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Derasa Clam ~ Tridacna derasaSmooth Giant Clam ~ Southern Giant Clam

Family: Cardiidae

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Tridacna derasaPhoto © Animal-World

     The Derasa Clam is one of the hardiest, most widely available giant clam for the aquarium!

     The Derasa Clam Tridacna derasa, along with its cousin the Gigas Clam Tridacna gigas, was one of the of the first aquacultured species of Tridacna clams. A popular food item, these clams have been hunted extensively throughout their natural habitats. Today both of these giant clams are cultivated in captivity. They are supplied as a food source, but are also readily available to the aquarist.

     The Derasa Clam has proven to be one of the hardiest giant clams for the aquarium. T. derasa's are also quite attractive. They usually have a mantle with a striped pattern of wavy lines or a spotted pattern. They sport various color combinations of orange, yellow, black, blue, and white, some can even have brilliant blue or green accents.

     Another claim to fame of the Derasa Clam, again shared with the Gigas Clam, is that it is one of the largest of the giant clams. The Derasa Clam grows fast and can double or even triple in size in a year. They will generally reach up to about 20" in length, though specimens have reportedly reached up to 24". The adult Gigas Clam is much larger, reaching over 3 feet in length.

Scientific name
   Tridacna derasa

Distribution / Background

     The Derasa Clam, also known as the Southern Giant Clam and the Smooth Giant Clam, has a broad range in the Indo-West Pacific, commonly found in Australia, the Philippines, and Indonesia. Tridacna derasa was first described by Röding in 1798.
     T. derasa's are found on the outer edges of the reef in waters ranging from 12 to 33 feet (4 to 10 meters). (One reference claimed finding these clams in waters as deep as 65.6 feet (20 meters).) Since T. derasa's lose their byssus glands early in their lives, they can often be found lying free on the substrate in lagoons. In protected areas (the Great Barrier Reef in Australia for example) they are sometimes found in densities of up to 30 clams a hectare (2.47 acres).

Status

     The Tridacna derasa is listed on the IUCN Red List as VU - Vulnerable. They are listed as threatened by the International Union for the Conservation of Nature.

Description

     The Derasa clams can grow to a maximum length of 20 inches (50 cm), although I did see one notation of 24 inches (60.96 cm). The Derasa Clams are also known by a couple of other common names, the Southern Giant Clam and the Smooth Giant Clam. The name Smooth Giant Clam resulted because of its thick smooth textured shell. This clam's shell has six to seven vertical folds, but lacks much in the way of ribbing or scales. Derasa's may develop scutes when grown in the aquarium. This may be a result of the artificial light and its effect on the expansion of the mantle. Some specimens are said to form scutes as a result of a genetic trait.
     Derasa Clams usually have a striped pattern of wavy lines or a spotted pattern. Various color combinations of orange, yellow, black, blue, and white can be found. Some can even have brilliant blue or green lines. T. derasa's seen in their natural habitat on the Great Barrier Reef in Australia are usually a vivid blue color. In other locations, their colors are shades of golden brown and green with a blue margin on the mantle.

     Some characteristics of Tridacna derasa are:

• Their shells are heavy and very plain.
• May not have scutes, or they may be tiny or sparser but larger.
• The inhalant siphon has clearly visible tentacles.
• Small, narrow byssus gland opening.
• Loses its byssus gland as it grows.
• The hinge is usually longer than half the shell's length.

     Comparing Derasa Clams to other species of Giant Clams:

     The Derasa Clam T. derasa is sometimes confused with the Gigas Clam T. gigas. Although as an adult the Gigas Clam reaches a much larger overall length than the Derasa, there are some other ways to tell the difference between these two:

• The Gigas Clam has a different mantle coloration.
• T. derasa's mantle extends further over the shell.
• The top edges of T. gigas' shell have triangle shaped projections which extend inwards.

Difficulty of Care

     The Derasa Clam is not only attractive, but can be a good choice for a beginning saltwater enthusiast. They are the most widely available and hardy of the Tridacna clams. They do need sediment-free water, so will need good filtration and do best if not kept with tank mates who need frequent feedings as this can pollute the water.

Foods / Feeding

     It is generally believed that giant clams do not require feeding in the aquarium. Most clams fulfill their nutritional requirements by filter feeding and absorbing dissolved organic compounds from the water. The Tridacna clams have gone even further than this, using zooxanthellae to manufacture food for themselves.
     The Tridacna clams receive the majority of their nutrition from their zooxanthellae. Whether additional feeding is required is still debated. Some enthusiasts believe they should be fed, going on the assumption that they are filter feeders like other clams. If you wish to feed your clam, it is suggested that they be fed micro-foods designed for filter feeders, especially when small. A yeast-based suspension is one suggestion, unless the tank has other fish and corals that are regularly being fed, or you can offer other micro-foods such as phytoplankton or commercially prepared micro-foods like 'marine snow' or 'reef snow'.
     Basic nutrients in the aquarium that these giant clams need are calcium, strontium, iodine, magnesium, and possibly a minute amount nitrate.

