In the fabulist world of Shark Heart, animal mutations function the same way that terminal illnesses do. Incurable and physically destructive, the mutations cut the human lives of Lewis, Angela, and Tiny Pregnant Woman short. During the appointment where Lewis receives his diagnosis, the language the doctor uses evokes the language real-world doctors use to describe conditions like cancer: “You’re in the early stages of a Carcharodon carcharias mutation. […] Chondrichthyes mutations, what we call the class of cartilaginous fish mutations, are usually fast-developing, aggressive” (10). This establishing scene indicates the metaphorical significance of the mutations and makes room for the rest of the novel to explore how Wren and Lewis will navigate the illness and their anticipatory grief.

Over the course of Part 1, Wren goes through several stages of grief as she braces herself to lose Lewis forever. At first, she copes by throwing herself into the athletic pursuit of swimming. She tells herself that “she, like Lewis, like the Tiny Pregnant Woman, would transform into something equipped for the water” (68). In this sense, her grief takes an empathetic form, and she tries her hardest to keep up with Lewis’s changing body.