What popular seafood item is actually a crustacean related closely to spiders and scorpions?

Crab

Who starred as Batman in Christopher Nolan's "The Dark Knight" trilogy? What famous composer wrote "Messiah," the musical work featuring the "Hallelujah" chorus often performed at Christmas? What U.S. state is famous for its annual Christmas-themed town named Santa Claus? Who established the Persian Empire in the sixth century BC? Who developed the polio vaccine?