steve, the species seen in the video is Lysiosquillina maculata, a spearer. It isn't capable of cavitation. It could still do major damage witth those great big sharp raptorial appendages and spined telson though.
If the mantis in question were a smasher, still no cavitation would occur. the claw is a small enclosed fixed volume space. No cavitation will occur within the claw or outside it because air is too thin to form such a steep pressure gradient. The claw may still be damaged simply due to the extreme forces the muscles exert on it. The damage would likely be severe if the animal had just moulted, but would also be more reluctant to strike as well. My guess is that the animal was freshly moulted or very unhealthy.
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