Does turbidity affect critical thermal maximum (CTM) of mussels : Behavioural changes in Unio tumidus under stress

Abstract: Freshwater mussels is one the most endangered groups of organisms worldwide and stand in front of several threats. Global warming along with anthropogenic influence affect the mussel’s natural habitat and stress leads to changes in their behaviour. The aim was to investigate if the behaviour of Unio tumidus exposed to thermal stress differs depending on fine particulate matter exposure. This study hypothesises that (1) the critical thermal maximum of mussels is lower in high turbidity than in clear water (2) mussels in highly turbid water extend their foot at a lower temperature than in clear water (3) mussels have a more frequent gaping behaviour in high turbidity than in clear water. Mussels were placed in each water filled beaker with sensors glued to their shells for monitoring gaping behaviour. Aquarium heaters increased the water temperature gradually until the mussels reached their CTM. One group were placed in clear river water and one group in water with high turbidity. There was no significant difference in the CTM between the highly turbid and clear water. The mussels in the river water extended their foot at a lower temperature compared to the mussels in high turbidity. One possible explanation is that the mussels remain shut to protect themselves due to higher stress levels. Valve gaping behaviour was visually investigated and displayed different patterns between mussels in clear and highly turbid water. This study contributes with information regarding mussel behaviour and response to thermal stress which can be of value for future studies.