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Giant Clam - Tridacna Gigas
Gigas Clam

Family: Cardiidae

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Tridacna gigasPhoto © Animal-World

     The Giant Clam, Tridacna Gigas, is the largest bivalve (mollusc) in the world!

     The Tridacna gigas clam is commonly called the Giant Clam or Gigas Clam. This impressive Tridacna clam species is popular as both a food source and a novelty. Its numbers in the wild have been greatly reduced because of over collection and hunting. The Giant Clam, along with its cousin the Derasa Clam Tridacna derasa, is one of the first aquacultured species of Tridacna clams. 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 Giant Clam, or Gigas Clam, has proven to be one of the hardiest giant clams for the aquarium. They are relatively easy to care for and can grow very rapidly when provided with adequate light and calcium levels. Giant Clams will need ample room in the aquarium, as true to their name they can can become very large, reaching over 3' in length.

     These are a very large, heavy bodied clams. They are also very colorful and attractive. They have thick smooth shells with 4 to 5 pronounced ribs. A juvenile Giant Clam may have some sparse scutes, but once an adult their shells are without scutes. The mantle is usually golden brown, yellow or green overall. It is decorated with many iridescent blue or green spots on its surface, especially around the edges. Larger T. gigas may have so many of these spots that the whole mantle appears blue or purple.

Scientific name    Tridacna gigas

Distribution / Background

     The Giant Clam, also known as the Gigas Clam, was first described by Linnaeus in 1758. It is native to the Indo-Pacific region, found from the Nicobar Islands in the west to Fiji in the east. There are doubts about T. gigas actually being found in Fiji. According to The Reef Aquarium Volume One, years of observations throughout Fiji have never turned up any living T. gigas or fossil shells. T. gigas has, however, been "re-introduced" there.
     Although the Giant Clams are usually found in shallow lagoons or on reef flats, they can be found as deep as 65 ½ feet (20 meters). T. gigas embeds itself into the substrate.
     Some Giant Clams in the wild are so over-grown with sponges, corals, and algae that their shape is unrecognizable. Also in the wild, they are also known to provide a protected living area within their cavity for a single pair of Xanthasia murigera, a small Pea Crab.
     It is interesting to learn that In past times, myths and fears about the idea of this unknow clam led it to be called the 'killer clam' or "man-eating clam'. Of course today these myths are absurd. A Giant Clam is totally non-aggressive, it is so heavy that it has to stay in one place, and it can not even completely close its shell when it gets large.

Status

     The Tridacna gigus 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 mantle of the Giant Clam is usually golden brown, yellow or green. The mantle will have many iridescent blue or green spots on its surface, especially around the edges. Larger T. gigas may have so many of these spots that the whole mantle appears blue or purple.
     T. gigas also usually have numerous pale or clear spots, mostly located near the center of the mantle. These clear spots, referred to as "windows", may serve to focus or allow more light through according to The Reef Aquarium Volume One.
     T. gigas have a maximum length of 4 feet (1.5m).

     Characteristics of the Giant Clam T. gigas:

• Adults have large, thick, heavy shells, without scutes.
• Juveniles may have sparse scutes.
• Juveniles may also have tubular projections near the umbo.
• When large specimens are viewed from above, the tops of the shells will have 4 to 5 large inward facing triangular projections.
• The inhalant siphon has no tentacles.
• As the clam grows, it will lose its byssus gland, relying on size and weight to hold it in place.

     Comparing the Giant Clams to other species of Tridacna clams:

     Young Giant Clams are sometimes confused with the Crocea Clam T. crocea or with the Derasa Clam T. derasa. To tell the difference:

• Use the mantle coloring of each species.
• A juvenile Giant Clam may have sparse scutes, but an adult does not.
• The ribs of the Giant Clam are more pronounced.
• (it should be noted that the triangular inward projections of the shell of the Giant Clam only become apparent as it becomes larger, not when small.)

     There have been possible hybrid crosses of T. gigas and T. derasa offered for sale. Characteristics and coloring of these hybrids has been divided between what is the norm for T. gigas and T. derasa. For example, a hybrid mentioned in The Reef Aquarium Volume One had tentacles on the inhalant siphon like the T. derasa. Also according to The Reef Aquarium Volume One, some of these hybrids have teardrop shaped clear windows along the edges of the mantle.

Difficulty of Care

     The Giant Clam is attractive, readily available for the aquarium, and is considered the most hardy of the Tridacna clams. It is, however, the fastest growing of the Tridacna clams and can easily outgrow the home aquarium. They don't like huge fluctuations in water parameters, need a low to moderate water flow, and are not tolerant of sudden increases in light intensity.

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 Giant Clams see What Do Clams Eat.

Aquarium Care

     If a healthy clam is obtained and proper light provided, these clams are quite hardy and relatively easy to keep. With proper lighting and careful attention, Giant Clams require require little else in the way of care. They are the fastest growing of the Tridacna Clams however, and they get very large, so adequate room must be provided for their growth. 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 grow very rapidly in the aquarium when provided with adequate light and calcium levels. Place it where you want it to stay as a several 100 pound clam will be difficult to move.
• Keep a watch for predators as this clam can be easy prey with its large and wide byssus gland opening.
• Giant Clams do well under moderate to relatively high lighting intensities. Provide a low to moderate water flow. As with all the Tridacna clams, you really don't want your water parameters to fluctuate too much!

Aquarium Parameters

     Giant Clams can be kept in a reef environment with live rock. Place them on the substrate in the bottom of the tank. Keep fluctuations in water parameters to a minimum and water flow low to moderate. These are the fastest growing of the Tridacna Clams, and will grow very rapidly when provided adequate light and calcium levels. A high pH and high temperatures can cause problems.

• Lighting:  Giant Clams will do well under moderate to relatively high lighting intensities. They don't particularly like intense lighting nor will they tolerate sudden increases in intensity. Take great care if attempting to acclimate them to intense lighting such as metal halides. T. gigas can be adapted to metal halide lighting, but this should be done over time as a gradual process..
• 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.
  
• 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 Giant Clam, low to moderate currents will be tolerated.

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

Compatibility and Social Behaviors:

     All the Tridacna clams are very stationary and peaceful, they are not aggressive towards other aquarium inhabitants. As the Giant Clam grows, it will lose its byssus gland. This is what other Tridacna clams rely on to anchor themselves in place, but the Giant Clam grows so large and heavy, it simply stays where is is put.
     They have the ability to close their shell as juveniles, but not as large adults. During all stages of growth 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

     Giant Clams, also know as Gigas 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 Giant Clam can be easy prey with its large and wide byssus gland opening. 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. Because of its use as a food item, the Giant Clam has been propagated for commercial uses for many years. Specimens have been introduced to the market for hobbyists. All specimens sold in the aquarium trade are aquacultured.

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