Necropsy of a 395kg shorten make shark captured off Montauk, New York revealed 2 healthy eels within the lumen of the heart. Close examination of the shark provided no clues as to the portal of entry used by these eels to access the heart. The eels, 21 and 24cm in length and both immature females, were identified as Simenchelys parasitica (Synaphobranchidae). The stomachs of both eels were filled with blood cells indicating that they had resided in the circulatory system at least long enough to ingest blood. Comparison of heart tissue from the eel inhabited shark with heart tissue collected from uninhabited makos showed alterations of the inhabited heart which might indicate that these eels resided in the heart long enough to cause histological changes. In light of this remarkable find and additional information on S. parasitica found in the literature, we consider this species to be a facultative parasite. This unusual finding also raises an insidious question. Why aren't there any truly endoparasitic fishes? In reviewing the physiological and life history plasticity seen within Pisces we conclude that reproductive barriers most likely have precluded the establishment of this lifestyle.