Shellback Crabs: these crabs create their own shields using discarded clamshells; they have semi-membranous bodies that can be pressed into the contours of the shell, producing a suction mechanism that holds it in place
Many Dromiidae crabs, including sponge crabs and shellback crabs, are equipped with a specialized pair of legs that allow each crab to hold a shell, sea sponge, or ascidian against its body as a defense mechanism, but shellback crabs (genus Hypoconcha) also have flexible, semi-membranous bodies that can be tucked more firmly into the contours of a bivalve shell, producing a suction mechanism that keeps the shell firmly locked in place.
The body of a shellback crab is also covered in a very fine layer of hair-like structures known as setae, and when the crab presses itself against the shell, those membranous "hairs" can take on an almost translucent appearance, particularly around the margins of the crab's body.
I recently wrote a post about a species of sponge crab known as Lamarckdromia beagle, which is another weirdly adorable member of the Dromiidae family. All of the crabs in this family seem to have their own unique adaptations that allow them to use specific materials for camouflage -- some of them use living sponges, clamshells, ascidians, debris, etc.
But I just could not stop laughing at the little faces on these shellback crabs. They've all got the same bemused and indignant expression...like some random jerk just walked up to them, flipped them over, and announced to the entire ocean that there's actually a crab hiding beneath that clamshell disguise; as if that actually happens to them quite a lot, and they're getting pretty sick of it.
Sources & More Info:
Crustaceana Monographs: Comparison of the Shell-Carrying Behaviors of Desmodromia, Conchoecetes, and Hypoconcha (p. 191)
South Carolina Department of Natural Resources: Shellback Crabs and their Larval Stages (PDF)
South Carolina Public Radio: Shellback Crabs