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Tridacnid clams are often beautiful animals, the shells may have a pleasing architecture, and their exposed tissues often have pleasing patterns and bright colors. Given the appropriate conditions, they can be quite hardy. However, clamrades, unless they are well nourished with supplemental feedings they are unlikely to thrive in a captive system.

The Tridacnids are one of the relatively few types of clams that may be successfully kept by aquarists. This group, which consists of the species Hippopus hippopus and several species in the genus Tridacna, are maintained with moderate success by many aquarists. These species all contain zooxanthellae in their tissues and for good health need a lot of light. In the real world they are seldom found deeper than about 30 feet and the majority of individuals are much shallower, even living on the reef flats in only a few inches of water. Given that they are living in the equatorial tropics, they receive and need a VERY significant amount of sunlight.

The zooxanthellae are maintained in the clams' tissues and, when sufficiently illuminated and otherwise cared for by the clam, use the light impinging on them and the dissolved carbon dioxide in the clam's tissues to produce sugars and other photosynthetic byproducts. Aeons of natural selection have fined tuned this association significantly. The clams actually have clear lenses in the tissues of their mantles to focus sunlight onto clusters of zooxanthellae maintained deep in the clam's tissues.

The brightly-colored, intricately-patterned, exposed tissue, visible between the clam's shells, is the mantle. The clam's mantle has many functions; it secretes the shell, has sensory structures to allow the clam to sense the world around it, and it creates an internal hydrodynamic space for the water flow around and through the filter-feeding organs, the gills. The colored patterns seen on the shell may have some function, perhaps as sunscreen, but if so, that function remains obscure. Zooxanthellae are greenish or greenish-brown, and don't contribute much to the color patterns seen in the clams.

While the sugars that are by-products of their zooxanthellae provide them with some energy, all the tridacnids are suspension-feeding animals and get the raw materials they need for new tissues from the phytoplankton they eat. In fact, until they get about four inches long, there simply is not enough volume of the zooxanthellate tissues for them get all of their energy requirements from the photosynthetic sugars, and need to get a lot of their energy, as well as raw materials, from feeding. When they are grown in aquaculture situations, the sea water containing the clams is often enriched with both nitrate and phosphate based fertilizer. As a result of this good water and lots of sunlight, the clams grow very rapidly. In the average reef aquarium, phosphates and nitrates are minimized and the light really isn't that intense. In nature these clams seem to feed primarily on unicellular green algae and bacteria; consequently, in aquaria the clams benefit from frequent additions of phytoplankton. Most tanks with a good deep sand bed, also have a lot of bacteria and natural microplankton in the water as well. These will assist in clam growth. Without supplemental feeding, Tridacnid clams typically appear to live well for three to six months after which they rather suddenly die. It appears that they utilize all of their accumulate energy reserves and in effect, live on borrowed time. When the reserves run out, they don't get enough nutrients from the system that they are in and perish.

Photos courtesy of Reef Central members.
Text by Ronald L. Shimek, Ph.D.

Many thanks to John Love (Rock Anemone) for his assistance with this project.


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