Just look at them.
The scientific name for the crabeater seal, Lobodon carcinophaga, translates to "lobed-tooth crab-eater," and one look at its mouth tells you why. This thing's so toothy its teeth have teeth:
These screenshots come from an old episode of The Brain Scoop called "Little Skeletons." In it, Chicago Field Museum Collections Manager Bill Stanley explains to host Emily Graslie how the teeth of L. carcinophaga are similar to the dentition of other seals – and how they are so very different.
The mouth of a crabeater seal is uniquely adapted to feed on Antarctic krill by acting like a sieve. A hungry crabeater will take a mouthful of water, close its jaws, squeeze the water out through its teeth and filter out all the krill, which it then consumes. These microscopic krill comprise over 90% of the crabeater seal's diet (despite its name, the crabeater seal does not actually feed on crabs). As Stanley explains, "the same thing that sustains 100,000 of these [crabeater seals] also sustains the 100-foot-long blue whales." Surprised? Don't be. With an estimated biomass of 500-million tons, Antarctic krill are, by some estimates, the most abundant animal species on the planet.
More nightmarish dentition here.