Glam Clams
© 2023 By Paul Erickson
June 14, 2023
Giant clam Solomons, Birgitte Wilms.
I’d like to tell you about the huge “killer” clams I’ve photographed on Indo-Pacific coral reefs. And I will. But first, I’d like to pay homage to our local softshell clam (Mya arenaria), famously harvested in my hometown of Ipswich, Massachusetts, which is located just a few flaps of a gull’s wing from Cape Ann.
Do yourself a favor. From Cape Ann or elsewhere, take a trip through Ipswich along Route 1A. If you spot a culinary landmark with a roof, festively resembling four upturned lids of a take-out-food-style paper container, you’ve arrived the Clam Box. As you pull into the restaurant’s parking lot, chances are you’ll be enveloped in the appetizing redolence of golden-battered, clams, scallops, calamari, and other mouth-watering beauties, bubbling and hissing in the Clam Box fryers.
The sweet smell of summer.
Even if you arrive at the Clam Box well before lunch or dinnertime, you’re likely to find a line of impassioned, fried-seafood fanatics extending out of the entrance and wrapping around the building.
In our town, fried clams are a major food group.
Small, Medium, and Ginormous
As the clam clan goes, our local softshell, aka steamer, is one of the little guys, with an average length of about three inches. (Even smaller are tiny freshwater fingernail clams, roughly the size of your smallest fingernail.)
Then there’s the softshell’s beefy, West Coast cousin—the geoduck (Panopea generosa), pronounced “gooey duck.” Also called the king clam, this delectable mollusk with a lifespan exceeding 150 years weighs up to 7 pounds.
But even the gooey duck is dwarfed by the massive giant clam Tridacna gigas. This tropical Indo-Pacific bivalve can grow to a length exceeding four feet and a weight as great as 500 pounds.
Giant clam, Paul Erickson.
Giant Clam, Birgitte Wilms.
Encountering a Giant
The first T. gigas I ever met had anchored itself on a remote coral reef in the Solomon Islands. As I photographed the enormous mollusk, whose partially opened shell pointed straight up toward the sun, I recalled the mythical killer clam in the 1948 film Wake of the Red Witch. As you may remember, in a dramatic undersea scene, John Wayne rescues a drowning pearl diver whose leg is trapped between the valves of a giant clam. (As bivalves, clams have one shell and two hinged valves.)
Giant clam mantle (detailed views), Birgitte Wilms.
However, in the real world giant clams are not dangerous. I’ve never been able to find a documented case of anyone ever being drowned or injured by one. In fact, an adult T. gigas is unable to close up completely because its fleshy mantle protrudes out and over the shell’s outer edges as the clam basks in the sun. For this animal, sunbathing is a matter of survival. Here’s why:
Like reef building corals, a giant clam’s mantle is a living greenhouse, providing an ideal microhabitat for plant-like photosynthetic, single-celled algae (zooxanthellae) located in vertical tubes just under the top surface of the mantle. Like land plants, the algae use solar energy to produce essential nutrients in the form of glucose, glycerol, and amino acids—most of which become food for the clam.
The Giant Clam’s Psychedelic Mantle
In addition to dull, brownish pigments of symbiotic algae, the mantle also displays psychedelic color patterns reminiscent of a 60s era Jefferson Airplane concert poster. These dazzling colors are created not by pigments, but by “diffraction structures” in cells called iridocytes. Roughly the same principle is at work in blue jays, whose feathers appear blue, yet contain no blue pigment.
Fiji giant clam green, Paul Erickson.
In a giant clam iridocyte, the diffraction structure at work consists of layered microscopic, crystalized guanine plates, alternating with layers of clear, gelatinous cytoplasm. This arrangement scatters the sun’s white light into the various electromagnetic wavelengths we see as the color spectrum (ROYGBIV).
Remarkably, iridocytes perform multiple optical tasks: The clam’s algal gardens are most productive under blue and red light. So the iridocytes have adapted to precisely direct beneficial blue and red wavelengths to the clam’s algal tenants. At the same time, iridocytes backscatter, or basically kick out, less beneficial yellow and green colors as well as harmful ultraviolet wavelengths away from the algae. And that is why the brilliant, mesmerizing reflected colors delight our eyes instead of being absorbed by the clam.
Finally, thanks to the friendly folks at the Clam Box for assisting me with the demanding culinary research involved in writing this piece. Please pass the tartar sauce.
Tridachna CC, Paul Erickson.
Paul Erickson is, among other things, Science Editor for Cape Ann Cosmos. Link to bio here.
Birgitte Wilms is a member of the Women Divers Hall of Fame. She was born in Denmark and began her diving career after moving to the United States in 1987. Birgitte co-authored In a Sea of Dreams, winner of the World Grand Prize for underwater photography books. Her photos have appeared in dozens of magazines internationally. Presently, Birgitte is working on a series of undersea books for children.
