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lower Colorado basin supported about 18 fish species. Six of these were small, short-lived minnows, and an equal number were larger, longer-lived minnows and suckers. Four even larger and more long-lived minnows and suckers, most common in "big-river" habitats, entered small rivers and may have spawned there, as is discussed later. Last, a pupfish and topminnow in the Gila basin ascended to slightly more than 1500 m elevation. Here, and at lower elevations, they occupied marshes and stream margins, as outlined below along with other low- desert species. The Rio Yaqui supports 20 species, only three of which are shared with the Colorado. Using the same categories as above, eight were small and short-lived, 12 are larger and longer-lived, but none (perhaps one or two, see later) are clearly "big-river" in ecology. A pupfish lives at higher elevation, and topminnows are represented by a number of species that live from relatively high elevation to the river's mouth. Short-lived minnows.-Among this group of minnows, spikedace, spinedaces, loach minnow/ charalito ardornado, and speckled dace are in the Colorado system. All are related to other species of western North America, and all but the last are endemic, commensurate with the long isolation of the Colorado River basin. Longfin dace/charalito aleta larga, another species of western relationships, is shared by the Colorado and Yaqui systems. Minnows with eastern relationships-beautiful shiner/sardinita hermosa, Mexican stoneroller/rodapiedras mexicanoJ and ornate minnow/sardinita ornata-are restricted to the Rio Yaqui and associated basins to the south in Mexico. The spikedace (Fig. 27) occupies midwater habitats of runs, pools, and swirling eddies, where it feeds on drifting aquatic and terrestrial insects. The species is mottled dorsally, silvery on the sides, and whitened beneath. Males in breeding condition become golden or brassy on their dorsal and lateral surfaces. Spawning is in spring and summer, with Figure 27. The spikedace, endemic to the Gila River basin of Arizona and Sonora, is listed as threatened. The specimens illustrated are ca. 70 mm in total length, from the Verde River, Arizona; female above, male below. younger females spawning once and older females twice each year. Life expectancy is two or three years, and the fish rarely exceeds 75 mm in total length. Spinedaces (Fig. 28) occur in the Little Colorado and Virgin rivers, and in isolated springs of the '-'II .. II II Figure 28. Above: Little Colorado spinedace, 80 mm in total length, from the Little Colorado River, Arizona. Below: Virgir spinedace, ca. 90 mm in total length, from Beaver Dam Creek Arizona. Both species are considered threatened by the American Fisheries Society endangered species committee. 15