It is surprisingly easy to mistake a molting hermit crab for a dead hermit crab, especially when they molt on the surface. A molting crab appears quite limp and lifeless and the body is often partway out of the shell. Sometimes, with very careful observation, you will be able to see small twitches from the hermit crab's body, but otherwise it can be very difficult to tell if a crab in this position is molting or if it has died.
Sometimes you may find an entire molted exoskeleton and think your crab has died and fallen out of its shell. Plus, if your crab has buried themselves in the sand and you haven't seen it for a while, it is natural to start wondering if they are molting or if they have died under there. If you are not sure if your crab is molting or dead, how you handle the situation may make the difference between life or death if your crab is indeed just molting.
The safest thing to do if you find your hermit crab in one of the aforementioned situations is to assume that they are molting. If you disturb a molting hermit crab at a critical time during their molt while trying to determine if they are alive or not, the results can be disastrous.
Caring for a Molting Hermit Crab
Since we are assuming your hermit crab is molting until proven otherwise, if your hermit crab appears lifeless and is in an isolation tank, leave him alone and watch to see what happens.
If your hermit crab is in the main tank with other hermit crabs (especially if they are on the surface), cut the ends off of a two liter pop bottle and sink it down into the sand to surround the crab with a clear protective barrier.
Do not disturb a crab that is limply hanging out of her shell (just protect her from other crabs).
If she is molting, she should continue through the process if given the time to do so. If she has died, she will start to smell very badly (rotting smell, often "fishy") within a few days. A hermit crab may take up to two months to complete the entire molting process so you will know far before that time whether or not your hermit crab is still alive (smaller crabs do not take nearly this long to complete the entire molting process).
A crab that is buried in his substrate is a bit trickier to care for and identify whether or not he is molting. Smooth the sand around his hiding spot and look for tracks to get an idea of whether or not he is coming up at night for snacks. Many crabs often disappear during the day but the tracks around the cage in the morning (and the sand in all the dishes) will let you know that they are still active. If it has been weeks since your crab buried itself and you still aren't sure whether or not your hermit crab is alive, you can carefully sweep off a bit of sand from around his hiding spot to check for a rotting smell.
If you find what appears to be a dead crab on the surface next to an empty shell have a closer look to see if it is just an exoskeleton (if is it hollow and crumbles easily, it is an old exoskeleton and your hermit crab has already molted and moved on to a new shell).
Have a quick peek in a nearby shell and you might find a molted crab hiding out. Similarly, a molting crab may leave limbs strewn about, which can be a bit startling.
Edited by Adrienne Kruzer, RVT