Hermit crabs spend most of their lives wearing protective shells - necessary tools to keep predators away from their soft, defenseless butts. Every once in a while, the crabs will grow so big that they have to abandon their shell for a new one. Sometimes, though, the crab can't find a shell that fits properly, so it has to get creative.
Ocean pollution is one of the more serious problems effecting marine ecosystems, but hermit crabs have managed to make the best out of a bad situation. People have been finding the animals encased in shells made out of trash and the hermits seem to be genuinely enjoying their new homes. Actually more closely related to lobsters than crabs, these crustaceans have managed to adapt to the effects of human pollution and come out the better for it.
Only time will tell if hermit crabs wearing trash shells will end up with resulting health issues down the line but for now it seems like one man's trash is another crab's treasure.