In this paper we present a particular history of Limulus polyphemus, the horseshoe crab, as a means of expanding on Haraway’s notion of companion species. Drawing on accounts of the horseshoe crab’s role, on the one hand, in work of the Serological Museum at Rutgers University that spanned the 1940s to the 1970s, and, on the other, in the development of the limulus amebocyte lysate test, we trace some of the complexities of human-limulus relations. These relations encompassed not only the horseshoe crab’s objectification (as a source of serum), but also the natural historical, the mythical, and the symbolic (in relation to its blue blood or its supposed status as a “living fossil”). We suggest that the horseshoe crab, and similarly alien or abjected species, can be valued as companion species if this concept is expanded beyond parameters such as intimacy, surprise, and “becoming-with” to include distanciation, wonder, and “becoming-because-of.”