Biogeography of Selected Spring Endemics in Texas Interglacial-Drought Refugia with Unexpected Insights from a Spring-Dependent Nematode Parasite

Abstract

The contemporary distribution of crenobiontic endemics in central Texas is enigmatic because only some springs are occupied by crenobionts despite other unoccupied springs having seemingly suitable habitats. In the absence of complete paleo-records, a cohesive and widely accepted explanation for this biogeography has eluded researchers for many years. We suggest that data on contemporary species with obligate coevolution, such as parasites with multiple obligate hosts in their life cycles, can help to fill intervening gaps in the paleo-record because the contemporary distribution of such a parasite indicates that its hosts cohabitated without interruption since the arrival of the parasite. To test this conjecture, we studied one such parasite endemic to a select few central Texas springs, Huffmanela huffmani. By studying the distribution of the intermediate host, geologic and paleo-climatic records, performing lab experiments with live animals, and examining archived museum specimens of the definitive hosts from the 1950s, we were able to test multiple predictions about how the distribution of H. huffmani became what it is today. Our results corroborate a narrative suggesting that several severe droughts since the Wisconsin glaciations are responsible for having sculpted the present-day distribution of central Texas crenobionts.