Arrow crab with trap door?

[blk822]

I just got my first arrow crab yesterday and it was missing a claw, But I now it will grow back. But I just noticed that it has a little bubble in the cennter between it's legs. Then to top it off it opened and the crab put it's claw in it and move it around like it was sturin somethin......Am I just seeing things or is this something that I should see????????

 

[elvis a.]

The lost arm will regrow when it molts..
As for the "trap door" that's where it's gills are located.. I'm not sure why they do that, I have no idea really.. But it's normal behavior for crabs.. Don't be frightened! It's normal!
Cheers,
Elvis