What is it? A beaded necklace? Red blood cells? No, it's the Portuguese Man o'War (Physalia physalis), magnified 30 times. Though it resembles a jellyfish, the Portuguese Man o'War is a siphonophore, a colony of organisms that work together. The sting of the venom in the tentacles' nematocysysts is incredibly painful, though rarely deadly. This photo, taken by Alvaro Migotto of the University of São Paulo in Brazil, won 6th prize in the 2009 Olympus BioScapes Interational Digital Imaging Competition.
Notorious for its painful, powerful sting, the Portuguese Man o' War has a gas-ﬁlled ﬂoating chamber that supports the tentacles, which bear sting cells. Shown are the pink batteries of stinging cells and a delicate muscular band responsible for the high contractibility of the tentacles.
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(Hat tip: Transcription and Translation)