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<br />431
<br />
<br />JAMES E. DEACON AND W. L MINCKLEY
<br />
<br />only their body shapes, but also the presence of thick, leathery skins, with
<br />scales reduced, absent, or deeply embedded (see Section III,A,I). This
<br />is especially pronounced on surfaces that first contact onrushing water,
<br />such as the leading edges of fins, dorsum of head, nape, and antero-
<br />dorsal surfaces of the sides. All the species have relatively small eyes, set
<br />in small, bony orbits, but none is as extreme as a chub from the highly
<br />turbid Missouri River (Hybopsis meeki) which often has an overgrowth
<br />of leathery skin ,almost covering the eye surface (Moore, 1950; Davis and
<br />Miller, 1967).
<br />
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<br />
<br />2. Fishes in Gradients of Springs
<br />
<br />Gradients of dispersion created by gradual changes in habitat downflow
<br />from a spring are somewhat more fixed, and considerably more evident,
<br />than those in streams fed by surface runoff. One, or perhaps two or three,
<br />factors present overriding forces in such a gradient. Thus the Moapa dace
<br />appears as a thermal endemic (Hubbs and Miller, 1948b), as do Crenich-
<br />thys spp. (Hubbs, 1932). Cyprinodon bifasciatus (Miller, 1968) and others
<br />discussed before are capable of completing their life cycles in thermal and
<br />chemical constancy, but are incapable of doing so downstream" either
<br />because of physiological constraints or because of competitive interactions
<br />with other fish species.
<br />Springheads are notoriously depauperate in species, and this holds, gen-
<br />erally, for most animal groups (Minckley, 1963). However, in large, com-
<br />plex limnocrenes, such as in the Cuatro Cienegas basin, heterogeneity may
<br />allow a substantial fauna to co-inhabit.
<br />When the spriogs are arranged by size (Fig. 7), faunal diversity may,
<br />however, be demonstrated to increase with decreasing constancy. At the
<br />terminus of the spectrum, the sumps of springs in desiccating lakes and
<br />pools, variability is so great that it acts as an overriding force, again limit-
<br />ing the fauna to one species, or a few species, capable of resisting, or per-
<br />sisting under, the prevailing regime.
<br />The patterns of dispersion in a spring:'fed environment therefore may
<br />theoretically result from the same basic factors producing longitudinal suc-
<br />cession of fishes in streams of classical physiographic gradient (Fig. 6).
<br />Small faunas of headwater creeks correspond to those in sumps of desert
<br />systems-only colonizers capable of resisting great' extremes can live in
<br />either place. Springheads correspond to the relatively greater constancy of
<br />large, lowland rivers of temperate zones. When the springs are large enough
<br />they may support a comparably large and diversified fauna. Intermediate
<br />habitats, by nature both diversified and relatively permanent, support the
<br />greater variety of fishes.
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