To move or not to move: taxis responses of the marine acoel Symsagittifera roscoffensis to different stimuli

Symsagittifera roscoffensis forms a photosymbiotic relationship with the alga Tetraselmis convolutae within the intertidal zone. Juveniles lack algal symbionts at birth and acquire them from the environment. Requiring light for photosynthesis, they position themselves within the water column while also balancing the risk of being washed away. To understand their behavioural adaptations, we conducted experiments on their movement in response to algal cues (chemotaxis), light gradients (phototaxis), and mechanical vibrations. Aposymbiotic juveniles showed three times more positive displacement towards algae. Adults exhibited positive phototaxis but retreated from high light intensity. When introduced to a column with a light source, the worms remained just below the surface. In the mechanical vibration experiment, worms only descended when vibrations exceeded a threshold. These findings suggest that S. roscoffensis has chemotactic abilities crucial for acquiring algae and acquires light for photosynthesis while minimizing dispersal risk and photoinhibition, facilitating its life cycle in the intertidal zone.