Tropical sea slug uses its prey against its predators

I imagine you've seen this image before; it's made its rounds of the Internet a few times, and rightfully so because this is a really cool critter.

Picture from Wikimedia Commons under CC Attribution-Share Alike 3.0

This is the blue glaucus (Glaucus atlanticus), a member of the nudibranch clade of invertebrates. Nudibranchs are often generally characterized as sea slugs, but the ones in this clade specifically are very brightly colored, as demonstrated by this one. However, what sets the blue glaucus apart is in its behavior. The streaky blue coloration we see here is actually the underside of the animal: instead of crawling around on the ocean floor like other nudibranchs, the blue glaucus swallows air to fill its stomach, which it uses to float (admittedly upside-down) on the top of the water. The side under the water (the top) is silvery gray, so the varied coloration helps it hide from predators above and below.

If that wasn't enough, the blue glaucus and a few nudibranchs can utilize jellyfish stingers to attack their prey. This one here nibbles on the tentacles of the Portuguese man o' war, but while these jellyfish in particular are well known for their very painful sting, the blue glaucus is protected in a number of ways. Their insides are coated with a mucus that prevents the cells containing the stinger (called nematocysts) from being triggered, and cells lining their stomachs contain spindles of chitin that neutralize the stinger. The nematocysts that do survive intact are then stored and relocated to the blue glaucus' appendages for use against predators that do happen to see through the camouflage. The ability to concentrate the acquired nematocysts onto its appendages allows the blue glaucus to pack a more powerful (and potentially deadly) sting than even the Portuguese man o' war it took them from.

Accidentally threatening one of these animals would probably be like shaking hands with a friend who is wearing one of those hand shocker things from the golden days of practical jokes, except the friend is a pretty blue slug and the hand shocker could kill you.