For many, many years out reach for marine aquarium keepers are the giant clams, Tridacna species. And to this day. though captive raised, giant clams are not for nascent keepers.
That said they can be kept if one has the intensity and quality of lighting, a steady temperature, and natural sea water levels of minerals, and sufficient alkalinity to buffer it.
The two most common reasons why giant clams fail in aquariums are insufficient light, and predation. In the former, clams contain a symbiotic algae species in their flesh. Like most all algae (other than cyanobacteria of course), the algae that give the vivid colors of giant clam flesh photosensitize light. The wastes from that photosynthesis are sugars that feed the clam. Also, giant clams need a brisk, chaotic flow, similar to that needed for Acropora species corals.
Like all clams in freshwater or marine species, the Tridacna species are also filter feeders, and their food of choice is phytoplankton (floating algae cells). One can see when a giant clam is in feeding mode when their ‘mouth’ opens and the feeding gills are displayed. Thus a phytoplankton drip maintained constantly will enhance the health and appearance of a giant clam or clams. The strong, random flow serves to bring the phytoplankton and other micro foods to the clam. Also, it prevents waste from accruing on the clam. One thing a prospective keeper should be aware of, the brighter the color of a giant clam mantle, the brighter lighting needed. Also, giant clams can consume Nitrate, making them a win-win option for a reef aquarium.
Predation can occur from varied sources. If one overfeeds the reef fishes, bristleworms proliferate in large numbers, and can easily consume a newly-introduced giant clam.
Also, Butterfly Fishes, some snails, some wrasse species, some dwarf angelfish, will pick at a giant clam to its ruin. Research very thoroughly what you are going to keep with what in a reef aquarium.
What makes giant clams available is captive raising. The Japanese developed the methodology to captive raise giant clams, since they view the clams as a delicacy. That made it necessary to develop captive raised giant clams, since the Japanese yen for them made those in the South China Sea giant clam species close to extinction. The Japanese method of captive raising giant clams by the thousands is still top secret.
That said, when shopping for a giant clam or clams for your reef aquarium make dead sure they have been captive raised, since it is highly, highly illegal to take a giant clam from nature. Make sure you only buy from reputable dealers. Those that cannot tell you where their stock came from, don’t buy from them, as there is a black market for wild caught giant clams. Giant clams are very highly protected these days, with daunting penalties to those caught dealing in illegal clams.
One should also be aware to site a giant clam where it is going to stay, since they produce tough, numerous tendrils from the bottom of the shell to anchor the clam to the site. Moving such an established giant clam is usually fatal.
Acclimation must be very careful and patient. Drip acclimation is highly recommended, at least four hours worth, with six better. The clam at no time should be exposed to air, and aquatic gloves are necessary to prevent oil from peoples’ hands on the shell.
Now some species.
By far the largest clam in the world is T. gigas.
Growing to four feet long and weighing an average of 500 pounds, gigas is truly a giant clam. A reef tank with at least an 8×4 footprint would be necessary to keep one. And the weight they can attain can stress a glass tank, making it highly possible for a collapse. I strongly recommend a cell-cast acrylic aquarium of a proper size before attempting a T. gigas.
Long-lived as are all giant clams, the largest measured weight was 710 pounds and it had an estimated age (counting rings in the shell) of 146 years. Thus, your Tridacna species clam may have to be written into your will for its continued care.
As they are increasingly rare in nature because of human pollution, T. gigas is perhaps the most vital giant clam species to be captive raised. Site a gigas clam on the substrate close to live rock. Gigas is the only Tridacna species that cannot close its shell completely.
Obviously not a clam species for any regular sized glass aquarium.
Almost half the size and a less than a quarter the weight of the above is T. derasa.
Growing to two feet with a fairly narrow shell, derasa is perhaps the most hardy of the giant clams. Thus, those with suitable light and current should have little trouble keeping a derasa species giant clam.
Though it does take some years for derasa to reach its maximum length, one should have an aquarium of a size to keep one; at least a six-foot tank.
Like the above, derasa should be placed on the substrate with sufficient space for it to grow. Keep in mind they will grow to two feet. Well kept and fed, a newly-purchased derasa can double in size in a year, and for each year after, until reaching its maximum size.
Found up to 60 feet deep in nature, one should very slowly acclimate derasa to the bright light necessary for other Tridacna species. Filtering the bright light, especially halide light, with transparent egg crate, is all but essential in the process of acclimating derasa to bright reef lighting. Mine are kept in the lower third in my metal halide and power compact based lighting.
It’s best to have some live rock rubble in/on your substrate, as it makes it easier for a derasa to attach and stabilize its position.
A good giant clam species for those with a suitable tank to keep.
Much smaller and thus more suitable to be kept in, say, a 55 gallon reef is T. crocea.
Unlike the above crocea needs very bright light. When two inches or less, the mantle is thin and can be damaged fatally by such lighting, so slow acclimation is needed as it grows.
Growing to six inches, crocea is also known as the rock borer clam, since they bore into live rock before anchoring themselves with filaments. Thus, it should be placed on a piece of live rock large enough for the adult clam before moving it over time to the proper high light, like that from metal halide or LED lighting.
Crocea attach permanently to live rock in just a few days, so I recommend placing the clam on the rock and wait about a week before starting the slow and careful move up to the final spot. Crocea grows and looks best when a phytoplankton drip is maintained. One of the commercial products can be used, but none can match live phytoplankton, since it is a natural food for many marine filter feeders.
Since crocea is a smaller species, keeping several in a large piece of live rock is a beautiful display. Mine are kept in the top third of my very highly-lit reefs.
Probably the most popular and coveted of the giant clams is T. maxima.
The reason is obvious, as their mantle is very wide and folded and most commonly in a rich blue color. Slightly larger than the above species at eight inches, maxima mantles fold over the edge of the highly convoluted shell.
The beauty of maxima tempts many reef keepers. Like all giant clams, a prospective keeper must be well versed in maintaining natural sea water levels of minerals and suitable alkalinity, and be able to keep a stable temperature. It should be 77 degrees and no higher, since maxima will fail if the temperature reaches low 80s’
Maxima is dependent on very bright light, like that produced by metal halide or higher end LED fixtures. Though it depends on its symbiotic algae more than most Tridacna species, for best results, start a phytoplankton drip so that glorious mantle will be well spread and growth and health maintained.
The current should be strong and chaotic, as laminar flow is disastrous. PH should never exceed 8.2. Given proper conditions and stability, maxima is quite hardy. As it is easier to maintain the correct parameters for maxima in larger tanks, I strongly recommend at least a six-foot aquarium should you wish to keep this giant clam.
As above, when small at around two inches and thus can be damaged by too much light, one should acclimate maxima very slowly to the proper lighting. Best is starting it on a fissure of a piece of live rock, where it will attach in a few days, then moving it just a few inches at a time, with three or four days between moves, until locating it in the upper third of your tank.
Like the above, multiple specimens of maxima can be kept together, with enough space between them for growth.
Giant clams are a fascinating addition for the reef aquarium for those with a suitable reef aquarium.