Those who keep a mixed reef or corals only type are the only aquarists who would be interested in clams, or at least in the clams that are being looked at here, which are in the family Tridacnidae. This includes some of the giant species from which stories of divers being trapped have been born, the stories no doubt being unsupported by fact. Not all of them have the potential to be giants, there are those that are much more aquarium sized.
The link shows some of these clams: http://www.poppe-images.com/images/search_results.php?keyword_mh=tridacna&x=73&y=11
Even if the aquarium system owned is a reef the successful keeping of these clams depends on adequate lighting, for within the flesh are zooxanthellae, the same single celled algae that reside within the flesh of most of the corals commonly kept. All reef aquariums do not have the required lighting, some use banks of fluorescent tubes. Unless the clam placement is very high on the reef this light is likely to be inadequate, and even if placed high the light could be insufficient. Insufficient light will prove to be fatal, and the sign for this is a brown coloured mantle. Once this has occurred then the clam is normally doomed and placing it under intense light in the hope of saving it is likely to fail. The best lighting for clams is properly powered (that is sufficient wattage) metal halide which can penetrate the seawater reasonably.
Placing a clam high on the reef in the hope of sufficient light has a downside in addition to the possibility there still won’t be enough light. This downside is seawater movement as the clams prefer this to be very moderate to weak. The seawater movement in an SPS reef is high and a soft coral reef is more than the preference of the clams. So at least an area where the clam is protected from high movement is required. In adequately lit aquariums (adequately meaning the lighting has sufficient power to penetrate sufficiently to the bottom) the clams should do well placed low down on the sand.
Despite the absolute necessity for adequate light the clams are easy in one respect and that is feeding. They are filter feeders, but the products of their zooxanthellae are sufficient to meet their requirements, which mean that the aquarist doesn’t need to worry about additional feeding. Any food that is placed in the aquarium that is suitable for filter feeders is likely to be useful to the clams as well.
There are three clams that are probably most of interest to the reef aquarist, and these are Tridacna crocea which grows to about 5 inches (circa 12.5 cm), Tridacna gigas which could grow to about 50 inches (circa 127 cm), and Tridacna maxima which could grow to about 14 inches (circa 35.5 cm). It would seem that the only two suitable to the aquarist are Tridacna crocea and Tridacna maxima, but all three are obtainable if searched for. If Tridacna gigas begins to become too large it could be moved very carefully to more suitable accommodation. It takes a long time to reach a large size.
The attraction for aquarists with these clams is their beautifully marked mantles. When healthy and fully open they are extremely attractive and automatically draw the eye.
If the aquarist has suitable accommodation, that is a reef aquarium with suitably powerful lighting, and also some experience has been gained the purchase of these clams is a very worthwhile consideration. (Experience means that high quality seawater can be maintained and the aquarist has an understanding of aquarium husbandry). They provide a very ‘reefy’ looking life form for the captive reef and one that has gorgeous colours when open. In addition feeding is not a problem so seawater quality will not be potentially downgraded.
Tridacna clams are now being commercially produced and if at all possible these should be the ones purchased, even though their size may be fairly small.