Creature feature: giant clams

Have you ever been mesmerized by the giant clams in the Aquarium’s Pacific Coral Reef exhibit? Learn more about them here, get even more details from our giant clam fact sheet, then come check them out on your next visit!

giant clam

How giant is giant?

There are actually several species of giant clam, all in the genus Tridacna. The largest species, Tridacna gigas, can grow over 4 feet long and weigh over 600 pounds! We don’t display that kind at the Seattle Aquarium—but we do have the species T. derasa, T. squamosa, T. maxima and T. crocea.

giant clam - pacific coral reef

Does big equal old?

Scientists don’t know how long these animals live, but here are some things we do know: All giant clams mature first as male, which can happen within two years, depending on the species. They later transform into reproductive females, which happens after several more years (again, exact timing depends on the species). Two of the larger clams in the individual window exhibits at the Aquarium have been with us for 20 years, according to Senior Aquarist Alan Tomida. One of the larger ones, also in an individual window exhibit, as well as the smaller ones in the main Pacific Coral Reef exhibit, came to us in 2008.

PCR-giant clam edited

Getting a grip

Most bivalves have a strategy for sticking down to the substrate, whether it’s rock or sand: mussels have byssal threads and many clams have a digging foot. How do giant clams stay in place on the reef? It’s different for the different species. The smaller species, T. crocea (which grows to 6 inches) and T. maxima (which grows to 12 inches), use strong byssal threads. T. crocea can also use its shell itself to bore into the rock. Those two species need a firm grip because they live in shallower water with more wave action, compared to the two larger species, T. squamosal (which grows to 17 inches) and T. derasa (which grows to 24 inches)—both of these live at the outer edge of a reef, in deeper, calmer water. They stay in place by virtue of their bottom-heavy shells and weak byssal threads.

PCR - giant clam

Shimmer and shine

The shimmering patterns on a giant clam’s mantle come from iridophores, cells that reflect the light. These cells aren’t just beautiful—they’re useful as well. Iridophores and pigments act as sunscreen, blocking UV rays that could damage the clam’s flesh (just as most of us would be damaged by too much exposure to tropical sunshine). These clams also have amino acids that serve this sun-blocking purpose, so that function doesn’t completely explain their brilliant coloration. Coloration is partly determined by genetics, but a species can’t be identified simply by the color of the mantle. Individual clams have their own unique coloration, a bit like an orca’s saddle patch or a harbor seal’s spots. Examining the shell is the definitive way to identify the species of a giant clam.

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