Thermal Tolerance, Physiologic Condition, And Population Genetics Of Dreissenid Mussels (Dreissena polymorpha and D. rostriformis bugensis) Relative To Their Invasion Of Waters In The Western United States
Morse, John T.
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Zebra mussels, Dreissena polymorpha, and quagga mussels, D. rostriformis bugensis, Eurasian freshwater bivalves, invaded the North American (NA) Great Lakes in the mid 1980's. Subsequently, they invaded water bodies east of the Rocky Mountains. Their more recent expansion into warm, isolated, southwestern U.S. water bodies suggests that both species are developing thermally-tolerant physiological races. Chronic upper thermal tolerance limits were determined for D. polymorpha populations from Winfield City Lake (KS) (WCL), Lake Oologah (OK) (OL) and Hedges Lake (NY) (HL), and the D. rostriformis bugensis population from Lake Mead (NV/AZ) (LM). Individuals from WCL had 12-d and 28-d incipient upper thermal limits of 31.7°C and 30.7°C, respectively, greater than any other D. polymorpha population in NA. In contrast, specimens from the warmer LO had a 12-d incipient upper thermal limit of 26.6°C, anomalously lower than that for individuals from the cooler HL at 29.0°C.To examine the basis for the low thermal tolerance of LO mussels, the relationship between dry tissue weight (DTW) and shell length was assessed for WCL samples of D. polymorpha periodically collected during summer and fall 2008. The results indicated that adults experienced significant DTW loss at ambient water temperatures >25°C. DTW loss was even greater at LO where adults on 29 June 2007 had a DTW roughly one-third that of WCL individuals on the same date in 2008. The LO population experienced nearly twice the number of degree-days >25°C than the WCL population, exacerbating their DTW loss by prolonged and intensified temperature-induced negative energy balance, and reducing their energy stores below that required to tolerate prolonged exposure to high temperatures experienced in chronic thermal tolerance testing. Prolonged exposure to negative energy balance may have been responsible for their extirpation from LO in 2007. Specimens of D. rostriformis bugensis from LM had an incipient 28-d upper thermal limit of 27.2°C which was approached or exceeded by the lake's summer ambient water temperatures at depths <12 m. However, their recent expansion into the surface waters of LM and the species' dispersal into warmer, shallow southwestern reservoirs indicate that, like the WCL D. polymorpha population, this species may be adapting to the warm waters of the southwestern U.S.The adaptation of southwestern U.S. dreissenid populations to elevated temperatures could reduce the efficacy of thermal mitigation treatments in this region and potentially require higher temperatures for hot-water spray mitigation of mussel fouling on recreational boats and submerged equipment. This study's genetic evaluation of both long and recently established of NA Dreissena polymorpha and D. rostriformis bugensis populations by Amplified Fragment Length Polymorphisms indicated a lack of founder effects and genetic bottlenecks in newly established dreissenid populations, a result strongly suggesting that successful invasion of a new habitat requires introduction of a large number of individuals. As such, dreissenid prevention and containment measures that are <100% effective may still allow uninfested western NA water bodies to remain free of dreissenid mussels if coordinated, integrated, and region-wide prevention, containment and management plans are adopted.