Physical Description:

• Small in size, deep-bodied 2.8 to 4.6 centimeters.  
• Body length 1.1 - 1.8 inches.  
• Color- usually varies from gray to brown.  
• The abdomen usually lacks scales, save for some in front of the pelvic fins and just behind the gill membrane isthmus (a narrow strip of tissue).
• There are 20 to 21 gill rakers and generally 3 or 4 preorbital pores on either side of the head.  
• The male Pupfish have dorsal and anal fins that are black.

The Pecos pupfish can usually be found and seems to thrive in saline bodies of water that contain a limited number of other fish species. It occasionally lives in fresher waters, but that is uncommon in such habitats. In the saline habitats, populations can be very dense.  Along the Pecos River, this pupfish has most often been found in backwater areas and side pools that do not have sunfish or other various other predators.
The Latin name for the Pecos Pupfish is Cyprinodon pecosensis. The name Cyprinodon translates carp tooth, or teeth like a carp. Pecosensis just referrs to the geographic area in which the fish lives.

Pics Courtesy of the University of New Mexico Biology Department

The pupfish is a very adaptable fish and can live in diverse conditions "from water saltier than freshwater to seawater" with no problems whatsoever.
 
 

• Aquarium Site:  http://www.aquariumsite.org/families.cfm
• The United States Environmental Protection Agency
• The Native Fish Conservancy:  http://www.nativefish.org/Articles/desertpupfish.htm
• University of New Mexico Biology Department: 

http://www.fw.vt.edu/fishex/nmex_main/species/010355.htm
• Desert Fishes Council:  http://www.utexas.edu/depts/tnhc/.www/fish/dfc/na/cyprinod/cyprinod/cyprino1.html
• http://www.desertusa.com/mar97/du_pupfish.html
• http://www.fguardians.org/news/n001020.html
• Fort Worth Zoo Texas Wild Exhibit