Giant Clams are Masters of Light

This is reprinted from New Scientist

Clever clams and algae show how best to harvest light

• 19:30 07 October 2014 by Andy Coghlan

Little and large they may be. But giant clams have evolved a unique trick for redirecting sunlight to their microscopic algal tenants.

In many species of giant clam, photosynthetic algae live in the clam's fleshy mantle, which is exposed to the sea and sunlight through the flaps of its shell (pictured). In exchange for their home, the algae secrete glycerol, which feeds the clam.

The association is one of many in which animals work symbiotically with plants and algae to harvest the power of the sun.

But giant clams have specialised cells called iridocytes that allow algae to grow in microscopic pillars, which go about 2 millimetres deep into the clam mantle.Alison Sweeney of the University of Pennsylvania in Philadelphia and her team have demonstrated that the iridocytes ensure that every last algal cell in the micro-pillar still gets its fill of sunlight, even though most of the 300 or so cells in each column have no direct access to the light.

"What makes this system in the clam special is that the design can extract every last photon from sunlight," says Sweeney.

By shining halogen lamps on clams, Sweeney found that iridocytes redirect the path of sunlight so that it fans out into a cone about 15 degrees wide, bathing entire pillars of algae in mild, but optimal, intensities of light. Moreover, they transmit mainly red and blue light, the wavelengths that the algae photosynthesise most efficiently, and deflect much of the green and yellow wavelengths.

"The amount and mix of light that gets through is just right for the algae," says Sweeney. "And it fans out just enough to reach all the cells in the column."

Typically, clams that live in shallow coral reefs are exposed to levels of sunlight that are enough to kill the algae.

Mimicking nature

"While earlier work speculated on the role of these iridescent cells, this paper clearly shows how clams use iridocytes to control and redistribute the light that reaches their algal symbionts," says Ryan Kerney of Gettysburg College in Pennsylvania.

Kerney says that the research also solves the puzzle of why many clams are iridescent – it's down to the green or yellow light that is reflected because it's of no use to the algae. "Animals such as starlings or butterflies generally use iridescence for display or camouflage, but giant clams do neither, instead optimising the absorption of light to suit tiny stacks of algal cells."

Sweeney and her colleague, Shu Yang, have now begun a project to try to artificially mimic the function of the iridocytes, and to test ways of growing pillars of algae.

It could drastically improve the efficiency with which algae can be farmed to produce biofuels , because it would allow the algae to be grown in layers hundreds of cells thick instead of as a single layer, or being constantly stirred to expose all cells to sunlight.

"The clams have shown us how to grow algae very densely, without having to stir them, which wastes energy," says Sweeney.