Tridacna Clams
Clams of the family Tridacnidae are some of the most amazing and beautiful animals available in the aquarium trade. Of the nine known species of tridacnids, only six are available to hobbyists. These magnificent creatures are native only to Pacific waters.
The presence of endosymbionts in the mantle of these clams has made them relatively easy to keep and to feed due to their ability to photosynthesize. Unlike corals however, the clam does not have these symbionts in the planktonic stage and must capture free-floating symbionts released by an adult clam.  
These clams are regarded as a delicacy in Chinese cuisine, many considering them to be an aphrodisiac.  This, along with their popularity in the aquarium hobby, has caused some species to become extinct in certain Pacific Islands.
The idea of propagating these clams was started by the Micronesian Mariculture Center (MMDC) in Palau.  Soon, organizations such as The International Center for Living Aquatic Resource Management (ICLARM) jumped in the game.  They founded the Coastal Aquaculture Center in the Solomon Islands and began producing clams and other marine organisms as well. Now, several species of clams are bred in captivity and available to hobbyists.
 Phylum:      Mollusca
 Class:         Bivalvia
 Order:         Veneroidea
 Family:       Cardiacea
 Subfamily:  Tridacnidae
 Genus:        Tridacna & Hippopus
                    1. Tridacna crocea
                    2. Tridacna derasa
                    3. Tridacna gigas
                    4. Tridacna maxima
                   5. Tridacna squamosa
                   6. Tridacna rosewateri
                   7. Tridacna tevoroa
                   8. Hippopus hippopus
                   9. Hippopus porcellanus

Of the nine species known, only the following six are commonly found in the hobby:

Tridacna crocea

Tridacna Crocea, which grows only to about 6”, is the smallest of the Tridacna species. These clams are found in colonies and live in shallower waters where more light is present. T. crocea is also noted as the “boring clam” because it can be found burrowed into rocks and coral heads.  The shell is relatively smooth with small furrows. This particular species has a relatively large byssus opening. This larger opening makes this species of Tridacna a little harder to keep due to the susceptibility of predation. It is also more light demanding than other species. The distribution of these clams ranges from Thailand to New Caledonia.

Tridacna maxima

T. Maxima can reach about the same size as T. squamosa, but is typically smaller. T. maxima is fairly easy to keep. In comparison to T. squamosa, the shell of T. maxima is asymmetrical with closer rows of scales and has a smaller hinge. T. maxima can be found from East Africa to Polynesia.

Tridacna squamosa

T. Squamosa can grow up to 16″ and are fairly easy to keep. The shells are very distinct in that the have rows of scales. The byssal opening of T. squamosa is fairly wide, but not like that of T. crocea.  These clams can live at depths of up to 18 meters and are found from East Africa through Polynesia.

Tridacna derasa

T. derasa is the second largest clam and can grow to about 24″. These are one of the easiest clams to keep. They can be collected in waters as deep as 20 meters and are commonly found in Australia, Philippines, and Indonesia.

Tridacna gigas

The giant of all clams, this species can grow up to one meter. Like T. derasa, this species is easy to keep. It can also be easily misidentified as a T.derasa. However, T. derasa has six to seven vertical folds where as T. gigas usually has four to five vertical folds. T. gigascan be found at depths of up to 20 meters. T. gigas can be found in the Indo-Pacific, but due to it’s overharvesting, this species is becoming endangered.

Hippopus hippopus

H. hippopus can reach 16″. The one distinguishing characteristic about this clam is that the mantle does not overhang the shell. The clams are relatively easy to keep. It is found in the Indo-Pacific region.

Positioning Your Clam

• Be sure not to place your clam close to any aggressive corals.
• Place your clam in an area of good light and low current. Too much current will cause your clam not to open. Light is very important to these animals. It is best to provide them with metal halide, PC, or VHO lighting.  Juvenile clams adapt to lighting variables more readily than adult clams.
• T. crocea and T. maxima are found in rocky habitats, so it is best to place them on rocks. Be sure not to place them in an area where they cannot fully open. T. squamosa, T. derasa, and T. gigas are best placed on a sandy substrate
• It is very important to place the clam on its byssus orifice and in the upright position. Failure to do so can cause death of the clam. If a clam falls over, re-position it as soon as possible. A clam can easily suffocate itself if not in the proper position. The byssus gland is a very important part of the clam. The gland secretes threads, which help the clam to position itself and to keep it from falling off of rocks. It also allows the clam to attach itself tightly to the substrate to prevent predators from attacking the clam. T. crocea, T. maxima, and T. squamosa use bysuss threads through out most of their life, others use them as juveniles. If you remove a clam from the substrate that is attached by its bysuss thread, it is important that you cut the thread and not pull the clam. Failure to do so can cause damage of the bysuss gland and cause death to the clam.

Water Quality

Water quality is important to clams. High pH and high temperature can be problematic.  Do not let the aquarium exceed 82 degrees or a pH beyond 8.3. Maintain a calcium level of at least 400ppm and dkh of 7-9. Salinity is also important, too high or low a salinity can cause the demise of a clam. Try to keep specific gravity between 1.023 -1.025.
The number one cause of a clam’s demise is usually water quality. Signs of an unhealthy clam include gaping (inhalant siphon remains wide open), listlessness (does not respond to shadows), or if the mantle does not fully extend beyond the shell (except in H. hippoppus). If your clam exhibits any of these symptoms, be sure to check your water quality first.

Clam Diseases and Predators

Damage to the bysuss gland can be a problem. Besides mishandling of the clam, predators can also attack the gland and cause a quick demise. If this is the case, there is not much you can do to help the clam. Predators of clams include certain wrasses, pygmy angels, shrimp, crabs, caulerpa, and crabs. Check your clam for parasitic snail, so which can burrow a hole through the shell and attack the clam. Also, be sure not to place the clam too close to any aggressive corals. Some corals can sting the clam, which will keep the mantle from fully expanding.  Air bubbles can be a problem too. They can become trapped inside the clam and cause the clams demise.

Purchasing a Clam

There are several things to look for when purchasing a clam. First, be sure that the inhalant siphon is closed and that the clam is not showing any signs of “gaping.” The clam should be responsive to changes in light. Clams are photosensitive and will close when shadows occur over the clam. If the clam is listless and does not respond to shadows, it is usually a sign that there is something wrong with the clam. Also be sure to check the bysuss gland for damage. There should be no fleshy tissue hanging from the opening.
If you take all these factors into consideration before purchasing a clam, you will have much success in keeping them alive.