The thermal relations of the freshwater triclad flatworm, Dugesia dorotocephala (Turbellaria: Tricladida)

Although Dugesia dorotocephala is among the most widely distributed and eurythermal triclads within North America, its thermal relations are poorly known. We compared the upper thermal tolerances and the temperatures selected in a circular gradient of planarians acclimated to 15°C or 22°C. We also compared speeds of planarians tested at 22°C after up to 10 days of starvation or after acclimation to 15°C, 22°C, or 30°C. We compared speeds of 22°C acclimated animals at temperatures ranging from 10°C to 30°C, and we examined the interaction between temperature and water viscosity. The triclads could survive temperatures close to 33°C for up to 12 h, but had incipient lethal temperatures around 30.5°C. Warm acclimated animals survived somewhat better at the higher, but not at the lower, temperatures within this range. The triclads selected temperatures between 12°C and 28°C, with the 15°C acclimated animals selecting higher mean temperatures (23.1°C) than the 22°C acclimated animals (20.9°C). The latter group showed a normally distributed selection pattern, whereas that of the former group was skewed to the right. Speeds increased during the first two days of fasting but showed no further changes between days 3 and 10. Speeds increased with body size with a scaling exponent of about 0.25. There was a significant effect of temperature, with a <f>Q10</f> of 2.31 between 10°C and 30°C. Thermal acclimation did have a significant effect on locomotion, with 30°C acclimated animals being markedly slower than 15°C or 22°C acclimated individuals. Although there was a slight indication of a viscosity effect, the resulting differences were not statistically significant. [Copyright &y& Elsevier]