Fossil of the Week

9/22 – Giant Clam

Our exhibition Science on the Half Shell: How and Why We Study Evolution opens at the Museum of the Earth this Friday, September 24. In recognition, this week’s “Fossil” of the Week is the living Giant Clam, Tridacna gigas.

With a weight reaching 270 kilograms (500 pounds) and a size of 1.2 meters (4 feet) across, Tridacna gigas is the largest living bivalve. Its average lifespan is more than one hundred years and one individual produced the largest pearl on record, weighing 7 kilograms (15.4 pounds) with a length of 23 centimeters (9 inches). These clams live in symbiosis with algae (zooxanthellae) that live in the colorful body tissues lining the upward-facing zig-zag margin of the clam. This exposes the algal cells to the light that they need for photosynthesis; this need also limits this species to shallow-water reef habitats. The zooxanthellae provide a major portion of the clam’s food (the clam also filter feeds) and also recycle its waste products.

Many myths surround Tridacna, which lives in the shallow waters of the southern Pacific and Indian oceans. Its height of popularity was arguably as a man-eating killer clam trapping unsuspecting divers, stories that persist today in cartoons and B-movies that portray drowning divers in the stranglehold of the vicious clam. But despite its gruesome reputation, nobody has ever been attacked by a Giant Clam; it lives solidly embedded in the reef and because of its bulk, closes its valves very, very slowly, allowing almost any potential victim ample time to escape its grasp. In medieval times, Tridacna shells were used in churches as holy water stoups and baptisteries. Tribes in the Indo-Pacific region made ceremonial figurines out of these clams, and Solomon Islanders used pieces of its shell as money and symbols of wealth. Today, the Chinese and Japanese enjoy the meat of Tridacna as a delicacy, and their large shells are still much-sought curios. The continuous popularity of the Giant Clam has lead to intense overharvesting and has put Tridacna on the list of worldwide endangered species.

The fossil record of Tridacna is only poorly preserved, but it can definitely be traced back to the Miocene Epoch (Quaternary). The subfamily Tridacninae (in the family of heart-cockles, Cardiidae) originated in the Tethys Sea – in the middle of modern-day Europe – but is now restricted to the Indo-Pacific region. The Tridacna gigas specimen pictured here (PRI 50338) was collected in 1946 from the lagoon at Bikini Atoll, eight years before the famous nuclear tests were conducted there. The name Tridacna gigas is attributed to Linnaeus, 1758 – this is the 10th edition of Systema Naturae, the first work to use binomial nomenclature (the two-part names, genus and species, for each kind of animal and plant).

Text by Judith Nagel-Myers (reprinted from “Fossil Focus” in American Paleontologist, Fall 2007)