Yes you can dampen a towel and cover the air holes. And we actually recommend glass tops a lot that stay completely closed! You can duct tape your whole screen, or Saran Wrap your whole screen as well. Don’t worry - the crabs will get enough air flow just from your daily clean up routine (removing uneaten food, straightening up the mess they make, etc.) and be just fine!

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Some additional and more long term ways to keep the humidity up and constant are: larger water pools, bubblers in your pools, and moss pits. You can use un-dyed Sphagnum moss, which you mist until moist with either Prime treated fresh water or salt water. Also, nice and deep, sand castle consistency substrate helps keep it up effortlessly. From personal experience, you want to try to avoid misting the enclosure, as misting too much can cause flooding in the bottom of the tank, bacterial blooms, and mold on your woods/decor.