"This squat, ivory crab has evolved a heightened sense of hearing in order to perceive its surroundings in the lightless nooks it inhabits."
Eyeless Crabs have high armor, so if your penetration is low you may have a very hard time damaging them.
Their claws, with which they can launch a Flurry of attacks, also seem to be very effective at punching through your armor, so it is best to take these deceptively robust critters seriously, even at higher levels.
On Butchering Edit
Eyeless Crabs have three pairs of legs and two pairs of arms plus pincers (which count as an extra pair of arms). which means you can gain the "Six Arms None the Richer" achievement by dominating one of them