Curing Live Rock and Hitchhikers

Hi the

I was reading in Vol.1 of Delbeek and Sprung's The Reef Aquarium, that
one way to rid yourself of unwanted hitchhikers is to put an octopus
in the tank with the live rock, and in short order, it will find and
eat a lot of unwanted stuff, mantis shrimp, etc. Since I've never
seen an Octopus in my city, are there any other creatures, i.e. crabs
that could perform that function?

thanks,
quietreef