The fountain darter is a small freshwater fish found in the headwaters of only two rivers in Texas

Texas

Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

: the Comal River

Comal River

The Comal River is the shortest navigable river in the state of Texas in the United States. Proclaimed the "longest shortest river in the world" by locals, it runs entirely within the city limits of New Braunfels in southeast Comal County. It is a tributary of the Guadalupe River...

 and the San Marcos River

San Marcos River

The San Marcos River rises from the San Marcos Springs, the location of Aquarena Springs, in San Marcos, Texas. The springs are home to several threatened or endangered species, including the Texas Blind Salamander, Fountain Darter, and Texas Wild Rice...

. It is generally smaller than 3 cm long and feeds on small invertebrates. It is a federally listed endangered species

Endangered species

An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...

 of the United States.

They live in the vegetation at the bottom of clean springwater rivers, the overuse of such springs being the main reason for their depletion. An extra worry for the future of fountain darters is the parasitic, non-native trematode (Centrocestus formosanus

Centrocestus formosanus

Centrocestus formosanus is a trematode parasite of Asian origin that has found its way in to North American streams and rivers. It not only affects the fountain darter but many species of commercially important fishes....

), whose impact on the darter population is not yet known. As of 2004 only the Comal population is seriously affected by the parasite. The National Fish Hatchery & Technology Center in San Marcos

San Marcos, Texas

San Marcos is a city in the U.S. state of Texas, and is the seat of Hays County. Located within the metropolitan area, the city is located on the Interstate 35 corridor—between Austin and San Antonio....

keeps a reserve population of 500 adults as a hedge against a man-made or natural event wiping out the fish in the wild.