R. R. Miller & Walters, 1972
R. R. Miller & Walters, 1972
The Catarina pupfish (Megupsilon aporus) is a diminutive species of fish in the Cyprinodontidae family, first described in 1972. It was endemic to a spring in Nuevo León Mexico, but has been extinct in the wild since about 1994. As of December 2012, one small captive population still remains. In addition to its small size, it is characterized by absence of pelvic girdle and pelvic fins, and by having different numbers of chromosomes in male and female fish. In 2013, its behavior was described based on very limited field observations of the previous wild population and more detailed observations in aquaria.
The Catarina pupfish is highly imperiled. It was found in the wild in only one spring in Nuevo León Mexico together with Potosi pupfish (Cyprinodon alvarezi). Contreras and Lozano (1996) reported that both species had become “almost extinct” in 1994 when their spring habitat essentially dried out; however inspection of a side spring in November 1994 indicated that a few specimens remained. Subsequent publications  indicated that both species had become extinct in the wild that year. The IUCN Red List also uses that designation, but cites an unpublished manuscript written earlier.
Mexico’s 2010 official list of species at risk (NOM-059-SEMARNAT-2010) indicates that Megupsilon aporus is category "E" defined as "Probably extinct in the wild". Species that are considered extinct by experts are given that designation. However, if a species were rediscovered alive it would be given legal protection status immediately.
The fate of the Catarina pupfish is just one of many ongoing conservation issues in Mexico. As of 2008, approximately 40% of more than 500 described freshwater fishes in Mexico are considered to be at risk and there have been 29 extinctions mostly in the previous 50 years. The extinctions and threatened status of many freshwater fishes are associated with overexploitation, dewatering, habitat disruption, and competition with alien species of diverse sources. 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.
Megupsilon aporus is the only known member of its genus. 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.
According to FishBase, this species grows to a length of 4 centimeters (1.6 in) TL. It has been highlighted as one of the smallest fish in North America. The nape and sides of nuptial males are iridescent blue with a dark blotch at the base of the caudal peduncle. Mature females are golden olivaceous with an indistinct lateral band. There are no pelvic fins or pelvic girdle.
Megupsilon differs from Cyprinodon in having fewer chromosomes in males than in females (47 vs. 48). Males have a large Y chromosome which appears to result from the fusion of two chromosomes: an autosome and the ancestral Y chromosome. The Megupsilon example was the first instance of autosome/Y chromosome fusion discovered in a fish. Subsequent research suggests that this type of chromosome fusion is relatively common; 35 examples have been found as of 2012. Among Cyprinodontidae species, Garmanella pulchra (Yucantan flagfish or Progreso flagfish) males also have one less chromosome than females.
Behavior and morphology
Liu and Echelle (2013) describe its behavior and unusual morphology as follows:
“We provide the first description of behavior in the Catarina pupfish (Megupsilon aporus). Aggressive, courtship, and spawning behaviors resemble those of other North American cyprinodontids. However, M. aporus [Catarina pupfish] differs from others in the group in absence of breeding territoriality in males. 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.”
Opercular rotation observed during aggressive displays was described as outward flaring of opercules and branchiostegal rays. Jaw-nudging observed during courtship was described as repeated protraction and retraction of male premaxillaries during which the jaw occasionally touched the females head (significance of the touching undetermined).
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. 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. Liu and Echelle (2013) theorized that the restricted habitat may have influenced its evolution. They also offered a contrary hypothesis that this species is a relict of a larger group of Megupsilon species in which miniaturization and absence of pelvic fins were characteristic.
Miniaturization and absence of pelvic fins in Catarina pupfish may be linked with each other. Studies indicate that miniaturization is often associated with morphological novelty. 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.
Some of the variability in behavior and morphology among pupfishes may be a response to harsh environmental conditions mediated by endocrine systems. Studies of specific endocrine systems which foster phenotypic plasticity in fishes and the evolution of endocrine pathways are underway.
- Contreras-Balderas, S. & Almada-Villela, P. (1996). Megupsilon aporus. The IUCN Red List of Threatened Species. Version 2014.2
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