Actual molting is difficult to document since it is supposed to happen underground. So technically a lot of these photos are pre molt or post molt.
Land hermit crabs must shed their hard outer skin in order to grow. The frequency of molting depends on the size of the crab. Small crabs molt frequently, larger crabs less frequently.
The hermit crab will dig down into the sand and create a cave in the sand. This is why you must use sand and it should be just damp enough to hold it’s shape. The crab will settle in to it’s cave when it finds the ideal spot. This is rarely in the same place where they dug down, which is why it’s never save to go digging in your sand with a molter down.
Once settled in to a cave, the hermit crab will use water stored in a molt sac on the abdomen to swell and help loosen the outer hard skin (called the exoskeleton or exo for short). The soft body of the crab then shrinks a bit so that the crab can slide backwards out of the hard skin. The soft body of the crab pulls back into its seashell, leaving the shed hard skin (exoskeleton) just in front of the shell opening. The crab will once again use it’s stored water to swell and stretch the soft exo before it begins to harden again. Hardening of the exoskeleton may take a couple days .
The crab will rest for a day or so after shedding and then begin to eat the shed exoskeleton to reabsorb the needed nutrients. When the hermit crab is ready it will dig out of its cave and return to the surface.
Molting can last a few days or a few months, it depends on size of course but for reasons unknown some crabs stay underground longer. NEVER dig up a molting hermit crab.
Some crabs display pre mot symptoms such as gorging on food and/or water. Sometimes the eyes appear foggy. Some hermit crabs will get a little cranky prior to a molt and some will start digging practice holes. If your substrate is too dry you may catch them dumping water from their dish.
A molt does not require a shell change. A pre molt shell change only happens if the current shell is too small to allow the crab to swell inside and grow.
Surface molting is not normal.
Leaving the seashell and molting while naked is not normal.
If your crabs have done either of these please seek assistance at The Crab Street Journal