Population and Reproductive Dynamics of Zebra Mussels (Dreissena polymorpha) in Warm, Low-Latitude North American Waters

Zebra mussels (Dreissena polymorpha), first reported in Texas during 2009, have infested 28 Texas reservoirs over 11 years. This species has not previously invaded water bodies as warm as those in Texas, where temperatures approach or exceed its previously accepted incipient upper thermal limit of 30 degrees C, raising the question of how such temperatures impact its population dynamics. Over 3-5 years, monthly collections of mussels, sampling for planktonic mussel veligers, juvenile settlement data, and water quality parameters, were undertaken at Texas lakes Texoma, Ray Roberts, and Belton to estimate mussel shell length growth rates, life spans, reproductive periods, and settlement patterns. Biannual spawning periods occurred at water temperatures of 18 to 28 degrees C, resulting in distinct spring and fall juvenile settlement cohorts. Growth rates were rapid, with both cohorts attaining mean maximum shell lengths of 20-25 mm within 8-15 months of settlement, compared to European and northeastern US populations that attained similar sizes after 2-4 years. Shortened life spans were demarcated by adult mussel die-offs during summer months of elevated water temperature the year after initial settlement, leading to short-term cyclical fluctuations in population densities. Large-scale mussel die-offs were caused by flooding and hypoxia events. Elevated temperatures appeared to facilitate mussel invasiveness by increasing spawning frequency and elevating growth rates, thus reducing time to maturity and allowing population recovery within 1-2 years after environmentally induced severe population declines.