Which sea creature can regrow lost arms?

Starfish, and some can do considerably better than regrowing an arm - in several species a severed arm with a piece of the central disc attached will grow an entirely new animal, so cutting one in half produces two. Fishermen who used to chop up starfish caught on oyster beds and throw them back were, in effect, farming them.

The animal is strange in most other respects too. It has no brain and no blood; it runs on a nerve ring, and instead of blood it pumps seawater through a hydraulic system that works the hundreds of tube feet on its underside. To eat a mussel it grips the shell, pulls steadily until a millimetre-wide gap opens, and then pushes its stomach out through its own mouth and into the shell to digest the animal in place.

Their regenerative ability is studied for the obvious reason. A starfish regrows nerve tissue, muscle and gut without scarring, from cells that revert to an unspecialised state and rebuild - which is exactly what human tissue does not do. A sea star wasting disease epidemic from 2013 also killed billions of them along the Pacific coast of North America, and where the sunflower star disappeared, urchin numbers exploded and kelp forests collapsed.