Hermit Crab Sea Sponge
Keep you crab hydrated! These natural sea sponges can be left in the water dish you use for your crab in their tank. The crabs will drink and chew/eat the sponge over time. This is fine and won't hurt the crab as these are natural sea sponges. Don't use any other kind of sponge for your crabs other than sea sponges. Plastic type sponges will kill your crab if he eats it.
Sponges are ping pong to golf ball in size. A few sponges generally last a year in a crab tank before the crabs eat them up.