MORPHOLOGICAL DIFFERENTIATION OF THERMAL SPRING FAUNAS IN THE CHIHUAHUAN DESERT: HYDROBIID SNAILS FROM CUATRO CIÉNEGAS, MEXICO

The Cuatro Ciénegas basin in the Chihuahuan Desert of northeastern Mexico harbors a unique aquatic system of thermal springs and associated outflows, marshes, and evaporative playas. Because of its isolation within an evaporitic terrestrial landscape, the remarkable levels of diversity and endemism among organisms, and the high degree of environmental, hydrologic, and geochemical variability, this system is an excellent evolutionary laboratory and a major target for astrobiological research. The endemic hydrobiid snail genus Mexipyrgus is a model organism for studying environmental and evolutionary influences on morphological variation in organisms inhabiting evaporitic environments because its wide distribution in the basin encompasses an array of thermal spring and associated aquatic environments. Previous research on Mexipyrgus have identified it as a group with highly variable shell morphologies and thus, it may provide information on the process of morphological adaptation, population differentiation and speciation in analagous environments.

We analyzed variation in Mexipyrgus shell morphology within and between populations, environments, and hydrological systems. Preliminary results from principal and relative warp analyses of our landmark morphometric data suggest that populations are discriminated primarily on the basis of shell size; specimen size differs significantly among the populations. Despite the differences in size, however, most specimens are morphologically similar and follow similar developmental pathways, even when the individuals come from hydrologically distinct and isolated systems. But these preliminary data also suggest that there are at least two distinct developmental morphologies present in our samples. If we are to understand the nature of diversification and evolution in aquatic systems such as these, we must characterize morphological variability in these populations and evaluate the potential relationships between morphological variability, evolutionary differentiation and environmental conditions.