Tiny animals in the sea are the most lethal, apart of this cute octopus also

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It can kill in minutes from the shock caused by the pain (Irucandji syndrom, no antivenom)

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A bather was pricked in the neck by this cone when he found it on the beach and held it to his ear to listen to the noise of the sea. He died in seconds.´

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The Box Jellyfish also can kill in minutes by cardiac arrest, worse, these jellyfish can see and actively attack