Marine clams hold a certain appeal and mystique for many saltwater aquarium hobbyists. When we talk about clams in the saltwater aquarium, we are really talking about clams from the genus Tridacna in the subfamily Tridacninae. While it is the often the stunningly colored, fleshy mantles that catch the aquarist attention first, there is way more to tridacna clams than their aesthetics. These are fascinating reef animals that display beguiling behavior in the right home aquarium with the proper husbandry. This article should help you decide if a tridacna clam is right for your tank.
You may have heard that clams are nearly impossible to keep, and while some are certainly harder to keep than others, there is no reason that the intermediate aquarist can not have success with a tridacna clam added to an established and stable reef tank. The two biggest reasons that aquarists don’t succeed with clams are light and predation. Tridacna clams, like many stony corals, host symbiotic algae in their tissue, and these algae require appropriate levels of light to survive. If the aquarist does not meet the requirements, the algae will not survive. The result is a browning of the mantle and the eventual death of the clam, which depends on the algae for much of its nutritional needs.
Predation is the other most common killer of clams from the genus tridacna. While clams have developed a pretty bullet-proof mechanism for defense, in the confined space of most marine aquaria, there are a number of predators (or simply curious fishes) who may cause the demise of a newly acquired coral. The most usual suspects include various snails, many butterflyfishes, angelfishes, and some wrasses. You’ve heard us say this before, but here it is again: When it comes to any animal you are considering adding to a saltwater aquarium, do your species-specific research before adding the animal to the tank. By doing so before buying a clam, you will go a long way toward setting yourself up for success. Questions? Send us an email or give us a call.
The clams commonly seen in the marine aquarium trade are generally collected from or aquacultured/maricultured in the Indo-Pacific. In the wild, most of these clams are found in shallow water. Different species of tridacna clams will be found in different biotopes. For example, T. crocea and T. maxima will usually be found living on the reef, while T. gigas, T. derasa and T. squamosa are substrate dwellers. All clams commonly imported for the aquarium trade are collected from (or cultured in) areas of high light and strong flow. Depending on the species, tridacna clams may grow quite large. For example, T. gigas can weigh more than 200 kilograms (441 lb) and be more than 120 cm across. Many, however, reain much smaller.
T. derasa, also commonly called the southern giant clam or the smooth giant clam, is one of the larger of the Tridacna spp. (up to 24 inches across in the wild). Sometimes confused with (and mislabeled in the hobby as) T. gigas, T. derasa is possibly the hardiest and most adaptable of all clams in the genus. It is, rightfully so, a true aquarium favorite.
In the wild, these clams may be found inhabiting substrate throughout the Indo-Pacific in water as deep as 60 feet, although usually much shallower. This clam often lives solitarily, although it may be found in small groupings. Bright light and high flow are the common denominator for these clams, although they are more tolerant of lower light levels than many other species in the genus (so are T. squamosa and T. gigas). In the aquarium, T. derasa clams should be placed on the substrate or in a shallow hollow on the live rock. If placed on substrate, place on a small rock in the sand so that the clam can be moved in need be. Take care that the clam is not in a position where it is shaded by rock or coral growth, and allow room for growth (this species can more than double in size in a year!).
As mentioned above, T. derasa specimens require appropriate lighting in order to sustain their symbiotic algae, although their lighting needs are not as intense as some other species in the genus (keep in mind that that the brighter the color of the mantle, the more light the individual clam needs). A T. derasa, for example, can be successfully kept at a depth of one foot or more with only reef-ready power compacts or T-5s. While T. derasa is more light tolerant than some other species in the genus, it is essential to take the time to light acclimate these clams, especially the smaller ones. The smaller T. derasa clams can be easily light-shocked if they are introduced to a reef tank with high intensity reef lighting without the appropriate acclimation time. To acclimate T. derasa clams, use layers of plastic screening between the lights and the animal to shield it from the full power of the lights. Over a period of a week, gradually remove layers of screening until the clam is exposed to the full lighting regiment.
Smaller T. derasa clams should be target fed several times a week, particularly if they are being kept in a reef tank with aggressive filtration. Feed phytoplankton or other foods specifically manufactured for filter feeding invertebrates. Feedings of two or three times a week are recommended. While water flow is important for this filter-feeding animal, it should be noted that it is possible to have too much flow. If the clam is not opening, and all other parameters are good, consider adjusting the flow regimen in the tank.
All T. derasa clams currently seen in the hobby are aquacultured.
Tridacna crocea Clams
T. crocea clams are another popular clam from the genus. Commonly called crocea clams or boring clams, these are the smallest of all Tridacna spp., growing to no more than six inches in the wild and make great aquarium animals. While they do require more light than many other species in the genus, they are still relatively hardy clams that make beautiful additions to a captive reef.
Unlike T. derasa, T. crocea clams actually bore into the rock (hence the common name “boring clam”) and attach themselves firmly via their byssal filaments. As such, this clam should be placed on the rockwork in the middle to upper third of your aquarium depending on the tank’s depth. In the wild, this species is often found living on Indo-Pacific reefs in large aggregations, and a beautiful aquarium display may include multiple specimens.
Like T. derasa clams, T. crocea clams should be light acclimated to their new environment (see above), especially if they are less than two inches in size (smaller clams have thinner tissue in their mantle). Because the clam will readily attach to the rock within a few days, it is generally not possible to light acclimate the animal by starting it low in the tank and gradually moving it to a higher location (unless the clam is attached to a small piece of rock already). Once attached, the aquarist should not attempt to move the clam, as the damage to the byssal filaments may result in the death of the animal. Keep in mind that, as mentioned above, the clams with more brightly colored mantles generally require higher intensity lighting (or higher placement in the tank).
While T. crocea clams derive most of the nutritional needs from their symbiotic algae, they will appreciate target feedings, especially when smaller. Plan to feed two to three times per week by broadcasting phytoplankton upstream from the clam’s mantle.
Cause for Concern
While the two clams from the genus Tridacna described above are generally hardy, there are some telltale signs of trouble for which the aquarist should be on the lookout. The most common issues are commonly referred to in the hobby as “bubble mantle”, “pinched mantle” and “gaping”. Bubble mantle is easily avoided by not directing laminar flow directly at the clam directly or dumping water directly into the display tank during water changes. Both of these practices may result in bubbles of air being caught it the clam’s mantle, and this can lead to big trouble and sometimes the death of the animal. Pinched mantle looks like it sounds—the mantle of the clam appears “pinched”. The causes are still being debated, but the treatment involves freshwater dips that will kill the protozoan that is likely responsible for the condition. Gaping is a condition in which the larger of the two holes visible inside the clam’s mantle becomes noticeably larger or “gapes.” This may be an indication of a variety of problems that are often associated with water quality. Like most reef animals, stability should be king, and if you see any of these warning signs, take a long hard look at your parameters and the stability of your system.
Overall, clams can provide a beautiful, behaviorally interesting addition to the reef aquarium. While they are often reported to be “impossible to keep,” the two species described here can be successfully kept my most intermediate aquarists with established reef tanks. As always, do your species-specific research before buying, but, by all means, if you have the right tank for a clam, do consider adding one to your tank today!