Watch this nice snail. It slowly moves across the ocean floor—lah-di-lah-di-lah!—until it finds a sleeping fish three times its size. Does it say hello, Mr. Fish, sorry to wake you up? No. It just opens its giant mouth and swallows it.
Apparently, cone snails are one of the deadliest fish assassins on the ocean floor, despite being so slow and peaceful-looking. If it gets near a sleeping fish, it will release chemicals to paralyze it. Then it will open its mouth and literally stretch its body until it engulfs the whole thing, finally killing it using a venomous barb.
The barb releases a neurotoxin more powerful than cyanide and it's capable of killing humans (in fact, there have been many reported cases). The venom contained in one of the most dangerous species, the Conus geographus, can kill 700 humans:
Severe cases involve muscle paralysis, changes in vision and respiratory failure that can lead to death. There is no antivenom, and treatment involves providing life support until the venom is metabolised by the victim.