what role do the sharks play in the novel? are they santiagos sole antagonists?

THe sharks are antagonists in the novel. In Santiago's list of creatures he dislikes, they are pretty high. THey come and take what is his. THey desecrate the great Marlin, Santiago's kill, his friend and his brother. Still the sharks aren't the only antagonist. THere are the other fishermen (society)who laugh at him. THere is of course the great Marlin who serves a multi-dimensional role including antagonist. THere is also the betrayal of Santiago's own body.

The story's main antagonist is nature, the sea. The sharks play an important role because they challenge Santiago's inner strength (his spirit).