A pigeon causes the horrible death of many people.
A shark saves the lives of several people.
Both of them did it the same way.
Both of them triggered great adventures across the seas.
Who wrote about the adventures they triggered?
Although pigeons are usually a symbol of peace or harmlessness, and the sharks are a symbol of danger, and in the riddle it's ironically the opposite, the pigeon is still a single literal pigeon, and the shark is a single literal shark, no mere symbolism.
They both delivered a message. Not figuratively, but a real message, written on real paper. They have no part in the story after this. Their only role in their respective stories is that they trigger the plot by delivering the message, and we never hear about them again.