Greg Smith, who like Dana Eveland came over in the Dan Haren trade, replaces Dan "Crab-Man" Johnson on the roster, and Justin Duchscherer in the rotation. Since Smith is replacing the Crab-Man, I think it's natural to turn to crab predators in search of a nickname:
Crabs are frequently preyed upon by halibut, dogfish, sculpins, octopus, otters, sea turtles, and even other species of crabs. Young crabs may sometimes be eaten by their own species. Crab larvae might be eaten by pretty much anything that eats plankton.