Snakelocks sea anemones (Anemonia viridis) in a rockpool off Plymouth, UK, with their tentacles pointing to the overhead sun. Photo credit: Callum Shaw.
The French call sunflowers ‘tournesols’, because that is exactly what the young blooms do. As the sun traverses the sky, they turn gradually each day, tracking it westward. Most plants and algae turn or move with the sun at some stage of life. But some algae have struck a symbiotic deal; instead of ranging free, they reside within animals, such as corals and sea anemones, providing their host with energy in return for nutrition and safety. Yet, these photosynthetic algal lodgers still require a reliable light supply. Fortunately, some symbiotic sea anemones are able to move and gravitate towards light, but how was not clear. To begin unravelling the mystery, Eliska Lintnerova, Callum Shaw, Colin Brownlee and Vengamanaidu Modepalli from the Marine Biological Association of the UK (MBA) began observing symbiotic snakelocks sea anemones (Anemonia viridis) in local rockpools to find out whether they gravitate towards light.
Initially, the team monitored the anemones on sunny days, photographing the animals to measure the direction in which they pointed their tentacles, and it was clear that the anemones were behaving just like plants, actively pointing their tentacles towards the sun, moving from East to West as the sun passed overhead. And when the team blocked the sun with a black sheet, the tentacles slumped back, only springing up again when the sunlight returned. The sea anemones were reacting to the sun, but how? The team transferred some of the symbiotic animals to their lab in Plymouth (UK) to find out.
This time the researchers shone bright and dim white lights on the sea anemones, shifting the lamp to see whether the anemones followed the light with their tentacles, in addition to checking which light colours the symbionts could sense. Sure enough, the anemones followed the white light and protruded their tentacles in the direction of blue and violet light, although red and green light left the tentacles immobile. But were the Symbiodinium algae that reside within the bodies of the sea anemones responsible for the animals’ ability to track the sun?
The team tested the reactions of bleached sea anemones that had lost their algal lodgers to see if they could still track light and found that they could not. And when they checked whether light-sensitive proteins in the anemones’ tentacles might be responsible for the light tracking, the genes encoding the proteins were equally active in the bleached and normal sea anemones. So, the sea anemones were not guiding their tentacles toward the sun, the tentacles were being guided by their Symbiodinium algal lodgers – but how?
To find out, the team bathed the anemones in a drug that prevents algae from photosynthesising, for 10 min, 2 h and 24 h, before monitoring the ability of the anemones to track the sun. As the length of time over which the drug was administered became longer, the anemones were increasingly unable to follow the sun. And when Matthew Keys (MBL) and the team recorded the amount of oxygen produced by photosynthesis leaking from the animals’ tentacles when illuminated under different coloured lights, they saw that the anemones produced the most oxygen when illuminated by blue light and none at all when they had lost their algal lodgers.
Snakelocks sea anemones definitely move with the sun, and photosynthesis by their algal lodgers directs their tentacles. However, Lintnerova and colleagues suspect that the anemones may not be positioning their tentacles to maximise the energy produced by the sun. Too much sunlight can destroy delicate algae, so the anemones may point the tips of their tentacles toward the sun to reduce their exposure and keep their algal lodgers safe.
