Ok, so you're curious as to what goes on in their tank after you go to bed or, more precisely, how do they manage to completely trash the tank overnight?
Good questions! As you probably know from your personal observations, land hermit crabs are nocturnal, which means they are the most active at night. In their native tropical habitat, land hermit crabs are usually dormant during the hottest hours of the day. Since their abdomens are so soft and covered with very thin skin, the hermit crab's first line of defense against dehydration is to stay out of the sun during the day. You might compare this to the way a fair-skinned, blue-eyed person should also stay out of the sun to avoid being sunburned. A hermit crab's natural 'sunscreen' is his hard exoskeleton -- which helps to minimize the evaporation of water from his body -- and his seashell. On overcast days, some hermit crabs might brave the warmer daytime temperatures and venture out to scavenge for food, but usually the crabs stay in their hiding places until sunset.
The Sun Has Set -- Let's PARTY!
In their natual habitat, a little while after sunset, when the air temperature begins to drop, land hermit crabs start to emerge from their places of concealment and scavenge for food.
I spoke with one of the residents of Key West, who told me that the crabs seem to all come out at once, and "they make a racket" clinking their shells together. The crabs look high and low for food, water, shells, etc.
In your crabitat, the usual routine varies. Some crabs wake up around sunset and head straight to the food or water dish. Other crabs 'make the rounds,' going around and around the tank looking for a way to escape. Still other crabs decide to go looking for empty shells to try on. Resourceful hermit crabs will bury bits of food in the substrate to eat later, or take a hunk of food or treat back to their daytime hiding place with them and quietly munch on it during the day. Sometimes they eat it all -- more often they don't, and you are left to clean it up. Energetic hermit crabs will forego all these other activities in favor of climbing on every object in the tank, up to and including the silicone sealant holding the corners of the tank together, any power cord you have going into (or coming out of) the tank, all plastic plants, driftwood, coral, sea fans, choya wood, cork bark, etc. Big crabs or a lot of crabs on the same item will cause it to tip over, making a banging noise as all the crabs' shells hit the aquarium glass. So if you've ever wondered how your crabs managed to move the 'crabitat furniture,' there is strength in numbers -- and the chances are if a few of your crabs really enjoy a certain item, you can bet the rest of them probably do too, and that is why that item is usually pushed all over the crabitat by morning! People in the southern United States who keep their crabs in screen-topped aquariums note that their active hermit crabs like to climb upside down on the screen lids. So don't underestimate the 'houdini-ness' of your hermit crab. If there is any opening, and a branch or other ornament that reaches that opening, a hermit crab will find a way to escape. Case in point: I have a glass lid on my crabitats. My jumbo crabs are strong enough to literally lift the glass lid, if they have a sturdy piece of wood to stand on near the top. So please try to keep all cage ornaments within the tank. I have some tall ornaments towards the back of my crabitat, but I've fastened down the lid with duct tape to both keep mites out and prevent the crabs from escaping.
Many crab owners can relate to the 'problem' crabs that tip over the water dish at night. Hermit crabs absolutely love to dig in moist substrate. At night, some crabs -- Ecuadorians especially -- will crawl into the water dish, and fill their seashells with water. Then they will crawl out of the dish to a preferred spot and actually 'drool' the shell water onto the substrate, then dig in. Most times the crabs don't crawl very far from the water dish, and choose to 'drool' right next to it. Then when they dig in, they upset the dish and cause it to tip over. Purple pincher (PP) crabs also like to dig under the water dish. If you want to avoid the crabs' tipping the dish over, try to sink the dish into the substrate so the lip of it is level with the top of the substrate. It is the crabs' natural instinct when they run up against something to either climb over it or tunnel under it. You might have noticed this behavior in a crab that confronts a piece of 'crabitat furniture' up against the glass wall of the aquarium. The crab will try his darndest to climb over or tunnel under the object, but won't even consider walking around it. So by sinking your water (and food!) dishes into the substrate, you help to eliminate an object the crab feels he has to dig under. If you have a very persistent crab, you can make a "tip-proof" water dish by using non-toxic epoxy to glue a water dish to a large, flat piece of slate or flagstone, and bury the slate/flagstone in the substrate to act as an "anchor." Dishes made this way are nearly impossible for the crabs to knock over. Alternatively, you can use a well-cleaned (or never used) heavy glass or marble ashtray for a water dish.
Maybe the dish is leaking?
Not likely. Most of the repti-type dishes you can buy in pet stores are made of resin and do not leak. If you suspect a water dish of leaking, here is a simple 'test' you can perform on the dish:
Take the dish and rinse it in tap water and dry it.
Place it on a clean, dry paper towel before you go to sleep at night. If you are really worried about it leaking, place the towel and dish inside a plastic dishpan or other plastic watertight container
Fill the dish to the brim with water. Be careful not to spill any, otherwise you might forget overnight and get a 'false positive.'
In the morning, check the paper towel. If no moisture seeped through the dish then you're good to go -- the dish is watertight. If not, you can perhaps use the dish as a food dish instead. There are no waterproofing materials on the market that can be applied to crab water dishes that are crab-safe.
What Else do Hermit Crabs do at Night?
Almost all hermit crabs eat at night, especially if you put a fragrant treat in their crabitat before you go to sleep. If you put the treat in after sundown, you will probably see the crabs start to 'wake up' and sniff the air and wander towards the food dish. Since crabs do most of their eating at night, you don't actually see the crab eating, and you may get the mistaken impression that the crab is starving to death. Not true! Two methods you can use to test this are: (1) smooth out the substrate around the food dish; and (2) smooth out the actual food in the dish. Hermit crabs eat little and very slowly, so it might be a few nights before you realize a crab has been eating.
Hermit crabs often molt during the night, as well. That is why it is strongly recommended that you check in on your crabs every morning to make sure none have molted during the night. Freshly-molted crabs need to be removed to an isolation tank for their safety. Refer to the molting page for more help on what to do with a freshly-molted crab.So there you have it, in a crab's shell, basically what they like to do at night while you're asleep. To experience your crabs at the peak of their activity, I recommend you stay up one night or two with the lights down low in the room and watch them. They are absolutely fascinating.
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© 2001 Christa Wilkin