Mystery of 'disco' clam's flashing lights solved

An unusual clam living in small caves in Indo-Pacific coral reefs produces flashes of light that help it fend off predators and perhaps attract prey, a new research has found.
The disco clam, Ctenoides ales, is an active, filter-feeding mollusk that lives in crevices or small caves in Indo-Pacific coral reefs.
The clam's flashing is so bright that it had been thought to be the result of bioluminescence, the production of light within the tissue. Hoewever, how and why these invertebrates produce their spectacular display remained a mystery.
New research by Lindsey Dougherty of the University of California, Berkeley, suggests that scaring off predators or luring prey may be the primary motivations for the disco clam's flashy behaviour.

Using high-powered transmission electron microscopy, Dougherty discovered that the flashes are caused by specialised tissues that form a double layer, reflective to light on one side, but absorbent on the other.
When the tissue is rapidly rolled and unfurled by the clam, the reflecting light gives the appearance of flashing. These tissues are so reflective that they can even flash using the low levels of blue light found in the caves.

Dougherty and colleagues had three hypotheses about why the clams flash: to attract a mate, to scare off predators, or to attract prey.
They tested these hypotheses using experiments on live clams in the lab. They examined the structure and proteins in the clam's tiny eyes using a powerful microscope and concluded that its vision is likely too poor to allow it to observe displays by other clams.

This suggests that the clams are not visually attracted to one another, and that the flashing is most likely used for repelling or attracting other species.
Next, they tested the effect of the flashing on predators, which in the wild might be octopi, predatory snails, or mantis shrimp.
For this study, they conducted 'looming' trials, during which the researchers moved a false predator in the direction of the clams, and gauged their reactions.
"In this case, the false predator was just a styrofoam lid. But it turns out a styrofoam lid is indeed pretty scary to the clams, because their flash rate almost doubled from just under 2 Hz to just under 4 Hz," said Dougherty.

Phytoplankton prey were also introduced into the tank. As the clam's tentacles began to sense the presence of prey, their flash rate increased significantly.
Many species of plankton are indeed phototaxic and therefore attracted to light, though it is unclear whether the disco clam's prey species are able to see their flashing.
Dougherty presented the research at the 2015 annual conference of the Society of Integrative and Comparative Biology in West Palm Beach, Florida.