New research finds evidence that sea anemones track the sun

It's been well established that many plants orient themselves to the sun's position as it moves across the sky. New research by Eliska Lintnerova, Callum Shaw, Matthew Keys, Colin Brownlee, and Vengamanaidu Modepalli of the Marine Biological Association of the UK has found evidence that some animals are also heliotropic. They studied photosynthetic sea anemones and found that they track the sun in ways very similar to heliotropic plants. Science explains:

Scientists observed wild snakelock anemones (Anemonia viridis, pictured above), a green and purple, tentacled species found along European and Mediterranean coasts that draws energy from sunlight thanks to photosynthetic algae in its tissues, and found that they kept their tentacles aimed at the Sun as it moved through the sky. Next, they mimicked the anemones' environment in a lab with a moving artificial light, which yielded the same result. Using different wavelengths of light, they learned the anemones were tracking blue light, the most important wavelength for photosynthesis. Like plants, this angling behavior could help the animals get more sunlight in dimmer waters. The researchers also noticed that by pointing its tentacles straight out, parallel to the incoming rays, the anemones decreased the area of their bodies exposed to direct light, which could also help individuals in especially sunny spots avoid dangerous overexposure. Other photosynthetic anemones may also have this ability, experts say, but further research is needed to confirm this.

The bioRxiv preprint of the study is available here. The abstract reads:

Being photosynthetic sessile organisms, plants established heliotropism to track the sun's position across the sky and allow their vegetative parts to orient accordingly. Here, we report plant-like heliotropic movement in a photosymbiotic sea anemone Anemonia viridis. Like plants, photosynthesis represents a key energy source in endosymbiotic cnidarians bearing microalgae. We observed that A. viridis in their natural habitats under sunlight displayed heliotropism or solar tracking by pointing their tentacles towards the sun while remaining sessile, facing east at dawn and west at dusk as they track the sun's relative position through the day, a phenomenon previously only observed in plants. Solar tracking movements in A. viridis are driven by a light wavelength that prompts photosynthesis in their endosymbionts. The heliotropic response was absent in both bleached (aposymbiotic) A. viridis and in symbiotic A. viridis with chemically inhibited photosynthesis. We revealed a direct correlation between heliotropism and endosymbiont oxygen production in A. viridis. Our findings suggest that photosymbiotic A. viridis has likely evolved plant-like heliotropism as an effective way to modulate exposure to solar irradiation for photosynthesis. The study exemplifies how photosynthetic organisms such as plants and photosymbiotic sea anemones, display similar behaviour in response to similar environmental pressures.

If you want to see some cool videos of the anemone in heliotropic action, Vengamanaidu Modepalli, Research Fellow at the Marine Biological Association of the UK and one of the authors of the study, posted some on his X/Twitter.