Me and three friends on my server have been farming crabs on and off for days, spread out to catch as many rare spawns as possible. Our longest time whitout a single sighting was one spectaculary dissapointing day of 9 hours of four people farming crabs in pretty much all the locations, and not seeing a single spawn. we have seen about 30-40ish crabs total now, and helped two of our casual members to pets, though the hardcore group hasn't seen a drop. There have been a few on the AH, so the friends who don't care about the achievement cracked long ago and bought it when it shows up there.
at this point, we've had every single crabrelated pun, we practically have a theme song for ourselfs, and we spend most of our time together wondering why we are killing crabs again. Moral of the story: you're not the only one suffering out there! keep at it!
(at this point, i'm convinced this and the death adder hatchling will be the last ones i'll need for the achieve *sigh*)