Meet the Butterfly Kingfish AKA Butterfly Mackerel: the hottest fish you've never heard of! This not-well-studied fish is the sole species in the monotypic genus Gasterochisma, and the oldest lineage of Scombridae (the mackerel, tuna, and bonito family). Katrina, Guy, and guest Fernando Melendez Vazquez from the American Museum of Natural History in New York City dig into the details about this large, endothermic oddball and the broader topic of warm-bloodedness in fishes.