The electric fire clam brings disco to the reef

By Bec Crew | March 4, 2019

Just when you thought you’d seen it all, the ocean serves up the electric fire clam.

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Bec Crew

Becky Crew is a Sydney-based science communicator with a love for weird and wonderful animals. From strange behaviours and special adaptations to newly discovered species and the researchers who find them, her topics celebrate how alien yet relatable so many of the creatures that live amongst us can be.

WITH ITS FLASHY maw and bright, crimson flesh, this bizarre tropical mollusk is unlike anything else in the ocean, and it’s keeping scientists guessing with its spectacular light show.

Found in warm, tropical waters throughout the central Indo-Pacific region, including across the north coast of Australia, the electric fire clam (Ctenoides ales) lives at depths of up to 20 metres, tucked inside small crevices formed by rocks and other debris on the ocean floor.

For such an extravagant creature, it’s been very little studied, but one thing we do know is that it is the only known bivalve – a group containing clams, oysters, cockles, mussels, and scallop – in the world with its own light display.

And before you go assuming that this is an example of bioluminescence, as is the case with many illuminated sea creatures, this is something else entirely.

Only recently has the electric fire clam’s dazzling display been properly investigated, and it turns out that, instead of employing light-emitting molecules and enzymes like certain species do to produce intermittent bioluminescence, this little sea creature’s flesh has a permanent glow to it.

Back in 2013, when Lindsey Dougherty was a graduate student at the University of California, Berkeley, she decided to study the species using a bunch of high-powered imaging techniques.

She found that part of the electric fire clam’s mantle is made up of minute nano-spheres of silica, which give it a brightly reflective glow that it can turn on and off at will.

As Dougherty and her colleague, Indonesian marine biologist Dimpy Jacobs, explain:

“The clams have a highly reflective tissue on the very outer edge of their mantle that is exposed and then hidden very quickly, so the change back and forth from the white reflective tissue to the red tissue creates the appearance of flashing.”

Here it is in action:

And here’s another one, spotted on the Great Barrier Reef:

Exactly why the electric fire clam evolved to wield such a flashy display is not clear, but as Dougherty pointed out to National Geographic, “Most animals don’t do something that’s energetically costly unless there’s [a payoff].”

When she tested whether the light show was there to attract mates, she found it made little difference in drawing in suiters. But when she tempted these ‘disco clams’ with plankton (their preferred food) and scared them with predators, the flashing increased.

When she let an aggressive mantis shrimp at it, something remarkable happened.

“We have 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,” Dougherty told Nat Geo.

It probably didn’t help that the electric disco clam happens to contain a whole lot of sulphur in its mantle and tentacles, which, as you’d imagine, tastes awful.

It could be that the hungry mantis shrimp ignored the flashing warning signal of the clam and went in for a bite anyway, ending up with a mouthful of sulphur.

Here’s another video of that crazy light display for the road: