Portuguese Man O' War (Physalia physalis), is a jellyfish-like marine invertebrate of the family Physaliidae. Its venomous tentacles can deliver a powerful sting. Despite its outward appearance, the Man o' War is not a jellyfish but a siphonophore, which differs from jellyfish in that it is not actually a single creature, but a colonial organism made up of many minute individuals called zooids.
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A jellyfish in Ningaloo Reef, Australia
The Lions Mane Jellyfish is the largest jellyfish in the world. They have been swimming in arctic waters since before the dinosaurs (over 650 million years ago) and are among some of the oldest surviving species in the world. The largest can come in at about 6 meters and has tentacles over 50 meters long. Pretty amazing when you think these things have been swimming around for so long.