Fig. 8. Morphological plasticity in pupfish may be driven by differences in environmental conditions that affect thyroid hormone physiology and, subsequently, morphological development. This model illustrates a possible scenario where high water temperatures elevate metabolism and cause a suite of physiological changes in the thyroid hormone system that are dependent on food availability and quality. If food resources are abundant or high in nutritional quality, pupfish develop a normal morphology with small head and eye sizes, a deep body, and pelvic fins (possibly occurring in the refuges for C. diabolis). If food resources are scarce or of low quality, however, thyroid hormone production may be inhibited resulting in development of the neotenous morphology (large head and eye sizes, small body depth, lack of pelvic fins) typical of C. diabolis in Devil's Hole.