Giant Clams Help Algae Partners Harvest Light

"Giant" tridacnid clams and their algae lodgers are a prime example of natural partnership at its best. New research has revealed that not only does this clam let photosynthetic algae take up residence in the flaps of its shell, but it actually helps redirect sunlight so that its tenants get their daily dose of energy.

According to a study recently published in the journal Interface, the giant clam boasts specialized cells called iridocytes that make them ideal hosts for photosynthetic algae, which grow in microscopic pillars that are literally embedded into the clam's fleshy mantle.

In this way, the algae gets a home and protection, and the clam can feed off the glycerol that the algae secretes. Interestingly, the relationship doesn't stop there.

According to the study, most of the 300 or so cells in each residential algae column have no direct access to the light largely because they are caught between clam shell flaps. This is both unfortunate for the lodgers and the landlord, as the algae need sunlight to survive, and the clam certainly wants its sources of food to be healthy.

Researcher Alison Sweeney at the University of Pennsylvania and her team have demonstrated that the specialized structure of the iridocytes have evolved to correct for this problem.

"What makes this system in the clam special is that the design can extract every last photon from sunlight," she recently explained to New Scientist.

By shining halogen lamp light on giant clams in controlled environments, the research team discovered that iridocytes redirect the path of sunlight so that it fans out into a cone, bathing entire pillars of algae at once. What's more, these little "reflectors" appear to be fanning the light in mainly red and blue light - the wavelengths that the algae photosynthesizes most efficiently.

Interestingly, the giant clam is not the only clam that can manipulate light to its advantage. Nature World News previously reported how the "disco clam" (Ctenoides ale) uses highly specialized reflectors on its shell lips, which are furled and unfurled in brilliant flashes to attract prey.