Well, after almost two weeks of the Under the Sea Contest I finally came up with an idea! This is my try at creating a (more or less) realistic hermit crab.
Harry's story starts when he was just a small crab. He didn't have many friends because of his personality. He rather liked being alone. So one day he went out to have a walk. His chores had already been finished and he wanted to be all alone and meditate. Just as he stepped outside of Warm Streams (the settlement he lived in) he detected a strange spot on the surface of the ocean. It was far away so he was unable to make out what it was and kept on wandering. He arrived at the place where he could be lost in his thoughts. Here, the coral mountain abruptly stopped and the sheer drop into the abyss was just a few steps away. The whole ocean could be seen from here (or so he thought). Harry sat down and thought about many things.
Only when it began to get dark did he look up again and to his concern he discovered that the spot was actually a boat and that many strange, four-limbed creatures were descending from it. He observed the dance of the air bubbles that rose behind them and waved when one of them approached. Suddenly he realized that the being was very large, much larger than Harry or in fact any other crab. It didn't seem so friendly anymore, even less so with the small fishing hook it held at the end of one of its limbs. Just as Harry was about to slowly retreat, the fish hook sped towards him and pierced his exoskeleton in the region of the chest. The pain was excruciating and Harry was bewildered as to why anything would do this to other creatures. He thought he was a dead man but then he was let go and Harry started to fall swiftly. With relief he determined that the creature was no longer after him due to the fact that a group of sharks were gnawing on various parts of its body.
As the water was beginning to fill with dark red blood, Harry overcame his fear and his pain and made his way back to Warm Streams as fast as his legs and pincers could carry him. When he still had a little to go, he noticed that more of these creatures were swimming away from the village with bags full of live crabs and fish. He found the town in ruin, blood and dead beings dispersed through the whole expanse. The corals that provided shelter for more than 3/4 of the population were crushed and/or broken off and taken. He crawled through the area that was one a thriving city as well as his home with a look of horror on his face.
He gathered enough material to build a shack and travelled back to the cliff where he decided to build a new home. He made a deal with the organisms in the area. They promised to cement the walls of his structure in order to make it sturdy and he would gather them some food once in a while. In this hut he stayed, full of despair, until the end of his days.
Here are some pictures to explain what everything is supposed to be:
[Edit] As requested by spacemonkiee, I made a pic where you can see the little guy crawl! Here it is: