Some species of crabs can live in pretty cold waters, they live as far south as the sub-Antarctic islands but not actually in Antarctica’s surrounding waters. Scientists have found fossils of them though, showing that they lived there long ago in a time it was warmer there.
As Wally stated, those are crabeater seals. They are actually the most numerous of seals living in Antarctica. Their exact numbers are unknown, and actually little is know about them compared to other seals as they pretty much just live on the pack ice and ice floats around Antarctica making researching them more difficult. They are also a bit more aggressive when approached by humans, compared to the weddell that will roll over on its back. As Osbourne said they do not eat crab, they mainly eat krill along with some fish and squid. It was probably krill “crumbs” around the seals that made early explorers think they were eating crab (not knowing there were no crabs around). Krill are a major food in Antarctica’s waters, they are small and shrimp-like, crabeater seals have specially shaped teeth for filtering them out of the water. Helping to keep their large population in control are killer whales and even leopard seals that feed on the smaller, younger crabeater seals.