Patzcuaro salamanders have been hybridized with axolotls, and used in mitochondrial studies for comparison. Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. Axolotls exhibit neoteny, meaning that they reach sexual maturity without undergoing metamorphosis. Amphibians such as axolotls prefer a hard watersolution. Tilapia and carp directly compete with axolotls by consuming their eggs, larvae, and juveniles. Axolotls have barely visible vestigial teeth, which would have developed during metamorphosis. He has studied cardiac mutant axolotls, transgenic mice, and induced pluripotent adult stem cells. Chlorine, commonly added to tapwater, is harmful to axolotls. Several species indigenous to the lake are now extinct or endangered (e.g. axolotls). Axolotls have four pigmentation genes; when mutated they create different color variants. Axolotls are used extensively in scientific research due to their ability to regenerate limbs. In contrast to Axolotls, in mudpuppies, these THs are normally expressed. Climate change has also immensely affected axolotls and their populations throughout the southern Mexico area. As of 2003, there were only 600 axolotls known to exist in the wild. However, some need to be maintained because the axolotls are using them for reproduction purposes.