An Introduction to the Giant Clams by James W. Fatherree, M.Sc.

There are several species of giant clams that are commonly called “tridacnids”, as all of them belong to the Family Tridacnidae. These clams are very popular amongst reef aquarists because they come in a wide range of sizes and interesting appearances, and can be quite unique additions to any healthy reef tank. The patterns and colors that cover the fleshy, exposed parts of their bodies can be absolutely gorgeous, but there’s more to them than just good looks.

Be aware that some species of giant clams, like this Tridacna derasa, can live under much less light than others. It is important to do some homework and find out who can live under what – before buying one.

One of the primary features that all of these clams share (other than being big for clams) is a fleshy structure called a mantle. This is a soft tissue part of them, which envelops the body of the animal and forms its shell by precipitating hard material around it. The shell material is calcium carbonate, which is the same thing that corals and many other organisms use to build hard parts, and the mantle produces it in the form of two shell halves called valves. However, when it comes to tridacnids, the mantle tissue does much more than build their protective homes. It actually helps them get the nutrients they need, in a way that you would probably never expect.

While the vast majority of clams are filter feeders that rely on straining tiny food particles from water, the tridacnids also carry a full compliment of single-celled photosynthetic algae in their mantles, which provide them with an additional, internal source of food. These algae, known as zooxanthellae, are the same sorts of microscopic creatures that live inside reef-building corals, providing them with much of their nutritional needs, as well.

In corals and tridacnids alike, the zooxanthellae live where they are exposed to sunlight, and are thus being able to produce food through the process of photosynthesis. And, under good conditions, far more food can be produced by the zooxanthellae than is needed for themselves, so the host that is providing them with a place to live (inside its mantle) gets the “leftovers”. Thus, while the lower part of a tridacnid’s mantle covers the clam’s body and produces the shell, the specialized upper portion of the mantle provides a home for the zooxanthellae and is built to capture sunlight.

Some giant clams, like this Tridacna crocea, are so giant. This species, Tridacna crocea, has a maximum size of just six inches.

To do this, the upper part of the mantle is greatly enlarged and extends across, and usually beyond, the upper edges of the shell. So, what we typically see are large, colorful flaps of mantle that hang out of the top of the shell leaving the greatest amount of zooxanthellae-occupied tissue illuminated as possible. All these clams need to do in order to get some food is sit upright in the Sun and leave their shells opened enough for the mantle to receive the light. A very different way for a clam to get its nutrients, indeed.

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As far as aquarium care goes, when it comes to taking good care of tridacnids, the recipe is essentially the same as it is for most any coral you’d plan on keeping. Good lighting is a must, of course. And, excellent water quality is absolutely required, too.

When it comes to lighting, tridacnids can be found in a wide variety of environments in the wild, and some species can thrive under more or less light than others. Many tridacnids may do just fine under fluorescent lighting (like V.H.O., Power Compact, or T-5 bulbs) when kept in relatively shallow tanks or up on the rockwork in deeper tanks, but high-intensity metal halide lighting is really the best way to go in most situations. Should you choose to get one tridacnid (or more), you should do some homework to find out which species are the best matches for the lighting you already have, or vice versa, find out what type of lighting you need to buy to keep a particular species that you really want.

This is the real giant of the bunch – Tridacna gigas. The largest found was about 4.5 feet in length!

As mentioned, most clams are filter feeders, and the tridacnids are no exception regardless of their relationship with zooxanthellae. While they may get some nutrients from the zooxanthellae, they do require other nutrients that the zooxanthellae don’t produce, as well, which can be acquired by filter feeding. However, tridacnids have other options that most things don’t, as they also have the ability to absorb nutrients directly from seawater and/or can also remove some of the zooxanthellae in the mantle and digest it if necessary. Yes, they can get the nutrients they need in four different ways, while we’re stuck with just one (eating food).

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The best part is that, under good conditions, they’re able to absorb enough nutrients directly from seawater to cover their needs for things that they can’t get from the zooxanthellae. In fact, if provided with enough light, tridacnids can completely forgo the need to filter feed and can thrive in particulate-free water as long as there are enough dissolved nutrients present.

I dedicated an entire chapter to the details of tridacnid nutrition in Giant Clams in the Sea and the Aquarium, which covers how they “work” in great detail, but I’ll keep it simple here by saying that, in aquariums, basically everything is taken care of by having good lighting and simply feeding the fishes. Some fish food is left uneaten and becomes particulate detritus, which tridacnids can filter out, and it also releases other nutrients into the water as it decomposes. But, most food is eaten by the fishes, which then give off dissolved nutrients (like nitrogen-rich ammonia) and produce solid wastes that can also become detritus, too. So, when the fishes are fed, the clams get fed, as well.

Many giant clams, like this Tridacna maxima, have absolutely stunning patterns on their mantles.

Still, in the case of one or more tridacnids being kept in a very sparsely populated aquarium, there may not be enough food/dissolved nutrients present, making feeding a requirement. This would be an usual situation, though. Basically, if all other conditions are acceptable, a specimen should grow, which can be seen when new, white shell material is added to the edge of the shell. If it does not, and you have a very low fish population, then you should use a quality phytoplankton food, follow the manufacturer’s directions for use, and see if it makes a difference. Conversely, you could also start using food from the beginning, then if you can see shell growth, you may want to cut back on the amount of food being used and see if a specimen continues to grow. If it does, you can cut back on the feeding more and more, until you are satisfied that no food is needed and quit using it altogether.

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Now, when it comes to water quality requirements for keeping tridacnids, again, parameters fall right in with what’s considered “standard” for reef aquariums in general. Temperatures between 77° and 82°C are optimal, as is a pH of 8.1 to 8.3. Alkalinity should optimally be kept in the range of 9 to 12dKH , and calcium should be maintained at 380 to 450ppm, etc. About the only thing in particular to note is that as a clam grows, it adds new shell material to the entire inner surface of the valves to thicken them, rather than just the upper edge. So, even a slow growing clam can use more calcium than you might expect, and you’ll need to do what it takes to keep it up.

A good sign of good health is the addition of shell material to the valves. Here you can see bright white, new material that has been added to the edge of this one.

Tridacnids can also produce an attachment structure, called a byssus, to affix themselves to hard surfaces. This structure is made by an organ (called the byssal organ) found on the underside of the body, which secretes a liquid substance that hardens quickly to form a number of tough fibers. These emerge from an opening in the bottom of the shell, with one end of the fibers solidly attached to a rock, coral, etc. and the other held inside the shell by the byssal organ. This keeps them from being knocked over or moved around by waves or predators. Thus, it is best to place small specimens directly onto a hard substrate, like a piece of live rock, or to place them on sand with a flat piece of rock just under the surface. They can reach down through the sand and still attach to the piece, even if it is shallowly buried.

Specimens that use a byssus will firmly attach themselves to the rock, typically within a few days. However, it is important to note that many clams stop using a byssus once they grow to a particular size and then slowly close off the byssal opening at the bottom of the shell. Oftentimes, they can stay in place simply due to their weight, and will then release their byssal hold. In any case, what you don’t want to do is place any specimen with an obvious byssal opening into a gap between pieces of live rock, with the opening left exposed and nothing for it to attach to. You should never place a clam in a spot that prevents it from opening its shell normally, either, or in a location where strong direct currents may keep it from fully extending its mantle tissue.

And that’s about it when it comes to the basics of tridacnid biology and aquarium care. If you find these animals as beautiful and interesting as I do, give one a try. Or a few…