Animals live in underground rivers and lakes!
Deep in caves where sunlight never reaches, ghostly fish, crustaceans, and salamanders live their entire lives in the dark. Many caves are made from limestone, dissolved by rain, forming pools of water deep underground and subterranean rivers.
Troglobites—animals that spend their lives in caves-are found in these bodies of water.
Examples of troglobites include ghostly salamanders, such as the olm, found in caves in Slovenia and surrounding areas of southern Europe. They have external gills that help them collect the scarce oxygen from the underground water. Their skin is transparent. Because they have little need for sight, they are born blind. Their eyes are barely visible and are covered by a layer of skin. Many species of cave fish have no need for pigmentation in the dark, so their internal organs can be seen through their skin. Some have no eyes at all. These animals explore the world through other senses, like sound, touch, and taste.
Culver, D. C. (1976). The evolution of aquatic cave communities. The American Naturalist, 110(976), 945–957. https://doi.org/10.1086/283110
Owen, J. (2015, September 11). How this cave-dwelling fish lost its eyes to evolution. National Geographic News. https://www.nationalgeographic.com/news/2015/09/150911-blind-cavefish-animals-science-vision-evolution/
Trontelj, P., Blejec, A., & Fišer, C. (2012, July 16). Ecomorphological convergence of cave communities. Evolution: International Journal of Organic Evolution, 66(12), 3852–3865. https://doi.org/10.1111/j.1558-5646.2012.01726.x
PBS. (2010, November 30). The olm and other troglobites. PBS Nature. https://www.pbs.org/wnet/nature/the-dragon-chronicles-the-olm-and-other-troglobites/4533
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