Rheotactic Responses in Bufo bufo Tadpoles-Insights Into Adaptation to Lotic Environment

Current flow is an important stimulus for amphibian larvae. The common toad (Bufo bufo ) typically breeds in stagnant waters or, less commonly, in slow-flowing rivers, where tadpoles further develop. This study is the first investigation of B. bufo tadpole orientation under varying flow conditions within their natural habitat. Using Structure from Motion techniques, high-resolution orthophotomosaics were generated for three stream sectors with differing flow velocities, enabling accurate measurements of the orientation of a very large number of tadpoles. Circular statistics were employed to examine tadpoles' orientation in relation to water flow, stream banks, and microtopography. We found that in stagnant water, tadpoles exhibited random orientation in open areas but showed perpendicular alignment to the shoreline near margins. In flowing conditions, tadpoles exhibited positive rheotaxis, more pronounced with increasing current velocity, and were able to maintain their position against the flow despite their typical adaptation to lentic habitats. Additionally, tadpoles manifested the tendency to shelter behind the shores' irregularities and boulders, where flow is reduced, reflecting adaptive strategies to mitigate displacement. This study highlights the ability of B. bufo tadpoles to actively respond to flow dynamics, suggesting an adaptive behaviour to cope with the current. Given the tadpoles' absolute dependence on water, behavioural flexibility is a crucial factor for population survival, especially in the context of climate change, where stagnant aquatic habitats are drying up more frequently than rivers. This adaptability could play a key role in enabling tadpoles to exploit more lotic environments, providing a potential resilience mechanism in response to breeding habitat loss.