Disco clams stage a fascinating light show

“Disco clam” is an eye-catching mollusc, nestled in the coral reefs off Indonesia. The animal generates brilliant flashes of light that earned it its festive name. Though fascinating, this flamboyant bivalve (Ctenoides ales) is still poorly understood, something Lindsey Dougherty, a graduate student at the University of California, Berkeley is trying to change. Earlier this year, Dougherty shed light on how the clam’s flashing works – by reflecting light through tiny bits of silica near the edge of its shell, and not through bioluminescence like other species.

Now Lindsey and her colleagues have gotten closer to why the clams put on their marine light show: to warn predators or lure prey. “Most animals don’t do something that’s energetically costly unless there’s a payoff,” said Lindsey, who presented the new research at the annual conference of the Society of Integrative and Comparative Biology in West Palm Beach, Florida. For instance, “light displays are often used to try to attract a mate or to attract prey, and some of the displays can also be defensive, like the (extremely poisonous) blue-ringed octopus,” she said.

Shedding more light
Dougherty and her team tested three hypotheses for the clam’s brilliance: attracting a mate to spawn eggs, catching the attention of light-seeking plankton, or sending a warning to potential predators.

The scientists tested the hypotheses by introducing threats, food and the opposite sex into tanks with captive clams and observing how the mollusks reacted. The team found little evidence that the disco flashing draws in suitors, since the clams’ eyesight is likely too poor to see the flashes. But the other two hypotheses bore (at least preliminary) fruit. When the team moved a fake predator toward a disco clam, its flash increased in frequency, from 1.5 to 2.5 hertz, she said. “So, it has a really obvious reaction to potential predators.” When plankton were introduced to the tank, the disco clams flash rate also increased, although not as much. In both instances, “they get excited, you could say. Excited or scared.”

Leaving a bad taste
The team was surprised to find sulphur in both the tentacles and mantle – the fleshy protruding part – of the disco clam. “Sulphur is the main ingredient in sulphuric acid, which is really distasteful to predators,” Dougherty said.

To further test the predator hypothesis, the team introduced the mantis shrimp, a type of aggressive crustacean, into the tank. The shrimp then started acting strangely: “We have some footage of a mantis shrimp sort of recoiling and then cleaning its mouth parts and then going into a catatonic state after interacting with the disco.”

Lindsey suggests that the predator ignored the clam’s flashing only to taste something it didn’t like, like sulphuric acid. She added that more research is necessary in the wild to determine whether plankton can actually see the disco’s visual displays, and whether similar types of clams also secrete sulphuric acid. That’s no problem for Lindsey. “For me,” she said, “the most fun place is underwater with the clams.”