Why does Hemingway compare santiago's left hand as a claw to the first shark's teeth as "shaped like a man's fingers when they are crisped into claws"?

Well, a few reasons here. The shark is a predator and Santiago is a predator as well. Although Santiago dislikes sharks (especially when they feed on his Marlin) he sees them both as hunters. They are both at the top of their respective food chains. Hemmingway also wants to show them as being old products of the sea. This is their element and Santiago's claw and the shark’s teeth symbolize this.