I have seen a lot of giant clams in my diving career. In my latest trip to Fiji where I saw many beautiful clams, I wondered: what makes these claims so colorful, with such beautiful patterns, and each one different? Further, I wondered, are they all the same kind but different ages and thus different sizes? These questions opened up the rabbit hole, and down I went.
I’ve done a lot of reading while in the rabbit hole, and learned way more than I think I should regurgitate here. I will try to summarize on a big-picture level. I’ll include some links to references if you want to read more. If you do, you will learn, among other things, that just like every other part of Nature, there are many things we humans don’t know about clams.
There are over 9,000 different species of bivalve mollusks, e.g., clams, oysters, scallops, anything with two hinged shells, aka “valves.” The largest of these is the giant clam genus Tridacna, and the largest Tridacna species is the Tridacna gigas—“T. gigas” or the “giant” giant clam. All clams have a mantle, the living tissue that envelopes the soft body of the clam and forms and grows the shell. Unlike most other types of clams, the Tridacna mantles also house zooxanthellae (microscopic algae) that use sunlight to produce nutrients through photosynthesis, which feed the clam. Tridacna, and all clams, are filter feeders, drawing surrounding water in through the incurrent (aka inhalant) siphon, removing nutrients, and expelling the water through the excurrent (aka exhalant) siphon. Thus, Tridacna have two food sources.
There are around seven kinds of Tridacna species that a diver might see, but most often it will be the T. maxima, the T. derasa, or the T. gigas. Each type has some combination of various characteristics, e.g., a number of ridges or ribs (usually between four and seven) on each half shell, scutes (stacks of tiny shelves) growing on each ridge (or not), simple or fancy tentacles around the incurrent siphon (or not), maximum size, and life span.
For example, T. gigas have 4-5 prominent ribs with no scutes, lack siphon tentacles, can live over 100 years, grow up to 54 inches wide, and weigh 500 pounds or more … but, they have to start small. At first I thought maybe all the fancy clams I saw were T. gigas, just different ages. Now I realize I’ve been seeing several different kinds of Tridacna. Considering all the variables, I’m not really sure what I’ve been looking at, although I have seen a few really big clams that I think could not be anything other than T. gigas.
The flesh of a T. gigas is considered a delicacy and is mostly protein, and you may have read that sometimes the shells decorate a rich person’s table. Because of overharvesting, climate change, and habitat destruction, the T. gigas are critically endangered, the last stage before they are determined to be extinct in the wild. This explains why I have seen maybe five full-grown T. gigas in my life, clustered in two places. Clams suffer from bleaching events, just as hard corals do. As the oceans warm, the Tridacna expel their algae. Although Tridacna can live without algae longer than hard corals can (I’m guessing because they also get nutrition through filter feeding), eventually they can starve to death.
As an aside, I also wondered why corals and clams expel their algae when the water warms. Apparently it’s because the algae can’t function properly in extreme heat and expel toxins when the water is too warm. The toxins are harmful to the host and the algae, so the host kicks the algae out. Earlier I was told it’s because the host is making space for different algae to take up residence, different algae that is better able to withstand the warm water. Now I read that some scientists think clams may excrete more heat-tolerant algae under warming conditions, making the algae available to other hosts, which could help counteract bleaching events in corals and giant clams … yet another climate change topic to study.
Clams provide many environmental benefits. Their shells provide habitat for other life. As filter feeders, they clean the water. A single littleneck-sized clam can filter 4.5 gallons of seawater per day. Improved water clarity allows for more sunlight, which aids in the growth of seagrasses and increases oxygen. Clams continually grow their own shells by converting carbon into calcium carbonate and in this way, sequester carbon. Clams expel unused nutrients, which contain some live algae providing food for larvae and transferring zooxanthellae to other clams and probably also to corals.
Giant clams reproduce by releasing both sperm and eggs into the water—more than 500 million eggs at a time. A clam cannot fertilize its own eggs, so the eggs are taken in by other giant clams who fertilize the eggs. The egg floats for about 12 hours until a larva hatches. As the larva swims through the ocean and/or after it settles (sources differ), it ingests algae and later develops a system of tube-like structures inside its body coated with the algae. After around 7-10 days, the developing baby clam settles on the ground, changing location in the first few weeks until it finds the perfect spot.
Now, on to the question that started this—what makes the Tridacna mantels so beautiful, varied, and unique? First, as I said, algae live in the mantle, and the algae make their own pigment. Also, the Tridacna clams themselves produce numerous pigments, used primarily as sunscreen for both the clam and the algae. These pigments can absorb and use some parts of the spectrum of visible light better than others. They don’t absorb much green or red light, which is why zooxanthellae tend to be brownish in color. Other pigments convert less usable colors of light into readily usable colors like blue, which are then absorbed by another pigment. The light is bounced around all these pigments such that the algae can use as much of the light as possible.
Next, there are iridophores. These are small groups of cells in the mantle that contain stacks of tiny reflective platelets which reflect various wavelengths of light to produce some of the colors of the mantle. These can also act as sunscreen. UC Santa Barbara conducted a study to see how these iridescent cells interact with the algae to enhance photosynthesis, hoping to learn more about light collection and applying those lessons to create more efficient solar cells. They specifically studied T. maxima and T. derasa and found that each species uses different methods for mixing colors to produce white. White in the T. maxima comes from tight clusters of differently colored iridescent cells while T. derasa has multicolored individual cells that appear white on a macroscopic scale. A later study shows how the photosynthetic geometry of the giant clam (how the algae is arranged, which is part of the intricate designs in the mantle) might help improve solar energy production through large-scale algal plantations.
Other adornments include several hundred to several thousand pinhole eyespots along the mantle’s margin, each consisting of a small cavity containing a pupil-like aperture and a base of 100 or more photoreceptors sensitive to three different ranges of light, including UV. This triggers the T. gigas to partially close their shells in response to dimming of light, change in the direction of light, or the movement of an object. T. gigas are unable to close their shells completely because of the size of their mantles. 
I have really only touched the surface, er, mantle of any of these topics. There is just so much to learn about something that is easily ignored and that could help human civilization if we only let it. But what we can do, without any trouble at all, is marvel at the exquisite beauty. Please see more Fiji clams and Indonesia clams in my galleries. I invite you to search the web for more photos of giant claims, and challenge you to find two that are the same. Like all other life on this planet, every one is unique.
Here are links to some of the fascinating articles I read as I prepared this post: