Attack of the crab monsters seems to do exactly what the title implies...

B-Movie maestro Roger Corman provides giant crab-related shocks on a remote island.  And the crabs look pretty good, considering it was made fifty years ago on a tiny budget.

Something not obvious from the trailer is that apparently the crabs talk, and one has a French accent.  It's got to be worth a watch if only for that.

Favourite bit:  All the parts with the giant crabs.