Worried about my Moon Crabs

[HolyVessel]

Does anyone have experience with Halloween Moon crabs (Gecarcinus quadratus)? I'm a bit worried about the ones that I have. I got my first one in October and had her in a ten gallon tank with sand substrate. In November I moved her to a 40 gallon and it took her about a week to burrow. About a week after she burrowed she stopped coming out to eat. I looked through a little window she made in her burrow but didn't see a molt. Two weeks ago I got three more crabs (three more females) and introduced them into the tank so she wouldn't get lonely. Those crabs burrowed right away and they haven't come out to eat either. They covered up the windows to their burrows, but sometimes I can hear their feet skittering across the glass at night. Is it normal for them to go so long without eating? I thought that they had to eat every day. I'll post some pics of their enclosure below.

 

[HolyVessel]

 

[Arthroverts]

Are they going into an aestivation of sorts? Has the temperature got real low lately, and thus caused them to burrow? That is all I can think of as only a crab neophyte.
Any other crabbers on these boards?

Arthroverts

 

[HolyVessel]

It has been cold recently, but I keep my apt around 75. I also have a heat pad on one side of the tank. They've never really hanged out outside if their burrows

 

[The Mantis Menagerie]

This is a species I want to keep, and I did some research on them a while ago. They love to burrow, and they are nocturnal. Therefore, if they have deep enough substrate, then you will not see them during the day. Molting also takes a long time, so your crab could stay burrowed for a while.