The boxer pupfish (Cyprinodon simus) is a small species of pupfish in the family Cyprinodontidae. It is endemic to Lake Chichancanab in Quintana Roo, Mexico. In almost all places, different Cyprinodon species do not overlap in their range, but there are two notable exceptions and one of these is Lake Chichancanab, which is inhabited by C. simus, C. beltrani, , C. labiosus, C. maya, and C. verecundus (the other place where several Cyprinodon species live together are lakes in San Salvador Island, the Bahamas). Living together, the Cyprinodon species in Lake Chichancanab have diverged into different niches. Pupfish typically feed on algae and detritus. In Lake Chichancanab, however, C. simus has become a zooplankton-feeder. It once occurred in large schools, but has declined drastically due