Photosynthetically mediated phototropic column bending in a symbiotic sea anemone Exaiptasia pulchella

Abstract Phototactic behavior, e.g., locomotion toward light direction, in symbiotic sea anemones is thought to be driven by algal photosynthesis, yet the mechanism by which photosynthetically derived signals influence host muscle activity remains unclear. This is especially challenging to study due to the semi-sessile nature of anemones, which limits their physical responsiveness to light. Here, we report a novel light-induced column-bending response in the model symbiotic anemone Exaiptasia pulchella , and assess the role of algal photosynthesis in mediating this behavior. Following light exposure, anemones exhibited a phototropic response with a latent period of 5.3 ± 2.7 s and a maximum bending angle of 40° ± 17°. This response was featured a refractory period of approximately 120 s. Application of DCMU (a photosystem II inhibitor) and acetazolamide (a carbonic anhydrase inhibitor) resulted in a dose-dependent suppression of the bending response, implicating photosynthetic activity in behavioral initiation. Inhibition by 2-hydroxyethanesulfonic acid (a taurine transporter antagonist) and MgCl₂ further suggests that taurine may function as a signaling molecule released from endodermal cells, triggering neural and muscular activation. While the complete signaling pathway remains unresolved, these results offer new insight into host-symbiont integration and photosynthetically induced behavior in cnidarians.