I brought home 3 blue reef chromis from my LFS
and they were fine. One day I noticed there were only 2, then 1, then none. All seemed healthy and were eating well. I talked with my LFS
about it, and they said that it was indicitive of a mantis shrimp. Sure enough I started noticing those beady little eyes looking at me through a hole in the live rock. I think the chromis were small enough, and slept close enough to its hole, to become prey at night for the mantis I caught. Meals of opportunity I guess.