Fishes: The Rio Tamesí/Pánuco system contains at least 85 species of freshwater fishes (although some of these occur in interior headwaters west of this region), where temperate and tropical taxa mingle and endemism is high. One source list 93 species in the Guayalejo-Temesí watershed of Tamaulipas alone, but this includes brackish-marine species. Many of the streams and rivers of this region are spring fed by karstic aquifers, providing consistent and relatively thermally stable water compared to other basins sourced largely by precipitation. The karstic environment of the Sierra Madre Oriental produces many caves and subterranean waterways that ultimately surface near the base of the mountains in the west of this region. Some fish like the endemic phantom blindcat (Prietella lundbergi) are adapted to caves and have been collected at depths of 50 meters in cave systems of the Rio Frio (in the Rio Guayalejo drainage). Some populations of the Mexican tetra (Astyanax mexicanus [= A. jordani]) also inhabit caves and are blind, although other populations of the same species living in surface streams have eyes, while still other populations are intermediate. The region is rich in platyfish and swordtail (Xiphophorus) diversity including sheepshead swordtail (Xiphophorus birchmanni), delicate swordtail (Xiphophorus cortezi), mountain swordtail (Xiphophorus nezahualcoyotl), and the endimic varialbe platyfish (Xiphophorus variatus). Cichlid diversity is also high in this area including the lowland cichlid (Herichthys carpintis) and endemic species like the chairel cichlid (Herichthys [Nosferatu] pantostictus) and Nautla cichlid (Herichthys deppii) among others. Other fishes include the pigmy shiner (Notropis tropicus), Forlón gambusia (Gambusia regani), gulf gambusia (Gambusia vittata), chubsucker minnow (Dionda erimyzonops), and the endemic lantern minnow (Dionda ipni). Conservation threats of fishes include, damming of waterways, water diversion for agriculture, runoff associated with agriculture and livestock, oil and industrial contamination, invasive species, and irresponsible recreational activities.