Firstly, the name- hermit crab comes from their unique habit of living in a shell that has been used by someone else.
Salt water hermit crab body is covered with a exoskeleton, which protects them from predators. But the exoskeleton does not cover the whole body completely. This is where shells come in. Hermit crabs have a soft spot which is not protected by exoskeleton- abdomen. Due the weakness of this part of the body, these crabs take some second hand shell and use it to protect their soft abdomen. The shell also helps in regulating moisture level of the crab.
Hermit crabs come from a super family Paaguroidae. They are not that close to real crabs.
Hermit crab sizes vary from few millimeters to sizes of a coconut.
Terrestrial crabs begin their lives in the sea, and gain the ability to breathe air only after molting. They can't stay in water for too long after molting, because they will drown.
Hermit crabs have to change quite a few shells in their lives. That's because crabs can't stay in a small shell, it decreases their growth speed, and increases their chances of not surviving an attack by a predator (they can't completely withdraw to a small shell).
The requirements for a shell are quite big. Not only does it have to be the right size and have a right form of opening, it also has to be intact, with no cracks on it (that would make managing moisture levels much harder).
Reproduction organs of the male at found at the base of the last pair of walking legs, while the female reproductive organs are found at the base of the middle pair of walking legs.
Female hermit crabs can lay their eggs shortly after copulating, even though sperm can be stored for many months. The eggs are fertilized as they are laid by passing through the chamber holding the sperm. The eggs are carried and hatched in a mass attached to the abdomen inside the shell. The number of eggs depends on the size of the crab.
Hermit crabs go through 4 stages of development, two of which occur while they are still in the egg. Most of the crabs hatch at stage three, as larvae, and reach the fourth stage which is called magelops.