How does the appearance of the shark contribute to the theme of the story?

The shark that circles the boat in the middle of the night symbolizes the awe-inspiring terror of nature. The fin is otherworldly as it activates phosphorescence in the water, yet the shark is a violent predator. The correspondent is both intrigued and frightened by the shark. It would be easy for the correspondent to project ill-will onto the shark, but it is impossible to say whether the shark is purposefully stalking the boat or swimming around indifferently.