out to catch tiny organisms. You are far too large to be food.

Tentacles are lined with microscopic stinging capsules called nematocysts. Each one is a coiled, venom-filled harpoon on a pressure trigger. Brush against it and thousands fire at once, injecting venom into the top layers of your skin. That's the instant burn and the red, welted line.

Critically: this is mechanical, not a decision. The jelly isn't attacking you and doesn't need to be alive. A broken-off tentacle drifting in the surf, or a jelly washed up dead on the sand, will still sting on contact. Don't poke beached jellies, and be careful where kids step.

It also means the goal of first aid is to stop unfired capsules from firing — which is exactly why a freshwater rinse makes things worse. See the Soothe the Sting for the step-by-step.

Atlantic sea nettleChrysaora chesapeakeiStings

The main event. Milky-white saucer bell, usually 4–8 inches across, sometimes with faint reddish-brown streaks, and long thin tentacles plus frilly oral arms underneath. Thrives in the bay and coastal bays and rides into the surf. Sting is a sharp burn, welts, and itching that fades over hours.

Moon jellyAurelia auritaBasically harmless

Clear, flat, translucent disc with four pale horseshoe shapes visible on top and only a short fringe instead of long tentacles. Very common, very mild — most people feel nothing or a faint tingle. If it looks like a floating contact lens, relax.

Lion's maneCyanea capillataStings

Reddish-brown, shaggy mass of hair-like tentacles. Cold-water species, so you'd see it in the off-season and early spring, not in August. Sting is stronger than a nettle's.

Portuguese man o' warPhysalia physalisSevere

Not a true jellyfish, and rare this far north — a blue-purple gas-filled float that sits on the surface like a balloon with a pink crest, trailing very long tentacles. Painful, potentially serious sting. Never touch one, on the water or the sand, and tell a lifeguard.

Comb jellyCtenophoraNo sting

Small, clear, oval blobs with shimmering rainbow rows of cilia. No stinging cells at all. Harmless — and they actually eat sea nettle larvae, so they're on your side.

Photos are illustrative references, not photos taken in Ocean City.

Sea nettles want three things: warm water, calm conditions, and the right saltiness. Numbers build through July, peak from late July through September, and drop off as the water cools in fall.

• Hot and still means more nettles. A stretch of hot, low-wind days lets them build up near shore. Big surf and strong wind break them up and scatter them.
• Rainfall matters. Sea nettles like brackish water — not fresh, not full ocean strength. A dry summer pushes the ideal salinity zone around and can bring them closer in.
• Bay side usually stings more. The coastal bays behind the island are warmer, calmer, and brackish — near-perfect nettle habitat, and where they breed. The ocean side gets what the tide and wind push in. That's why every report on this site is tagged ocean or bay.
• Nettles often arrive in patches, not evenly. Two blocks apart can be completely different — which is the whole point of the map.

• Long trailing tentacles under the bell → sea nettle. Keep your distance.
• Short fringe, flat clear disc, four rings → moon jelly. Harmless.
• Rainbow shimmer running in lines → comb jelly. Harmless.
• Blue balloon on the surface → man o' war. Get out and tell a lifeguard.
• When you can't tell, treat it as a nettle and give it room.

• "Rinse it with fresh water." No — fresh water changes the pressure balance and makes leftover stinging cells fire. Rinse with seawater.
• "Pee on it." No. It doesn't work, and depending on its makeup it can trigger more firing. It's just an unpleasant myth.
• "Rub it with sand or a towel." No — friction fires more capsules and grinds venom in. Lift tentacles off with a card edge or tweezers instead.
• "Dead ones are safe." No. Beached and broken jellies sting fine.

The actual first-aid sequence — seawater rinse, remove tentacles, baking soda slurry, heat — is in the Soothe the Sting.