Glaucus Atlanticus. Source
This particular mollusk usually resides off the coasts of places like South Africa, Australia and Mozambique. A little bag of gas on the underside of the blue dragon causes it to float upside down on the ocean surface, where it hunts most of its food. Though these things normally only grow up to around three centimeters, they are ferocious predators, feeding on things many times their size. They are so awesome that they feed on the deadly Portuguese Man O' War. Normally, a Man O' War is fatally poisonous, but not only is the blue dragon immune to the Man O' War's poison, it actually commandeers that poison for its own use, storing the lethal stuff in the tips of its finger-like appendages.
Note the finger-like appendages. Source
Glaucus feeds on surface dwelling creatures like the Man O' War (left) and velella velella (right). Source(1) Source(2)
There's just something fascinating about these hermaphroditic slugs. For one, they look pretty cool. Their underbellies, which face towards the surface of the ocean, are colored with blue stripes and their backs, which face downward, are silvery, camouflaging them from all sides. What's more, the poison they take from their prey is concentrated in their appendages, making them much more deadly than the infamous Man O' War.
The ocean is filled with bizarre, cool and scary stuff. I think the blue dragon sea slug fits into all those categories.