Instead of blending in with their surroundings for safety, some brittle stars advertise their presence with bright pinpoints of light arranged along their arms. But their ostentatious spectacle is more than misplaced bravado. It seems likely that the brazen display is designed to divert predators. According to Jérôme Mallefet from the Université Catholique de Louvain, Belgium, brittle stars shed the brightly illuminated tips of their arms with ease when under attack, allowing victims to crawl deeper in the mud while their attacker is distracted. However, it wasn't clear how these echinoderms come by the chemicals – known as luciferins – that give them light, until Mallefet realised that captive Amphiura filiformis seemed to lose their glow after a year or two. Knowing that the luciferin coelenterazine provides the illumination for these brittle stars, Mallefet wondered whether it would be possible to restore their glamour by boosting their coelenterazine levels. But first he needed a new batch of the animals so he could follow the decrease before refuelling them.
Mallefet travelled to Sweden to collect the delicate animals. ‘It's an ideal place to go; within half a day of ship time we can collect 500 specimens’, he says. He then returned to Belgium carrying the fragile animals in a large ice box packed with chilly seawater and mud to keep them comfortable, ‘So the animals can hide, which they do in natural conditions’, he explains. Back in the lab, Mallefet placed the brittle stars on a strict coelenterazine-free diet of aquarium fish food for more than a year while keeping track of their luminosity.
Mallefet extracted coelenterazine from brittle star arms each month to keep track of their levels. However, luciferins are only one-half of the glow-in-the-dark equation; they emit light when triggered by an enzyme known as luciferase. To measure the quantity of coelenterazine in each captive brittle star arm, Mallefet added luciferase to the coelenterazine samples and measured the amount of light they produced. After 5 months of captivity, Mallefet and his colleagues Laurent Duchatelet and Constance Coubris saw the A. filiformis glow dwindle until the animals were indistinguishable from a close relative, Amphiura chiajei, which never glows. And when they tested how much luciferase the coelenterazine-deprived A. filiformis were producing, they found that the enzyme activity surged briefly as coelenterazine levels declined, but then fell back to normal as the brittle stars’ spark finally fizzled out. But the real question was would the dimmed brittle stars regain their glow when coelenterazine was reintroduced into their diet?
Excitingly, the gamble paid off. ‘To see the brittle stars eating the food we provided was a very exciting moment’, says a relieved Mallefet, who wasn't sure they would take to the change in their diet. And when he checked the animals’ coelenterazine levels, he was overjoyed to see their glow returning rapidly, peaking 18 days after the new diet was initiated.
Amphiura filiformis extract the key light-giving chemical from their diet and the researchers are now eager to find out which morsels provide the brittle stars with their shots of light. But that will be easier said than done; ‘Imagine searching the stomach contents of a 3 mm diameter animal to identify what is inside’, chuckles Mallefet as he and Coubris embark on the next chapter of their bioluminescent odyssey of discovery.