Basic Anatomy Tridacnid clams have shells consisting of two halves, or valves, which are joined at the top. Muscles and ligaments on each side of the shell hold the shell closed and pull the shell open. Species of tridacnid clams can be easily identified by the shape, symmetry and texture of their shells. All clams have a tissue structure called a mantle. One function of the mantle is to increase the surface area of the clam so that it receives maximum exposure to the light. It also precipitates calcium carbonate which forms the clam's shell. The mantle contains symbiotic algae (zooxanthellae) and light sensory organs which contribute to a clam's coloration and pattern. The zooxanthellae produce photosynthesis-derived food for its host while the light sensory organs serve to position the clam for maximum light exposure, to protect the clam against excessive light and UV radiation and to warn the clam of potential predators. The mantle is whole except for where it is broken by two siphons; the inhalant siphon, which is often fringed with fine tentacles that strain larger particles out of the water, and the exhalent siphon which is tube like. Photo of T. crocea showing the mantle, the inhalant siphon, the exhalent siphon and the gills (photo courtesy of A. Carriglio) Clams are filter feeders. Water and food are drawn in through the inhalant siphon to the gills where food is caught. The gills also draw oxygen from the water. From the gills, the food is transported along a groove to an organ which pushes it into the clam's mouth. The exhalent siphon carries away the water. Clams get some of their food from filtering the water, however, they get a lot more of their nourishment from their symbiotic algae. The byssal gland of a clam secretes filaments called byssal threads which hold the clam to the substrate. Because the larger clams, such as T. gigas, T. derasa, and H. hippopus get so large, they will eventually lose the gland and rely on their size and weight to hold them in place. T maxima and T. crocea are rock boring clams, therefore, their byssal glands excrete a substance that dissolves the substrate to which they are attached which helps them to burrow into it. Lighting requirements Some tridacnid clams require moderate to high lighting and some require intense lighting. Those that require moderate to high lighting are T. squamosa, T. derasa, H hippopus and T. gigas. Those that require intense lighting are T. maxima and T. crocea. If lighting in the aquarium is not sufficient, the clam may display higher than normal mantle extension. This usually means that the clam is trying to extend its mantle for maximum light exposure because of insufficient lighting. A Guide to Buying and Caring for Tridacnid Clams Make sure that you have enough light and space in your aquarium to support the clam long term. Only purchase clams that respond quickly to shadow, touch or other stimulus by closing its shell. The mantle should not have any tears or other damage to it. The mantle should extend past the edge of the shell (H. hippopus being the exception). Check for signs of gaping (shell is open wider than what is considered normal, little or no mantle extension, intake siphon open wider than what is considered normal, slow closing response to stimulus). A gaping clam should not be purchased. You should not see any torn or loose tissue hanging from the bottom of the clam. Some byssal threads may be visible, but no solid tissue should be hanging. Place clams on the appropriate substrate and away from any corals or other inverts that may sting it. Do not place the clam where it is constantly blasted by strong and direct water currents. Because the exhalent siphons of a clam are capable of quickly expelling large amounts of water, you'll want keep this in mind if you decide to place your clam high up in the aquarium, close to your lighting system. If you need to move a clam to another location in your aquarium, it is very important that you not tug at or pull on the clam if it is attached to the substrate. It's best to cut the byssal threads as close to the substrate as possible with a razor blade until the clam is no longer attached and then relocate it. One way to avoid having the clam attach to something that cannot be moved or attach in a place where you cannot reach the byssal threads, is to place a flat piece of live rock or a shell underneath the clam. Once the clam attaches itself to this piece of rock or shell, it can be easily moved. The rock/shell also prevents predation of the byssal muscle from worms and other sandbed fauna. Certain angelfish, triggerfish, puffers, butterfly fish, wrasses and other fish have been know to eat clams as do certain crabs and shrimp. Therefore, spontaneous fish or invert purchases should be avoided if you are keeping clams or plan to keep them in the future. Many clams can harbor parasitic boring (pyramid) snails which can wreak havoc in a tank full of tridacnid clams. Look for rice grain-sized, cream colored spots near the base or in the scutes of the clam, or, at night, along the upper edge of the shell. If the clam is attached to a rock, check by lifting the clam a short distance off the rock and look underneath. You are looking for small (0.08-0.2 inch / 2-5 mm long) snails. Remove all of these snails. If you have a quarantine tank, quarantine the clam until you are sure that all of the snails are removed. Also check for small, jelly like egg masses and remove them as well. Don't confuse the jelly-like mass some clams excrete around their byssus opening for these egg masses. When introducing the clam to your aquarium, it's common for your fish and shrimp to check it out for any desirable food that may have come in on the clam's shell. This should only be a concern if the animal begins to bite and/or tear at the mantle. The number one cause of a clam's demise is usually water quality. Calcium is the main building block for clams and should be present in the water at levels of at least 280 mg/L for growth to occur. More rapid, natural growth is seen when calcium is in the range of 400-480 mg/L. Strontium is incorporated in the shell along with calcium and should also be provided for optimum growth. The addition of iodide to the aquarium will also enhance growth and color in clams. High pH and temperatures can cause problems. Do not let the aquarium exceed 82 degrees or a pH above 8.3. Maintain a dkh of 7.9. Too high or low a salinity can cause the death of a clam. Try to keep specific gravity between 1.023 and 1.025. Photo of T. Gigas showing well defined ribs (notice lack of scutes). (Photo courtesy of A. Carriglio) Photo of T. maxima showing well defined ribs and tightly spaced, well defined scutes. (Photo courtesy of JP Dias at www.justphish.com) Photo of H. Hippopus showing distinct asymmetrical shell and well defined ribs. (notice lack of scutes) (photo courtesy of JP Dias at www.justphish.com) Photo of T. crocea showing well defined ribs and tightly spaced, less noticeable scutes. (photo courtesy of JP Dias at www.justphish.com) References Identifying the Giant Clams, James W. Fatherree, M. Sc., Reefkeeping On-line Magazine. Identifying The Tridacnid Clams - by James Fatherree - Reefkeeping.com Fish & Chips, A Monthly Marine Newsletter, Elizabeth M. Lukan 8/17/99. Fish & Chips August 1999 Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams, Daniel Knop, Ricordea Publishing, July 1996.