The Disco Clam That Puts on a Mind-Bending Light Show

The dazzling display of the disco clam is almost certainly not fueled by cocaine.

Ah, disco. That lamentable fad turned the Bee Gees from a kinda cool, kinda psychedelic band into a conduit for polyester, all that damn color, roller skates, and strobe lights. And what’s left of it now? Disco Stu and John Travolta’s chest hair. Also, it has given scientists a shorthand way of describing a flashy little mollusk that lives in the Indo-Pacific and fires beautiful bands of light across its flesh. Technically it’s Ctenoides ales, but to the researchers who know it best, it’s the disco clam.

And unlike the ’70s, it is excellent.

For a long while, scientists reckoned the disco clam was throwing off bioluminescence. After all, the ocean is positively aglow with it—in the deep sea, perhaps 90 percent of animals produce some kind of light, either with their own chemical reactions or in cooperation with onboard bacteria. Down in the dark, that light does everything from confusing predators to attracting mates or prey. But the disco clam makes its home in shallower waters where bioluminescence isn’t as useful. And indeed, the clam’s brilliant flashes aren’t the work of bacteria, but instead are tricks of sunlight and some peculiar tissue.

Lindsey Dougherty, a biologist at UC Berkeley, was among the people who figured this light show out. She likens that strip of tissue to a scarf with one side covered in foil: “One side reflects all visible light, so it’s white. The other side only reflects long, low-energy wavelengths, which is why it appears red.” By rapidly furling and unfurling this tissue, as often as six times a second, the disco clam looks to be producing a mollusk-sized Laserium show, when in fact it’s simply reflecting available light at your eyeballs.

So how exactly is this “foil” side of the scarf so brilliantly white? Well, it turns out it’s covered with highly refractive nanospheres of silica. Silica is everywhere, from sand to shells to those little packets manufacturers put in shoe boxes and bags of food and such to absorb moisture. (By the way, they’re emblazoned with those serious warnings not to eat them not because silica is poisonous, but because apparently some folks need to be reminded that you can’t just go around eating whatever you feel like.)

The disco clam uses those tentacles to “sniff” the water, and as we’ll see in a bit, somehow drive mantis shrimp to try to get busy with it.

“The silica spheres are in a dense arrangement, and both their size and their density in the tissue are nearly optimized for scattering light,” says Dougherty. “They’re even better at this in blue-light conditions, which is what most of the ocean is composed of.” How exactly the disco clam is synthesizing this silica, Dougherty isn’t yet sure, but she’s working at it. The creature could be pulling the stuff from sand or straight from the water, or even consuming tiny plankton known as diatoms (clams are filter-feeders), whose shells are made of silica.

Go Into the Light

But why bother? Is the disco clam just trying to get laid, like so many dudes at Studio 54? Dougherty doesn’t think so. “All of the experiments we’ve done surrounding communication haven’t supported the idea,” she says. “The clams have about 40 eyes, but after studying their structure, it is unlikely they can actually perceive the flashing.” The intensity of the flashing, though, did seem to increase when Dougherty introduced plankton to their tank, suggesting the display may help the clam lure prey.

The clear dividing line between the clam’s reflective and non-reflective tissue.

But the flashing appears to increase—twice as much as normal—when a predator takes an interest, so maybe the display confuses villains. To find out if that was true, Dougherty introduced a disco clam to one of the ocean’s most ferocious pound-for-pound predators: the mantis shrimp. And hilarity ensued.

While the flashing may effectively drive off less tenacious predators, the mantis shrimp was having none of it. The mantis (which isn’t actually a shrimp, but a crustacean known as a stomatopod) comes equipped with two deployable club-claws, which it fires at its victims with such speed that it briefly heats the water to the temperature of the surface of the sun. It demolishes everything from clams to other crustaceans, blowing apart any shell that stands between it and flesh.

A mantis shrimp recoils from a disco clam moments before saying, “Eh, screw it. I’ll just hump the thing.”

And so it made its move on the clam, disco lights be damned, fiddling a bit with it and giving it a few taps. But something was amiss. The stomatopod suddenly recoiled and backed away, approached once more, and recoiled again. Another approach, and another recoil, before the predator began frantically cleaning its mouthparts and eventually going catatonic. When it recovered, it came in for one more attack before deciding to just try to mate with the clam instead. (A mating attempt actually happened four times, but Dougherty says she only included the first in the video so “as to not parade his failures.”)

The problem, Dougherty reckons, is that the clam’s mantle and flowing tentacles—which are chemosensory structures, similar to our noses—are packed with sulfur, which stomatopods don’t seem to take too kindly to. “We often see them go into a sort of catatonic state after interacting with the clams, so something suspicious is definitely going on,” she says. But when she gives the stomatopods flesh from inside the clam, the muscles and such, they don’t hesitate to dig in. There’s apparently just something different about the makeup of the mantle and tentacles. “This is the area where we found high amounts of sulfur,” Dougherty adds, “although we’re still working to figure out what type of sulfuric compound it is using mass-spectrometry.”

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