Thermotaxis in an apolar, non-neuronal animal

Neuronal circuits are hallmarks of complex decision making processes in the animal world. How animals without neurons process information and respond to environmental cues promises a new window into studying precursors of neuronal control and origin of the nervous system as we know it today. Robust decision making in animals, such as in chemotaxis or thermotaxis, often requires internal symmetry breaking (such as A-P axis) provided naturally by a given body plan of an animal. Here we report the discovery of robust thermotaxis behavior in Trichoplax adhaerens , an early-divergent, enigmatic animal with no anterior-posterior symmetry breaking (apolar), and no known neurons or muscles. We present the first quantitative and robust behavioral response assay in placozoan, which present an apolar flat geometry. By exposing T. adhaerens to a thermal gradient under a long-term imaging setup, we observe robust thermotaxis that occurs over timescale of hours, independent of any circadian rhythms. We quantify that T. adhaerens can detect thermal gradients of at least 0.1 ° C/cm. Positive thermotaxis is observed for a range of baseline temperatures from 17-22.5 ° C with motility trajectories to be well-described by a Lévy distribution. Interestingly, the organism does not maintain a fixed orientation while performing thermotaxis. Utilizing natural diversity in size of adult organisms (100um to a few mm), we also demonstrate a critical animal size above which thermotaxis behavior is hindered. Several TRP family homologs have been previously reported to be conserved in metazoans, including in T. adhaerens . We discover naringenin, a known TRPM3 antagonist, inhibits thermotaxis in T. adhaerens . The discovery of robust thermotaxis in T. adhaerens provides a tractable handle to interrogate information processing in a brainless animal. Understanding how divergent marine animals process thermal cues is also critical due to rapid temperature rise in our oceans. ### Competing Interest Statement The authors have declared no competing interest.