- Humidity: make sure the humidity is high enough, but not too high either (should be 70-80 percent relative humidity -- invest in a hygrometer to be sure).
- Temperature: too high or too low, too much fluctuation, or hot spots (should be 72-78 F or 22-26 C).
- Chlorine in the water (use a dechlorinator on all water in the tank).
- Residual chemicals from cleaning in or around the tank (use only hot water for cleaning the tank).
- Condition of substrate: not too damp, no mold.
- Cleanliness of the tank: clean the tank regularly to remove and wastes or uneaten food.
- Bathing too often: once a week is plenty under most conditions.
Check the tank and the crabs for mites which will appear as tiny specks moving around. Hold your crab up by the shell to the crab extends itself a bit (blow gently on the crab to entice him or her out if necessary), and look at its body to best dectect mites on your crab. If you think your crabs and tank have mites, see Vanessa's Crabarium for help.
Shell Fights / Aggression
Hermit crabs are usually pretty peaceful but sometimes they will fight over resources, especialy coveted shells. If your crabs are fighting over shells, you need to increase the number of appropraitely sized shells, in a variety of styles to make sure there is enough selection to cut down fighting. You might also want to consider larger tanks, more hiding places and more feeding/water bowls if your crabs are fighting. If the problem persists, you might want to separate the crabs.
Illness and Severe Stress
If you have a crab that is very stressed or ill, it may drop multiple legs. If your hermit crabs has lost more than one leg in a short period of time, isolate the crab (still maintaining proper conditions especially temperature and humidity). The causes of dropping legs are not usually contagious, but this will help keep the crab comfortable and free from the stress of competing with other crabs. Crabs that drop multiple legs are often so stressed that they cannot be saved.
Sometimes new crabs start dropping legs shortly after you get them home; this is more likely due to the conditions they experienced during collection, shipping, and their time at the pet store than anything you are doing wrong. Just make sure your conditions are ideal and hope that the stress reaction is reversible.
Now What: Regeneration of Legs
Usually, the dropped leg will be regenerated. At first a "gel limb" forms which starts out as a little bud or bump, then develops over the course of a few molts into a new leg (or claw), though it may be smaller than the original. Your crab may undergo more frequent molts until the limb is regenerated.