In the vastness of the ocean, there are many so-called animal to animal symbionts, seemingly odd-fellow relationships from which both species benefit. Finding Nemo made famous one such partnership, that of the clownfish and anemone.
But what about symbiosis between an animal and a plant? Or more specifically, a plant-like alga called zooxanthellae? It’s a surprisingly common phenomenon, especially in the shallows of warm equatorial reefs where there’s abundant light for photosynthesis. Corals, jellies, even sea slugs participate.
And so does Tridacna gigas, the giant clam of the Indo-Pacific, the largest bivalve mollusk on Earth and the world's only sun-powered clam. The giant clam hosts a thick layer of zooxanthellae in its tissues and gets up to 90 percent of its nutrition from their photosynthesis. Imagine if we could do that? Just stand outside on a sunny day and photosynthesize? That's pretty close to a free lunch.
The giant clam does its part, too, by providing the zooxanthellae with a habitat protected from hungry predators.
During the day, the giant clam extends its mantle tissue, allowing sunlight to reach the zooxanthellae. In fact, full-size giant clams cannot fully close their shells. You're less likely to get your foot stuck in a giant clam like in one those ‘50s-style horror movies than to get a hernia from trying to pick one up.
Because of their symbiotic relationship with the zooxanthellae, giant clams can photosynthesize their food like plants, even as they carnivorously filter feed, sieving out plankton from the water as all clams do. No Omnivore's Dilemma for the giant clam.
And with this abundance of nutrition, giant clams have gone turbo—at least in terms of size. Giant clams grow and grow. In the wild, they can reach a length of four feet, weigh up to 500 pounds, and live for a hundred years.
Scientists have also discovered the giant clam can even “farm” its zooxanthellae. At night, specialized cells called amebocytes search out and digest old algal cells, keeping the "farm" clean and healthy, and in the process aiding the entire reef. The giant clam's vigorous filtering keeps reef water crystal clear and free of fouling organisms.
But these giants are becoming rare, and near some Pacific Islands, are already locally extinct.
There is a huge demand for every single part of the giant clam. For Pacific Islanders, who rely on the ocean’s bounty for most of their diet, giant clams have been a traditional food source for millennia. The clam's mantle and dried abductor muscle are considered a delicacy in Asia.
Further, every year approximately 200,000 live giant clams are taken for the ornamental aquarium trade. Their shells are, of course, sought-after as souvenirs. The zooxanthellae make the clam's mantle look glamorous, in hues of electric blue to malachite green. Each clam's pattern is unique and has long caught the eye of humans.
To save the giant clam, and the reefs on which they make their home, mariculturists are learning to farm them on Fiji and other islands, much the same way oysters are raised in the Chesapeake Bay. The goal is to reintroduce them into the wild, where they can filter and photosynthesize to their hybrid heart’s content. As happy as clams.