The tiger barbs probably ate them. I had tiger barbs before they loved eating the shrimp.
no there isn't any exoskeletons... do they bury in normal sized gravel?
I don't believe they do. Brianna's probably right about the barbs eating him.
They can't survive out of the water for long, check all around your tank. I've had amano shrimp escape and they got less than a foot from my tank. They are either hiding or like they said eaten. Just because one fish eats them doesn't mean all do, it depends on the fishs personality. Angels are supposed to eat shrimp, mine don't. Watch your tiger barbs and see if they notice them at all.