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Hernandez, M. R. and R. F. McMahon, 1996. Tolerance of Zebra Mussels (Dreissena polymorpha) to Daily Temperature Fluctuation Above and Below Their Upper Thermal Limit, The University of Texas at Arlington Abstract from The Sixth International Zebra Mussel and Other Aquatic Nuisance Species Conference, Dearborn, Michigan, March 1996 Tolerance of Zebra Mussels (Dreissena polymorpha) to Daily Temperature Fluctuation Above and Below Their Upper Thermal LimitZebra mussels are dispersing into southern U.S. freshwaters. Dense mussel populations have developed in the lower Mississippi River, near Baton, Rouge, LA, even though daytime, ambient, surface water temperatures approach or exceed this species’ incipient, upper thermal limit of 30°C during summer months. Surface water temperatures in lotic habitats display extensive daily variation. In the Trinity River at Fort Worth, TX, average, daily surface water temperature fluctuation was 5.5°C during June, July and August in 1993 and 1994, with maximal daytime water temperatures generally falling above the 30°C upper thermal limit of zebra mussels and minimal evening water temperatures falling below it. Utilizing daily temperature fluctuation in the Trinity River as a model, we exposed samples of zebra mussels in the laboratory to four daily temperature fluctuation regimes: 26°/33°C; 28°C/33°C; 27°/32°C; and 25°/32°C. All four regimes cycled every 24 h such that mussels were exposed to 6 h at the low temperature extreme, followed by a 6 h period of temperature increase to the high temperature extreme which was maintained for 6 h, and followed by a 6 h period of temperature decline to the low temperature extreme. Replicate samples (n ~ 40 in each sample) were simultaneously exposed to a fluctuating temperature regime and to constant temperature regimes representing the extreme high and low temperatures of the fluctuating temperature regime. During exposure, media was continuously aerated and renewed every two days. Mortality among mussel samples was recorded daily until all individuals in the fluctuating and constant high temperature regimes had died. Mussels exposed to constant low temperature extremes did not achieve 100% mortality over the duration of the exposure periods. Multifactor analysis of variance of the ln of survival time with shell length (SL) as a covariant indicated that temperature regime (P < 0.0001), but not sample replication (P = 0.294) significantly affected survival time and that survival time was negatively correlated with SL (P = 0.011). Mean survival times recorded in the 25°/32°C; 27°C/32°C; 26°/33°C; and 28°/33°C fluctuating temperature regimes were 287 h (s.d. = ±191), 368 h (±99), 74 h (±43), and 42 h (±31), respectively. Mean survival times under the corresponding high constant temperature regimes of 32°, 32°, 33° and 33°C were 88 h (±67), 145 h (±56), 47 h (± 16), and 49 h (±18), respectively. Least squares difference testing indicated that samples exposed to fluctuating temperature regimes of 25°/32°C, 27°/32°C and 26°/33°C survived significantly longer (P < 0.05) than did specimens continuously exposed to the corresponding high temperature extreme. In contrast, specimens exposed to the 28°/33°C fluctuating temperature regime had significantly reduced survival times relative to those continuously exposed to 33°C. Mean survival times of mussels exposed to the 25°/32°C and 27°/32°C were 3.3 and 2.5 times greater than that of specimens held continuously at 32°C. The greatly extended temperature tolerance of specimens of D. polymorpha exposed to daily temperature fluctuations in which the maximal ambient temperature did not exceed 32°C, relative to continuous exposures to 32°C suggests that this species may be able to inhabit lotic waters in the southern U.S. where midday, summer, maximum surface water temperatures exceed this species upper thermal limit of 30° by 1° or 2°C, if surface water temperature falls below 30°C during evening hours.
Keywords: Zebra_mussel, Basic_biology, Inland_lakes/rivers |