COS 92-2
Tolerance of the invasive apple snail Pomacea maculata to low temperatures
Indigenous to Brazil, Pomacea maculata and other closely related species of apple snails are widely introduced and invasive in tropical and subtropical regions around the world. Many of these apple snail species (including P. maculata) eat aquatic macrophytes and damage both natural and agricultural systems. Recently, populations of P. maculata have become established in the states bordering the Gulf of Mexico and there is concern about their potential to expand northward under both current climatic and projected future climatic conditions. Because of their tropical origins, apple snails are not cold tolerant. To better estimate their potential for northward range expansion, we tested the lower temperature tolerance of P. maculata. Individual snails were weighed, placed in 1L beakers containing 500 ml of dechlorinated tap water and then placed in temperature chambers for ten days at 20°, 15°, 10°, 5° or 0o C. Ten individuals were exposed to each of the temperatures. Every 24-hours the animals were examined for viability and a 2 ml water sample was analyzed for ammonia as an index of metabolic activity. At the conclusion of the experiment on day 10, the osmotic concentration of the hemolymph from the surviving animals was measured with an osmometer.
Results/Conclusions
There was 100% survival at 20° and 15o C. Approximately 50% of the snails died at 10° and 5o C. There was 100% mortality at 0° C after five days. Ammonia excretion was 1.1 + 0.5 µmol/g/day at 20o C and 0.3 + 0.1 at 15o C. At lower temperatures, ammonia excretion was less than 0.1 µmol/g/day, indicating little metabolic activity at 10° C and less. The osmotic concentration of the hemolymph increased with decreasing temperature (20°, 15°, 10°, and 5o C; 95.1 + 9, 91.1 + 25.1, 101.7 + 26.1, and 127.0 + 38.0 mOsm/kg, respectively). Differences among the means were not significant but variability increased an order of magnitude, which might indicate a stress response. Our results show that 100% of the P. maculata can survive water temperatures as low as 15° C and that a significant proportion can survive temperatures as low as 5°C for as long as 10 days. We conclude that P. maculata have significantly greater potential for northward range expansion than previous temperature tolerance studies would suggest.