I’m a bit late to this party but just wanted to add a few comments about keeping mantis shrimp. Many species including the one in this thread are intertidal and tolerate a wide range of salinity and are even resilient to swings which would be less than ideal for coral. I can’t see the meral spots on the mantis in the pictures above but he is either Gonodactylus smithii or one of several species from the genus Gonodactylaceus.
Other species are found in deeper water, like the popular Peacock mantis shrimp. But this species, too, thrives in salinity ranges of 1.021 - 1.027. They don’t tolerate swings as well as intertidal species, but salinity is really not the most important factor as some in this thread have insinuated. Normal reef conditions in terms of alkalinity, calcium, etc are ideal for Peacock mantis shrimp.
Peacocks and other species do quite well in reef tanks and normally don’t bother coral that is well-attached. They will move loose frags, however.
The only difficulty in keeping Peacock mantis shrimp is controlling nitrates. Prolonged exposure to nitrates above 10ppm will cause shell rot, especially in older, larger males.
Otherwise, Peacocks and other mantis shrimp species are excellent candidates for a predator-only reef tank.