Who or what is the antagonist in the colomber by dino buzzati

In the story, The Colomber, the colomber was portrayed as the antagonist of the story. 

A colomber is a mysterious shark which symbolizes doom to the person who sees it. It is said to pursue the person until the very end. However, at the end of the story, there was twist. The twist uncovered the real antagonist, it is the fear of the unknown or the unproven fact.