I need to borrow a crab trap

[AquaPin]

Depending on what you are trying to catch; couple of options beside the crab trap are:

Inverted bottle as mentioned in previous reply

Placing a clear glass cup by the rock, critter falls/crawls in but can not get back out

Placing a piece of frozen shrimp in nylons near the mantis shrimp, it will get it's claw caught in the nylon and then you just pull him out.

Have used all the above depending on what I was after... good luck with your hunt.