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Marine Invertebrate of the Month
Issue: May 2009

Tridacna maxima

(RÖDING 1798)

Tridacna Maxima 1 (BG)

Bob Goemans

Common Names: Maxima clam, Rugosa clam

Phylum: Mollusca

Class: Bivalvia

Order: Veneroida

Family: Tridacnoidea

Range: Tropical Western Pacific Ocean, and Red Sea to Eastern Africa

Natural Environment: Inhabits the shallow coral reef flat.

Captive Care:

The giant clams (bivalves) of real interest fall into two genera—Tridacna and Hippopus. Of the eight well-known species of giant clams, Tridacna maxima, T. derasa, T. gigas, T. crocea, T. squamosa, and Hippopus hippopus are the most widely kept in aquaria. Tridacna mbalavuana and Hippopus porcellanus are rarely seen in the trade. As for a possible ninth species, Tridacna rosewateri, it may not be an actual separate species, but rather an oddly shaped T. maxima.

Size ranges from 6 inches for T. crocea and 12 to 16 inches inches for T. maxima, to 4 to 5 feet for T. gigas, which has been recorded at lengths up to 53 inches! The more colorful clams, such as T. maxima and T. crocea, are naturally found high up on reef areas where there is intense light and excellent water movement. They have highly pigmented mantles that shield them against damaging UV radiation, with the protective blue, green, yellow, and brown hues of their mantles indicating that they are shallower-water species. The olive-green mantle of T. gigas and H. hippopus indicate a deeper-water species.

In general, two shells form their enclosure and are hinged and held together by a powerful muscle, and its mantle edges have primitive eye spots that can detect changes in light intensity, such as when possible predators come their way. The mantle also has two openings—one is an inlet siphon, and the other is an outlet siphon. Water and some waterborne nutrients enter the clam through the inlet siphon, which is a slit-like opening in the mantle. Excess water and wastes are expelled through the tube-like outlet siphon.

These photosynthetic animals are also filter-feeders. Strong water movement brings suspended particles and nutrients, and at the same time its zooxanthellae utilize a portion of the clam’s own waste products for food. In return, the algae’s waste products provide a natural food source, e.g., carbohydrates/glycerol. In fact, these clams can meet about 90 percent of their own food requirements from photosynthesis, and when excess algae cells exist, they too can serve as a food supply. Therefore, adequate lighting, especially for the shallow water species (e.g., about six to eight hours per day) is extremely important for the health of these miniature algae scrubbers. Excellent lighting, good water movement, properly dosed calcium, strontium, and iodine additions will provide long-term success.

Keep in mind these clams attach themselves firmly to a substrate surface with special byssal threads, generally referred to as a foot. If not sure of the placement of a new clam, always place it on some loose gravel, possibly in a small gravel-filled dish, until the permanent location is decided. In fact, juvenile clams generally require less light than adults because they have thinner tissue, so the dish method is a better way to maintain juveniles at a lower light intensity until they are ready for permanent placement at brighter locations. And once attached to a rock, never simply pull the clam off. To remove, use a credit card or razorblade to cut the byssal threads as close to the attachment surface as possible.

There’s a small, white, cone-shaped snail Pyramidellidae sp. that feeds upon these clams and can multiply quickly at a very young age. In large numbers, they can be a serious threat to any clams in the aquarium. They can typically be found along the clam’s upper edge where the mantle overhangs, especially during evening hours. During the daytime, they may be located at the foot area or tucked away along the ridges on the shell. There is also another predatory snail, Cymatium muricinum, which clam keepers should be aware of. It enters the clam through the byssal opening, and after consuming the juices inside the clam and killing it, moves on to other clams. As for other clam predators, be forewarned—bristleworms, starfish, and crabs are also a danger!

The very popular Tridacna maxima and T. crocea are mostly very colorful and fairly easy to maintain, and they have become widely admired by reef aquarists, yet remain somewhat expensive. However, Tridacna clams from various mariculture farms are finding their way into the aquarium market, and prices for these beauties are becoming quite reasonable. For more information about these beautiful animals, see Giant Clams in the Sea and the Aquarium by James Fatherree (Liquid Medium, 2006).

Water Requirements: Calcium 380 to 430 ppm, alkalinity 2.5 to 3.0 meq/l, pH 8.1 to 8.2, nitrate 10 to 25 ppm, specific gravity 1.024 to 1.026, strontium 1.0 ppm, phosphate <0.015 ppm, magnesium 1290 ppm, and a temperature range of 75° to 83°F (24° to 28°C). Iodine additives—

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