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First published online August 30, 2006
Journal of Experimental Biology 209, 3499-3509 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02417

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Testing an ecophysiological mechanism of morphological plasticity in pupfish and its relevance to conservation efforts for endangered Devils Hole pupfish

Sean C. Lema^* and Gabrielle A. Nevitt

Center for Animal Behavior and Section of Neurobiology, Physiology and Behavior, University of California, Davis, Davis, CA 95616 USA

View larger version (16K): [in a new window]   Fig. 1. The growth rate of pupfish from the high food (100% rations), medium food (50% rations), and low food (20% rations) treatments differed (P<0.0001). Tukey pairwise comparisons showed no difference (ND) in body length among the three treatments on the 1st measurement day (15 days post-fertilization) when fish were initially assigned to treatments. Body size deviated significantly by day 36 and remained different among treatments through the conclusion of the experiment on day 141. Values are means ± s.e.m. (N=7-8).

View larger version (15K): [in a new window]   Fig. 2. Relative (A) head size (P=0.0443), (B) eye size (P=0.0119) and (C) body depth (P<0.0001) differed in pupfish from high food (100% rations), medium food (50% rations), and low food (20% rations) treatments. Values are means ± s.e.m. (N=7-8). Letters indicate pairwise comparisons among groups (Tukey HSD test). SL, standard length.

View larger version (18K): [in a new window]   Fig. 3. Effect of food ration on pelvic fin development. (A) A significantly smaller proportion of fish in the low food ration treatment developed pelvic fins when compared to the medium and high food treatments (P=0.0048). Values are means ± s.e.m. (N=7-8), and letters indicate pairwise differences (Tukey HSD test). (B) In all treatments, there was a trend toward a low proportion of fish with pelvic fins when reared at higher temperatures (P=0.0392). Each data point represents the mean of two replicated buckets per temperature value (except the point indicated by asterisk, which represents a single bucket).

View larger version (13K): [in a new window]   Fig. 4. Food ration treatments differed in whole-body levels of the thyroid hormone T4 (P<0.0001). T4 levels are plotted against body mass (g) for each treatment bucket.

View larger version (20K): [in a new window]   Fig. 5. Effect of goitrogen treatment on pupfish morphology. Relative (A) head size (P=0.0053), (B) eye size (P=0.0013) and (C) body depth (P=0.0049) of pupfish varied significantly among the methimazole, KClO4, and control treatments. Relative head size, eye size, and body depth are presented as percent of standard length (SL). Values are means ± s.e.m. (N=7-8). Letters indicate pairwise comparisons among groups (Tukey HSD test).

View larger version (18K): [in a new window]   Fig. 6. Elevated rearing temperature increased (A) relative eye size (P=0.0035) and decreased (B) relative body depth (P=0.0221) in the methimazole, KClO4 and control treatments. Each data point represents the mean of two replicated buckets per temperature value (except the point indicated by asterisk, which represents a single bucket).

View larger version (18K): [in a new window]   Fig. 7. Effect of goitrogen treatment on pelvic fin development. (A) A significantly smaller proportion of fish in the methimazole and KClO4 treatments developed pelvic fins when compared to the control (P=0.0009). Values are means ± s.e.m. (N=7-8) and letters indicate pairwise differences (Tukey HSD test). (B) The proportion of fish with pelvic fins plotted against rearing temperature for each treatment. Elevated rearing temperatures resulted in a lower proportion of fish developing pelvic fins (P=0.0196). Each data point represents the mean of two replicated buckets per temperature value (except the point indicated by asterisk, which represents a single bucket).

View larger version (23K): [in a new window]   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.