Conus
A group of highly poisonous stinging shells found in coral reefs in the Indo-Pacific, Australia and the Mediterranean. They are quite small (biggest reported specimens have been just under three inches long) and have colourful patterned shells much coveted by collectors. The cone shells can be divided into three main groups based on their primary diet: worms, mollusks or fish. Of all the cones, the fish-eating variety is the most dangerous to humans.
All species in the Conus family are capable of firing a barbed projectile from a hole in their shell to defend against predators and to obtain food. Moreover, the fish-eating cone shell has teeth strong enough to pierce cloth or even skin. Both the bite and the 'harpoon' fired by the cone contain neurotoxin which competes for acetylcholine in the nerve synapses of the victim. The effect can be fatal within 15 minutes, as there currently is no antitoxin available.
That said, most victims seem to recover in 48 hours - at least this is what the doctor said to my uncle, who got hit by cone's arrow whilst scuba-diving... and survived.