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Figure 1 Collecting sites of Mexipyrgus churinceanus from spring-fed habitats in the Cuatro Ciénegas valley located in the center of the Chihuahuan desert of northeastern Mexico. These pools and streams are arrayed around the Sierra de San Marcos that bisects the center of the valley. Population numbers are shown on the map and referred to in Table 1.
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Figure 2 Representative images (A-C) of the hydrobiid snail, Mexipyrgus churinceanus, collected from nearby populations to illustrate small scale variation in size and shell pigmentation. Herichthys minckleyi papilliforms (right figure D) exhibit gill arches modified into more gracile pharyngeal jaws with small muscles and pointed teeth that are ineffective at crushing snails while H. minckleyi molariforms (left figure D) have robust muscles and enlarged crushing teeth on their pharyngeal jaws that seem clearly modified to crush snails. Figure E portrays heterogeneity in habitats with Nymphaea (top) and without Nymphaea (bottom).
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Figure 3 A) Average size-adjusted crushing resistance (95% Confidence Intervals) for M. churinceanus populations. B) Frequency of banded snails for M. churinceanus populations. Populations along the Y-axis are arranged on a transect from southwestern populations to the northern Rio Mesquite down to the southeastern lobe.
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Figure 4 A and B Relationship between Moran's I and cytochrome b pairwise distance classes (correlograms) for two Mexipyrgus traits: size-adjusted crushing resistance (triangles and dashed line) and frequency of pigmented shells (closed circles and solid line).
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Figure 5 Relationship between frequency of molariform cichlids and both adjusted marginal mean crushing resistance (triangles and dashed line) and frequency of pigmented shells (circles and solid line).
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