What is the exoskeleton?
Hermit crabs have a tough outer skin called an exoskeleton. The exoskeleton is the hard part of the crab, including legs, claws, antennae, and head/back, which joins the abdomen, or soft part that normally stays in the shell. The exoskeleton is mostly made up of calcium, a nutrient found in human hair, teeth, and fingernails.
The more active the hermit crab is, the more it eats; as a result, that crab molts more frequently than an inactive crab. Calcium causes the growing exoskeleton--the one underneath the top one that we see and feel--to strengthen. A strong exoskeleton will grow bigger in the period just after a molt, when the crab is still soft.
Why do hermit crabs need calcium supplements?
A healthy exoskeleton will strengthen and harden faster. A healthy, calcium-infused crab will also have an easier molt, and will be much more likely to survive the experience. An example of the effect of calcium on the animal hermit crabs borrow shells from: aquatic snails' shells are made up entirely of calcium, and they can be seen to devour calcium pills when they are provided; they then grow large and healthy shells. Commercial hermit crab food (such as the FMR brand) should contain some calcium in it, as well as other nutrients crabs need to get from molt to molt.
I've seen Kali IV, post-molt, scaping the inside of an empty shell with her little claw, possibly in search of another calcium source, since snail shells are also made of calcium.
Having observed that they don't seem to eat much when compared to their water intake, I decided to give them calcium supplements in their water. I use calcium pills--the ordinary kind that can be bought as a vitamin in any drugstore. The pills dissolve into the water, giving them much-needed calcium with every drink of water they take. Hermit crabs grow in layers, forming new exoskeletons below the old one, shedding the top layer periodically in order to get bigger.
Another good (and all-natural) source of calcium is cuttlebone. It's the dried skeleton of the cuttlebone fish, found in the bird section of the pet store. Birds, like hermit crabs, need extra calcium in captivity; they scrape their beaks on the cuttlebone to acquire it. Hermit crabs can use their claws to scrape it off, too, as it very easily becomes a powder. Right after a molt, Kali IV was seen scraping a cuttlebone and eating it. Some hermit crab owners use this instead of calcium pills. Simply powder the cuttlebone in your fingers, into the waterdish, or into their food. Remember--right after a molt is when they're at their hungriest and in need of the most calcium. It's up to you to help your hermit crab start forming a healthy new exoskeleton right away! I've also heard of people using crushed-up egg shells, but I haven't tried this myself. I've heard it works--the crabs just eat the egg shells. Don't use milk; hermit crabs might be allergic to dairy products such as milk and cheese.
Hermit crabs can also regenerate lost limbs during a molt. Kali IV, a medium-sized crab who lost a leg in her first, traumatic molt, eventually grew back her missing limb. A month or so prior to her molt, she grew a "gel-limb" in the spot where the leg was; this is a whitish sac shaped like the limb. When Kali molted, she lost the sac along with the old exoskeleton, revealing a smaller version of the leg she was missing. In successive moltings, the limb became bigger, until it was full-sized again. You couldn't tell it was ever missing. One of my "charity crab" adoptees, Link, was missing an eyestalk from a previous molt, and had since grown the stalk back. The eyestalk will grow back, but not the eye; looking at Link she had one stalk ending in a normal black eye, and one eyestalk covered with clear exoskeleton that was hollow inside. Another crab, a jumbo named the AuraCrab, has this same interesting characteristic (which does explain why she's a little clumsy!)