Background/Question/Methods . The Chihuahuan Desert is a unique region with high biodiversity. There is a high degree endemism of freshwater biota, but relatively few studies have been focused on zooplankton. Bdelloid rotifers are one component of these ecosystems and often dominate very temporary habitats. Bdelloids are usually assumed to be cosmopolitan species due to their potentially high dispersal rates. Distribution records of the bdelloid Philodina megalotrocha extend from North America to New Zealand. However, little is known concerning its ecology or genetics. The aims of this study are to: 1) determine whether the distribution of P. megalotrocha in the Chihuahuan Desert is associated with ecological characteristics of habitats, 2) investigate whether P. megalotrocha is a truly a cosmopolitan species and not a complex of cryptic species and 3) determine patterns of gene flow among populations of P. megalotrocha with respect to their geographical distribution. Redundancy Analysis was applied to investigate relationships between rotifer species and 23 environmental factors. Variables were added when they provided extra fit to the model at a significance level of p<0.05. To determine the extent of genetic differentiation among geographically isolated populations, cox1 sequences were used to construct phylogenetic hypotheses using maximum parsimony and Bayesian Analysis.
Results/Conclusions . The first 4 conical axes in the Redundancy Analysis explained only 8.6% of the variance in the species data. Although environmental factors did not explain much variation of the distribution of P. megalotrocha among ecological habitats, this analysis indicated that P. megalotrocha is associated with the Cattail Spring pools (Big Bend National Park) and somewhat associated with flowing waters and the summer season. High levels of genetic variation among populations of P. megalotrocha were found in the cox1 sequences (ranging from 0% to 19.6%). Levels of sequence divergence among some P. megalotrocha Chihuahuan Desert populations were equivalent to or greater than those of other well-defined bdelloid species. This may indicate that P. megalotrocha populations in the Chihuahuan Desert are, in fact, members of a complex of cryptic species. Analyses of additional gene regions and morphological features are needed to corroborate the cox1 sequence findings.