In an attempt of saving the rapidly declining species, some were brought into captivity in the late 1980s and early 1990s, but it proved very difficult to maintain.

[4][5][6] In addition to its small size, it was characterized by absence of pelvic girdle and pelvic fins, novel male reproductive behavior of jaw-nudging, a darkened dorsal patch, and by having different numbers of chromosomes in male and female fish due to a recent chromosomal fusion event.

[9] In an attempt of saving the Catarina pupfish, small numbers were brought into captivity in 1987 and 1992, but the species proved very difficult to maintain.

In 2014, the last male individual of this colony died in Chris Martin's laboratory (photograph in inset) and with its demise the species was fully extinct.

The extinctions and threatened status of many freshwater fishes are associated with overexploitation, dewatering, habitat disruption, and competition with alien species of diverse sources.

[12] The inland fish fauna of Mexico is particularly vulnerable because many species (such as the Catarina pupfish) are endemic to isolated springs or small drainage systems as extensively described by Robert Rush Miller.

[2] Based on a molecular clock analysis of mitochondrial DNA, it has been estimated that Megupsilon and Cyprinodon diverged from a common ancestor approximately 7 million years ago.

Male M. aporus often perform opercular rotation during aggressive displays and jaw-nudging during courtship, behaviors that, among other North American cyprinodontids, are absent or known only in Floridichthys.

Some unusual features of behavior (lack of territoriality) and morphology (dwarfism [=miniaturization]; absence of pelvic fins) in M. aporus might have been shaped by interaction with a cohabitant, the Potosi pupfish Cyprinodon alvarezi.

[7] The hypothesis that Megupsilon behavior and morphology might have been shaped by interaction with the other pupfish stems from the observation by Miller and Walters (1972) in the original description of the genus and species.

[2] In aggressive interactions between them, the larger Cyprinodon species dominated Megupsilon, which seemed to restrict its distribution to shallow, highly vegetated parts of the spring.

[18] Also numerous examples of adaptation of bone growth to miniaturization in fish, amphibians and reptiles have been noted; these include skeletal reductions such as reduced ossification or complete loss of the pelvic girdle.