Which is funny, cuz if you hit their legs, they split at the stomach AND the crab dies. If you kill any zombie with an objects with the gravity gun, no matter where it hits them, as long as the die, the crab dies.
Gravity Gun was also useful in that area as far as saving ammo goes because it had the same effect on normal and fast crabs as the pistol did (2 hits from grav gun on easy mode killed them) and it also took down poison crabs in 2 hits on easy mode. That was especially nice since it took 5 or 6 shots from the pistol. If you did gravity gun, shotgun, and a wee bit of pistol, you'd make it through ravenholm easily.