Jellyfish and Portuguese Man o' War at the Beach: What They Are and Why Not to Touch Them
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How we wrote this: A plain-English summary of NOAA's National Ocean Service pages on jellyfish and the Portuguese man o' war, checked October 29, 2026. This isn't medical advice. For a serious sting, an allergic reaction or trouble breathing, call 911.
Jellyfish: 95% water
NOAA says jellyfish are about 95 percent water, which is why one washed up on the sand looks like a clear blob. They have no brain, blood or heart. Their bodies have three layers: an outer epidermis, a thick jelly-like mesoglea in the middle, and an inner gastrodermis. A simple nerve net lets them sense light and smells. They belong to the phylum Cnidaria, along with sea anemones and corals.
Jellyfish sting with their tentacles. NOAA notes that sting severity varies, and that most jellyfish stings cause only minor discomfort in people.
The Portuguese man o' war: not a true jellyfish
The man o' war (Physalia physalis) is often called a jellyfish, but NOAA explains it's a siphonophore: a colony of genetically identical individuals (zooids) that work together as one animal. Different parts float, catch prey, feed and reproduce.
How to recognize one:
- A balloon-like float, blue, violet or pink, sitting up to 6 inches above the water.
- Tentacles averaging about 30 feet (10 m) and reaching up to 100 feet (30 m), so the stinging part can be far from the float you see.
- Found mostly in tropical and subtropical seas. NOAA says they're moved by winds and ocean currents alone, sometimes floating in groups of 1,000 or more.
The tentacles carry nematocysts, microscopic capsules with barbed tubes that inject venom strong enough to paralyze small fish. NOAA says the sting is rarely deadly to people but "packs a painful punch" and causes welts on exposed skin. NOAA also warns that a man o' war may still sting weeks after washing ashore.
Beach sense
- Don't touch jellyfish or men o' war on the sand. Keep kids and dogs away. Stranded ones can still sting.
- Watch for the purple flag, which some beaches fly for dangerous marine life (see beach warning flags), and ask the lifeguard.
- Swim near a lifeguard (see drowning prevention), and get medical help for a severe reaction.
Related: sea turtle nesting beaches, marine debris.