The Conservation and Biogeography of Freshwater Mussels (Bivalvia: Unionida) in Texas: The Big Picture and a Central Texas Case Study

Abstract

Understanding how processes shape the occurrence and life history of species is important to their management and conservation. Freshwater mussels are one of the most imperiled groups in North America, with 90 of the ~300 species known are listed for protection under the U.S. Endangered Species Act. In Texas, 52 species are thought to occur in the state and 16 are imperiled, which has resulted in a statewide effort to protect remaining populations. As part of this effort, there have been 17 taxonomic revisions over the last decade which has altered the knowledge of Texas mussel biogeography. Likewise, age and growth have been conducted to better understand mussel performance in response to stressors. Two major attempts have classified Texas mussel biogeography, however both efforts relied on limited distribution data, outdated taxonomy, and lacked molecular data. Based on this knowledge, I re-examined the biogeography of mussels in Texas by performing hierarchal cluster analysis (HCA) and non-metric multidimensional scaling (NMDS) to identify biogeographic groupings. I conducted similar analysis using molecular sequence data for my target species. Results from the HCA and NMDS using community and molecular data identified five geographic provinces. These findings shed light on the biogeographic relationships amongst the mussel fauna in Texas, which will be important for the conservation across the state. Age and growth data for mussels in Texas are almost nonexistent. To address this, I estimated age and growth parameters from thin sections of 2 central Texas endemic mussel species, Cyclonaias petrina and Lampsilis bracteata. To validate growth estimates, I used cross-dating, which compares interannual growth across a cohort and measures growth synchrony. Growth and longevity estimates differed across 3 populations of C. pretrina (K = 0.065-0.100; L∞ = 55.03-94.43 mm), while L. bracteata were similar across both population (K = 0.187-0.208; L∞ = 61.40-61.52 mm), respectively. All 5 populations of mussels had positive interseries correlation (R = 0.400-0.573) which shows synchrony among populations. I was able to demonstrate that C. petrina is a K-selected species and L. bracteata is a r-selected species. These differences are important when considering risk of extirpation and actions to mitigate declines.