Factors influencing the upper temperature tolerances of three mussel species in a brackish water canal: size, season and laboratory protocols.

Mussels are the most problematic organisms encountered in the water intake systems of electrical power plants. Various fouling control measures are adopted, among which heat treatment is considered the relatively more attractive from economic and ecological points of view. Thermal tolerance experiments were carried out to determine the effects of mussel size (2-20 mm shell length), season (breeding vs non-breeding), nutritional status (fed vs non-fed), acclimation temperature (5-25 degrees C) and acclimation salinity (1-35%o) on the mortality pattern of three important mussel species, viz. a freshwater mussel Dreissena polymorpha, a brackish water mussel Mytilopsis leucophaeata and a marine mussel Mytilus edulis under different temperatures (36-41 degrees C). The mussels in the 10 mm size group exposed to 36 degrees C showed 100% mortality after 38 min (D. polymorpha), 84 min (M. edulis) and 213 min (M. leucophaeata). The effect of mussel size on M. edulis and M. leucophaeata mortality at different temperatures was significant, with the largest size group of mussels showing greater resistance, while no significant size-dependence was observed in the case of D. polymorpha. All the three mussel species collected during the non-breeding season (June-October). Nutritional status had no significant influence on the thermal tolerance of the three mussels; fed and non-fed mussels showed 100% mortality at comparable rates. Acclimation temperature had a significant effect on the mortality of all three species. Survival time at any given target temperature increased with increasing acclimation temperature. The acclimation salinity showed no significant effect on the thermal tolerance of the three mussel species. In comparison, M. leucophaeata was more tolerant to high temperature stress than the other two species. The present studies clearly show that various factors can influence the mortality of D. polymorpha, M. edulis and M. leucophaeata to elevated temperatures. The results, therefore, suggest that if heat treatment were to be used as a control measure for these mussels, it has to be employed judiciously, depending on the mussel species, mussel size, breeding season, water temperature and salinity.