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Figure 50. Sonoran chub/chara/ito sonorense, 12.7 em in total length, from its only locality of occurrence in the United States in Sycamore (or Bear) Canyon, Arizona; it is listed as threatened. headwaters of essentially all the major tributaries to the Gila River. It is becoming rare, especially where land use practices such as overgrazing lead to incision of floodplains and lowering of water tables, which, in turn, drain marshlands and other stream- associated habitats. Sonoran chub (Fig. 50) scarcely enters the United States in a single stream that drains southward across the international boundary. It is widespread in the small Rio Concepcion basin in Mexico. When in breeding condition, it is one of the most brightly colored species of chubs, with intense red on the lips, opercles, fin bases, and belly. It occupies erosive creeks, sometimes living under severe intermittent conditions in remnant pools under cliffs. It seems more omnivorous than other chubs, feeding extensively on algae as well as invertebrates. This apparent opportunism may relate more to the severe habitat than to any basic preference-anything tends to be eaten when provisions are scarce. Other, Special Fishes.- Two special groups of fishes, pupfishes and topminnows, are distributed in part in streams of intermediate elevation in the Colorado and Yaqui systems. Although both these groups are often thought of as "desert fishes" inhabiting isolated springs, neither is exclusively so. Pupfishes are especially misrepresented in this regard. In fact, both groups are quite commonly represented along margins of small to large streams in habitats well above the desert floor. The original elevational range of desert pupfish/cachorrito del desierto (Fig. 51) in the Colorado basin was from well below sea level in the Salton Sea area to at least 1500 m elevation in the upper San Pedro River of Sonora. In Mexico, the undescribed, whitefin pupfish/cachorrito aleta blanco Figure 51. Male (above) and female (below) desert pupfishl cachorrito del desierto, 46 and 35 mm in total length, respectively, from Estuario de Santa Clara, Sonora. This endangered species is now restricted to three small, natura localities in the United States, and about the same number of sites on the Colorado River delta in Mexico. 25