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Giant Clams Provide Beauty and Benefits
Veterinary & Aquatic Services Department, Drs. Foster & Smith
Tridacna Clams
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T. squamosa clamGiant clams have been a focus of human interest for many thousands of years. They are harvested for the food industry, their shells used for decoration and jewelry, and within recent years have been collected for the aquarium industry.

Unfortunately, the demand for these wonderful invertebrates has decreased their numbers in the wild to the point of some species being placed on the International Union for the Conservation of Nature's threatened species list. In response, many aquaculture facilities have come into existence throughout the Indo Pacific, and have been able to produce enough clams to meet the demands.

The clams of interest for the aquarium industry are within the Tridacna genus. These beautiful clams include: T. maxima, T. crocea, T. squamosa and T. derasa. The variety of colors and patterns within these species is amazing and oddly artistic in some specimens. Among the species mentioned, the T. derasa is the hardiest and fastest growing clam when given the right conditions.

Clams benefit an aquarium

Besides their beauty, clams offer one important benefit to a saltwater aquarium: Their ability to filter nutrients from the water. Nutrients in a closed system, like an aquarium, often build to the point where they become problematic. Excess nutrients, like nitrate, lead to problems with low pH, algae and cyanobacteria blooms, and ultimately, poor health with the livestock in the system.

T. maxima clamThe colorful, fleshy part of Tridacna clams that is displayed when the shell is open is called the 'syphonal mantle.' This mantle contains both an incurrent (intake) and an expelling orifice (opening). The clams use these openings to move water through their internal organs, where nutrients and plankton are filtered out and consumed by the clam.

Tridacna clams not only use nutrients from the water to satisfy their nutritional needs, but also employ algae cells called zooxanthellae within their mantle. These algae cells use nutrients from both the clam and the water, and along with light, they photosynthesize. The energy that these cells produce through photosynthesis is then returned back to the clam which uses this energy source to aid in its growth.

The nutrients that clams remove from the water are mainly ammonia and nitrate. When a clam feeds upon ammonia, it eliminates this nutrient from the system before it is processed by the biological filtration. The biological filtration is what facilitates the nitrogen cycle, and by not allowing ammonia to enter into this cycle, the end product nitrate will not be formed. This ultimately reduces the biological load and lowers the overall nitrate level in the aquarium.

By incorporating these beautiful, fast growing clams into your saltwater aquarium, you will increase the biodiversity of that system, thereby creating a more stable environment.

Environmental requirements

T. derasa clamLighting: The T. derasa clams are the hardiest of the Tridacna clams, and will adjust to a wide variety of conditions within the aquarium. The most critical parameter that is essential to their well being is the level of light. They require a moderate to high level of lighting that can be supplied by power compact fluorescents up to metal halides. Simply place the clam higher in the aquarium if using fluorescents. If a more powerful lighting system like metal halides are used on an aquarium of 24 inches or less, these clams can be placed just about anywhere in the aquarium, as long as they are not blocked from the light.

Water Movement: Like all Tridacna clams, the Derasa clams do not like strong direct water currents. This kind of water movement seems to inhibit the clam's ability to adjust the amount of water passing through it. Be sure when placing the clam, that it is in a location where the water movement is moderate, and indirect.

Placement: The clam's placement within the aquarium is the last part of caring for Tridacna clams. After evaluating your lighting system and water movement within the aquarium, and have chosen a location for your clam, place the clam so that the mantle is facing upward towards the light. This is important, because the clam obtains a majority of its nutrition from photosynthesis and ultimately from the light. Do not place the clam in a crevice where it may have difficulty fully opening its shell. Make sure the location is sturdy and offers enough room for the clam to grow.

Clams have the ability to open and close their shells very rapidly in order to move their position. They will do this when they do not like their position. If at any time the clam moves and is laying on its side, right the clam back to the correct position. Do this immediately because the mantle must be in full illumination in order to provide the clam with nutrition. Laying on its side, the clam will slowly starve to death.

Tridacna clams are not only some of the most beautiful additions to a reef aquarium, but because of their superior filtering abilities, they offer an added stability and nutrient control to the system. T. derasa clams are one of the best choices out of this genus. They are aquacultured, hardy, fast growing, and adapt to a wide variety of environments. These clams are a great choice for not only the beginner reef aquarist, but also offer diversity, beauty, and rapid growth for even the most advanced reef aquariums.

Trace Elements: Giant clams use calcium and strontium in order to build their shells, and these elements should be present in the aquarium at or near those levels found in nature.

  • Calcium, which should be added to all reef aquariums in order to maintain a high pH, should be maintained between 380 and 420 ppm, with a high stable pH of 8.3 to 8.5.

  • Strontium is typically replaced fast enough during routine water changes, but may need to be supplemented in an aquarium containing a large number of hard corals. This level is more difficult to test for, but should be maintained in the range of 6 to 10 ppm. Although slightly higher levels of strontium have proven not to be harmful to most invertebrates, it can be a source for unsightly cyanobacteria outbreaks.

  • Iodine is the last major trace element utilized by giant clams. Iodine aids these invertebrates in their ability to break down the oxygen that is produced by the zooxanthellae cells, and ultimately helps the clam maintain their bright coloration. In many systems, regular water changes and feedings are sufficient to replenish the iodine levels. However, aquariums with large numbers of soft corals or if aggressive chemical filtration is used, may need iodine supplementation. If the clam shows poor coloration, and the soft corals within the system show poor expansion, test the iodine level and use a supplement to maintain this level in the range of .04 and .08 ppm.

References and Further Reading

Delbeek, JC; Sprung, J. The Reef Aquarium Vol. 1. Ricordea Publishing. Coconut Grove, FL; 1994.

Knop, D. Giant Clams. Dahne Verlag GmbH, Postfach 250, D-76256 Ettlingen, 1996. Translated from German by Dr. Eva Hert and Dr. Sebastian Holzberg.

Tullock, J. Natural Reef Aquariums. T.F.H. Publications, Inc. Neptune City, NJ; 2001.

Tridacna Clams
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