College of Science and Health Theses and Dissertations

Date of Award

Summer 8-23-2015

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Windsor Aguirre, Ph.D.

Second Advisor

Tim Sparks, Ph.D.

Third Advisor

Jason Bystriansky, Ph.D

Abstract

Most aquatic vertebrates are ectotherms. As a consequence, they are particularly susceptible to variation in environmental conditions during development, which can impact their phenotypes and survival. In fishes, vertebral morphology and body shape are particularly susceptible to changes in temperature and water flow regimes, two variables that are often correlated along altitudinal gradients in streams. This study addresses the impact of variation in these ecologically important environmental variables on the phenotype of the emerging model species Astyanax mexicanus (Teleostei: Characidae). Specifically, this study examines how variation in temperature and water flow impact vertebral number and body shape variation. Fish from seven different lab-reared crosses were pooled and subjected to four temperature treatments (20°C, 23°C, 25°C, and 28°C) for one month. They were then split into two different water flow regimens (flow and no flow) for an additional four months. A significant difference in the number of precaudal vertebrae was documented for fish reared under the different temperature treatments, with fish reared at 20°C having significantly more precaudal vertebrae than fish reared at higher temperatures. However, the number of caudal vertebrae and the total number of vertebrae did not differ significantly between temperature treatments, although the latter was only marginally non-significant. No effect of water flow treatments on vertebral number was detected, which is consistent with vertebral number being set early in development. A higher number of fused vertebrae was also documented in the 20 and 28°C temperature treatments, consistent with the breakdown of canalization mechanisms outside of the range of normal development. Body shape variation was significantly associated with body size, temperature, water flow treatments, the interaction between water flow and temperature, total vertebral 2 number and the ratio of precaudal to caudal vertebrae. Allometry accounted for the greatest component of body shape variation, followed somewhat surprisingly by temperature treatment. Temperature treatment was associated with variation in the upper jaw, the ectocoracoid, an increase in the size of the caudal peduncle and an increase in the dorsal fin and anal fin base lengths. Water flow treatment was associated primarily with variation in body depth. Fish subjected to the additional water flow were more streamlined than fish reared in still water, consistent with expectations from ecomorphological models. Variation in vertebral number within temperature treatments was associated mostly with body elongation, with fish having more vertebrae having more elongate bodies. Finally, the ratio of precaudal to caudal vertebrae was also significantly associated with variation in body shape, with fish having a greater proportion of precaudal vertebrae exhibiting an expansion of the abdominal region and fish having a greater proportion of caudal vertebrae exhibiting an expansion of the caudal region. The findings of this study, could apply to other aquatic species of characids, possibly including those experiencing rapid changes in their habitats. Additional studies are needed in order to more completely understand the influence of environmental factors in generating morphological variation through phenotypic plasticity. Examining how the morphological changes documented here impact other traits affecting fitness like swimming performance would provide greater insight into their functional significance.

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