When people think of dangerous animals, they tend to think big. Megalodon-size sharks, giant squid capable of sinking ships — that sort of thing. But the deadliest creatures in the sea, pound for pound, are so small, you probably wouldn’t think twice. The secret weapon? Venom. For instance, a small, often painless, bite from the beautiful blue-ringed octopus can contain tetrodotoxin, which it uses to hunt. A 25-gram octopus carries enough of this powerful toxin to fatally paralyze 10 adult humans.