Behavioural and metabolic responses of the freshwater mussels Anodonta anatina and Unio pictorum to environmental stimuli with a focus on food and light availability

Benthic invertebrates play a crucial role in sediment mixing, nutrient cycling, and oxygen fluxes in benthic ecosystems. Despite the broad global distribution and high abundance of unionid mussels in both lentic and lotic ecosystems, the role of their locomotion behaviour and metabolic activity on the aforementioned processes remain understudied.
In this thesis, Anodonta anatina and Unio pictorum were exposed to a range of Chlorella vulgaris concentrations between 0.5 and 20.0 mg Ash Free Dry Mass (AFDM) l-1 at 11.0, 15.0 and 19.0 ± 1.0 °C in laboratory experiments. Unionid behaviours were recorded by time-lapse photography. Mussel locomotion probability and duration, opening behaviour, posterior tip movement and the position in relation to the light source were extracted image and video analysis. The diel rhythm was assessed as well as the responses to the light intensity at ~230, 450 and 1200 lux on a horizontal light gradient. The oxygen consumption (OC) during digestion was quantified at 0.05, 6.0 and 12.0 mg of AFDM of Chlorella vulgaris l-1 at 19.0 ± 1.0 °C.
The locomotion probability was significantly higher for A. anatina, compared to U. pictorum, increased with increasing temperature (lower for 11.0 ± 1.0 °C), and decreased with increasing algal concentrations. Locomotion duration decreased with the increasing algal concentration in both species, with U. pictorum showing a shorter locomotion duration than A. anatina. Valve opening peaked at algal concentrations of 3.0 mg l-1. A contrasting locomotion pattern was recorded between the two species with A. anatina crawling on the sediment and U. pictorum breaking through the sediment with is umbo covered. Both species showed a significantly higher probability of locomotion in the absence of light and a decreasing locomotion path distance with the increasing light intensity. The specimens which moved towards the light source covered a longer distance than those which moved away. Additionally, A. anatina showed a net movement towards the light source while all activities were recorded in the absence of light. Digestion significantly contributed to unionid metabolism in both species. In addition, the mean OC rate per dry soft-tissue mass (DM) increased with the algal concentration, with A. anatina showing a significantly higher rate. In A. anatina OC DM-1 decreased with the increasing DM.
The findings are discussed in the context of eutrophication, unionid bioremediation potential, and the development of species-specific remediation models. A conceptual model developed demonstrates the ecological interactions between unionids and their environment in lentic systems.

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