Skunk Cleaner Shrimp Eating Snails?

[Gwynhidwy]

I have five skunk cleaner shrimp (Lysmata amboinensis) in my 155G reef. A couple of times now I've seen a shrimp (not necessarily a particular one, but I can't tell them apart), with a snail turned over on its back apparently eating the snail. What I'm hoping to find out is if they are eating dead snails or attacking live snails.

The first time I assumed it was simple a dead snail, but I've seen it a couple of times now in the last few weeks and since I'm not watching the tank all the time its probably happening when I'm gone too. I can't think of any reason that the snails would be dying off either as everything in the tank looks good. This tank has been up and running for about six months.

Water parameters as follows:
pH: 8.3
temp: 80F
Ammonia: undetectable
Nitrite: undetectable
Nitrate: undetectable
SG: 1.025

Any ideas?