Not all corals are attached to the substrate; some taxa are solitary and free-living, allowing them to migrate into preferred habitats. However, the lifestyle of these mobile corals, including how they move and navigate for migration, remains largely obscure. This study investigates the specific biomechanics of Cycloseris cyclolites, a free-living coral species, during phototactic behaviour in response to blue and white light stimuli. Our results indicate a strong positive phototactic response to blue light, with 86.7% (n=15) of samples moving towards the light source, while only 20% (n = 15) of samples responded similarly to white light (400-700 nm). Locomotion, characterised by periodic pulses lasting 1-2 hours, involved distances up to 220 mm in blue light trials, whereas significantly shorter distances were observed in white light trials. Trails with two light sources reinforced the preference for blue light over white, with all samples consistently moving towards the blue light and away from the white. High-resolution time-lapse captured the biomechanics of forward motion that appeared driven by three key factors: tissue inflation, which increased contact surface area for lift and friction; the ventral foot/pads, adjusting substrate interaction/friction; and the contraction and twisting of lateral peripheral tissues, which propelled the coral forward in a coordinated manner resembling the pulsed swimming motion of jellyfish. Our findings provide new insights into coral mobility mechanisms, emphasising the role of tissue inflation in active locomotion, with potential implications for coral neural systems, vision, and habitat selection.

Trials were performed using either a white light source (shallow water; ~ 420nm to ~ 755nm, PAR 115 to 120 µmol m-2s-1) or a blue light source (deeper water; ~ 460nm, PAR 115 to 120 µmol m-2s-1). Spectral quality was measured using a CL-S10w spectroradiometer (Konica Minolta, Tokyo, Japan) (Fig 1) while intensity was recorded in available photosynthetically active radiation (PAR) and measured using the Underwater Quantum Flux reader (Apogee, North Logan, Utah), on the floor of the aquarium directly under the openings for light. Individuals of C. cycloseris were placed at the shaded end (opposite to the end with the opening) and 1 to 3cm from the aquarium walls to avoid interaction with adjacent surfaces.

Measuring distance travelled

To quantify distance travelled and frequency of mobilisation over 24 hours, time-lapse videos (n = 16) were recorded using an iPad (Apple Inc., USA) at a resolution sufficient for tracking movement, with playback set at 30 fps. The experiment consisted of 16 replicates under controlled colour conditions: 6 white, 7 blue, and 3 combined (blue vs white). The distance travelled was measured by tracking the position of the coral’s mouth at the start and end of each experiment using ImageJ® software (Wisconsin, USA). This region was selected to minimize errors caused by peripheral tissue expansion, which could obscure true displacement. Data visualisation and analysis were performed using MATLAB (MathWorks, USA) and Coral Draw (Alludo, Canada), ensuring an accurate representation of movement. Additional replicates (n=17) were conducted without time-lapse imaging, with manual measurements taken by comparing the coral’s mouth position to the walls of the aquarium at time zero and after 24 hours. This method was employed to validate the time-lapse data without potential light exposure from the iPad screen. A chi-squared test was used to determine if the movement frequency values between blue light and white light were significantly different.