Found out from other players - you have to kill the rock crabs ON Tortuga. Enter the Misty Mire swamp from the Rat's nest, there will be a rock crab just over the bridge. It will reappear after you kill it, but other critters will be trying to eat you at the same time.
I had to kill it five times to get my 2 pearls, but at least the game acknowledged it was looking for them
