Shellback Crabs: these crabs create their own shields using clamshells, and they have semi-membranous bodies that can be pressed into the contours of the shell, producing a suction mechanism that holds it in place
Crabs of the genus Hypoconcha, commonly known as shellback crabs, are able to conceal and protect themselves by carrying discarded seashells against their backs. Many Dromiidae crabs are equipped with a specialized pair of legs that allow them to hold sea sponges, ascidians, or shells against their bodies as a defense mechanism, but shellback crabs 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.
This article describes how the shellback crab carries and deploys its clamshell disguise:
Instead of a hard carapace protecting its back, this little crab is covered above with a parchment-like skin. The underside and the hands are rough and granular. Three pairs of walking legs are developed as such, but the last pair are reduced and held above its back. With these legs, this bit of a crab hoists half of a small bivalve shell on its back, tucks its angular rear into the hinge side and carries the shell for its protection.
Walking along, it resembles a wave-washed shell, but when danger threatens, it squats down and drops the protective shell to the bottom, and so it goes unnoticed.
Each crab is also covered in a very fine layer of hair-like structures known as setae, and when the crab presses itself against the inner surface of a clamshell, those membranous "hairs" can take on an almost translucent appearance, particularly around the margins of the body. This effect makes the crab itself look like nothing more than a clump of wet sand stuck to the underside of the shell.
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, including discarded seashells, live sea sponges, ascidians, or bits of debris.
I just could not stop laughing at the faces on these little shellback crabs, though.
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; like that happens to them quite a lot, actually, and they're getting pretty sick of it.
Sources & More Info:
The Great Outdoors Book of Seashore Life: Hypoconcha arcuata
Crustaceana Monographs: Comparison of the Shell-Carrying Behaviors of Desmodromia, Conchoecetes, and Hypoconcha
South Carolina Department of Natural Resources: Shellback Crabs and their Larval Stages (PDF)
South Carolina Public Radio: Shellback Crabs
iNaturalist: Genus Hypoconcha