• Calcium: 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: Strontium is incorporated in the shell along with calcium and should also be provided for optimum growth.
• Iodine: The addition of iodine to the aquarium will also enhance growth and color in giant clams.
• Magnesium: Magnesium aids in maintaining proper calcium levels and in the formation of skeletal material in clams
• Nitrate: They require some nitrogen for proper growth. Nitrate can be added if levels are extremely low, but be careful as nitrates should never exceed 2 mg/L.

     For more information about the feeding process of Derasa Clams see What Do Clams Eat.

Aquarium Care

     If a healthy clam is obtained, these clams are hardy in captivity and relatively easy to keep. With proper lighting, good filtration, and careful attention the Derasa Clams require require little else in the way of care. It is important to make sure they are not being irritated, not being fed upon by other organisms, and good water quality must be maintained.

• If you have a healthy specimen, it will generally attach itself to the substrate in less than a day. Keep this in mind when placing your clam in the tank. Make sure you put it where you want it to stay.
• Keep a watch for predators as this clam can be easy prey with its large and wide byssus gland opening.
• Derasa Clams can do well under a variety of lighting intensities and occasional stronger currents will be tolerated. They are intolerant of changing salinity, they need sediment-free water, and you really don't want your water parameters to fluctuate too much!

Aquarium Parameters

     Derasa Clams can be kept in a reef environment with live rock and you can place them almost anywhere in the tank. Keep fluctuations in water parameters to a minimum. They are not very tolerate of sediment or pollutants in the water, nor do they like changes in salinity. A high pH and high temperatures can cause problems.

• Lighting:  They will do well under a variety of lighting intensities. Of course, the more light you supply, the faster they will grow. It is not unheard of for a 2.5 inch (6 cm) clam to double or triple their size in less than a year as long as they are given plenty of calcium (more than 400 mg/L).
       However, some care should be taken with placement of the clam depending upon the clam's color. Clams showing the iridescent gold color will typically handle the higher light intensities.
       If the clam is not showing the gold pigmentation and the mantle is mostly brown, it is better to put the clam in the lower part of your tank. Definitely avoid placing these brown colored derasa's directly under strong metal halides.
• Temperature: Mid to Upper 70 ° F. (mid 20 ° C.). Do not let the aquarium exceed 84 ° F.
• Salinity / Specific Gravity: 1.024. Salinity is also important, too high or low a salinity can cause the death of a clam. Try to keep specific gravity between 1.023 and 1.025.
       Be careful when replacing evaporated water, derasa's do not like fluctuating salinity.
• pH: 8.3. Do not let the aquarium exceed a pH above 8.4.
• Hardness: Maintain a dkh of 7.9.
• Water Movement: For the Derasa Clam, occasional stronger currents will be tolerated.

     For more in depth information on caring for Derasa Clams see, Caring For Tridacnid Clams

Compatibility and Social Behaviors:

     Clams are very stationary and peaceful, they are not aggressive towards other aquarium inhabitants. In the wild Derasa Clams are often associated with hard corals and they may be kept in an aquarium with them.
     Although they have the ability to close their shell, they do need protection from anemones and some corals. They should not be kept near any stinging cell creatures and must be kept away from any sweeper tentacles. Anemones need to be watched, as they can move close to a clam and sting or eat it.
     Be cautious with other tank inhabitants as well, those that may pick at the clam or eat its mantle, such as Trigger Fish and Puffers. Blennies, Butterfly Fish, Clown Gobies, Angelfish and shrimp may disturb Tridacna clams.

Breeding and Reproduction

     Derasa Clams have been propagated in captivity. The demand from aquarists has raised interest in producing colorful varieties of all the tridacna clam species.

     For detailed information of tridacnid propogation, see Giant Clam Breeding and Reproduction

Potential Problems

     Keep a watch out for predators! The Derasa Clam can be easy prey with its large and wide byssus gland opening. Derasa Clams need sediment-free water, so will need good filtration and do best if not kept with tank mates who need frequent feedings as this can pollute the water. They are also quite sensitive to chemicals or toxic substances dissolved in the water, so be sure to maintain good water quality.
     As mentioned under aquarium care and aquarium parameters above, high pH, high salinity, and high temperatures can also cause problems.

     For in depth information on potential Tridacna clam problems, see: Tridacnid Clams: Friends, Enemies & Ailments

Availability

     Clams obtained from aquaculture systems are readily available and are very hardy in reef aquariums. The Derasa Clams you purchase today are the result of aquaculture projects, not wild collecting. This is because T. derasa's, along with T. gigas, were some of the first clams to be commercially bred.

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