Giant Clams Use Iridescence to Harness the Sun's Light

Giant clams live within tropical waters, growing to impressive sizes over their lives. Now, scientists have shown how these mollusks use iridescent structures to operate as living greenhouses that grow symbiotic algae as a source of food.

"Many mollusks, like squid, octopuses, snails and cuttlefish have iridescent structures, but almost all use them for camouflage or for signaling to mates," said Alison Sweeney, one of the researchers, in a news release. "We knew giant clams weren't doing either of those things, so we wanted to know what they were using them for."

The iridescent structures on giant clams are known as iridocytes. While researchers weren't sure exactly what they were used for, they did have a theory; they believed that they were used to help cultivate the symbiotic algae that grow within their flesh. Because there's a lot of light at the latitudes at which these giant clams live, researchers thought that it was possible that the clams were using their reflective surfaces to optimize light absorption.

In order to learn whether or not this was correct, the researchers made precise cross sections of the clams. In the end, they found that the algae within the clams were organized into pillars, with a layer of iridocytes at the top.

"When we saw the complete picture, we understood that the pillars are oriented exactly the wrong way if you want to catch sunlight," said Sweeney. "That's where the iridocytes come into play."

The iridocytes scatter many wavelengths of light in a cone-like distribution pointing deeper into the clam. Red and blue wavelengths, the most useful to the alge, spread the widest and impact the side of the pillars. This, in turn, optimizes the photosynthesis occurring.

"We see that, at any vertical position within the clam tissue, the light comes in at just about the highest rate at which these algae can make sue of photons most efficiently," said Sweeney. "The entire system is scaled so the algae absorb light exactly at the rate where they are happiest."

The findings are published in the Journal of the Royal Society Interface.