There are many different ways of capturing a Mantis Shrimp,
but most methods involve removing the LR from the tank. A
very easy method which is minimal invasion of the tank is
a trap.
You first need a small plastic soda bottle; 16oz is perfect.
Cut the top of the bottle neck off at the widest point.
Drop some bait into the bottom portion of the bottle and
invert the top back into the lower portion so that it looks
like a funnel.
Fill the trap with tank water and put it as near to the rock
where you know the Mantis is living.