Giant Clams and Other Shell Builders

A giant clam is a bivalve: an animal with two shells. Unlike other clams it sits with its shell opening upward and spreads a sheet of colored tissue, the mantle, into the light. The mantle is packed with the same kind of algae corals carry (Module 3), and those algae are the clam's main source of energy.

Two openings show in the mantle. Water is drawn in through one siphon, passes over the gills, and leaves through the other. Under the shell is the byssus, a bundle of threads the clam uses to anchor itself to rock. Giant clams range from a species that reaches about 15 cm to one that passes a meter.

Tridacna crocea is the smallest, at about 15 cm. It lives only in shallow water, bores into rock, and needs the most light. T. maxima reaches about 35 cm and also lives attached to rock. T. squamosa reaches about 41 cm and carries large leaf-like scales on its shell. T. derasa has a smooth shell, reaches about 60 cm, and is the hardiest and easiest to keep. T. gigas is the largest: its shell can pass 130 cm.

Know which species a clam is before it's bought. Its adult size decides whether a system can hold it (Module 24), and its species decides its light and where it's placed. Choose clams raised on farms where possible.

Light is a giant clam's main food, and too little of it's the most common reason clams fail. T. crocea and T. maxima need the most; T. derasa, T. squamosa and T. gigas a little less (Module 18). A clam sits upright with its mantle facing the light.

A clam is moved toward stronger light in steps, with a week or two between moves. A clam under too much light that can't move away may lie on its side to shade its mantle: lower it or shade it. A mantle turning pale or white is bleaching, and a bleached clam isn't bought.

A giant clam has four sources of food. Its algae make sugars, which supply carbon and energy. It filter feeds, catching phytoplankton and other fine particles: a giant clam holds on to most particles between 2 and 50 microns. It absorbs dissolved nutrients from the water, nitrogen as ammonia or nitrate and phosphorus as phosphate. And it digests some of its own algae.

The algae supply energy, not the nitrogen and phosphorus that new tissue is built from. Those come from filter feeding and from the water. Filter feeding matters most in small clams: in one study it supplied about two thirds of the carbon a small T. gigas needed and about a third in a large one, and it was needed as a source of nitrogen to keep young clams growing.

Feed giant clams phytoplankton (Module 26). In trials, young T. maxima fed a mix of two phytoplankton grew fastest and unfed clams grew slowest, and in a trial across four species fed clams grew more and fewer of them died. A clam can live in a well-stocked system without added food, on fish waste and what's in the water, but systems differ in how many fish they hold, and clams grow faster with phytoplankton. Add it in measured amounts: too much feeds nuisance algae, which can overgrow small clams. Food never replaces light.

A clam's shell is calcium carbonate, built from calcium and alkalinity in the water (Module 19). It thickens on the inside as well as growing at the edge, so more shell is being made than you can see. A large clam, or several small ones, can pull calcium and alkalinity down surprisingly fast, so both are tested often. A healthy clam adds at least a little new white shell at its edge every month.

Clams are kept in reef water: 26 to 27 C, steady salinity, and calcium and alkalinity in this course's ranges (Module 6). They want low to moderate flow. A strong current hitting the mantle keeps it from opening fully.

T. crocea and T. maxima go on rock, where they stay attached for life. T. squamosa, T. derasa and T. gigas can go on rock or on sand. On sand, put a flat rock just under the surface so the byssus has something to hold, and don't bury the clam: sand grains irritate it when it opens.

Use a flat or slightly bowl-shaped spot much wider than the clam, with room to grow. The clam sits upright with its mantle open to the light. Don't wedge it into a crevice where it can't open fully, and don't set it on a steep slope where it can slide into the rockwork. Keep it out of reach of stinging corals and anemones (Module 24). Then leave it: repeated moves stress a clam and slow its growth.

A newly placed clam attaches with its byssus within a few hours to a couple of weeks. Larger clams of some species stop using it, close the opening under the shell, and stay in place by their weight.

Never pull an attached clam off its rock. The threads are joined to an organ inside the clam, and tearing it out is often fatal. To move an attached clam, lift it together with the rock it's on. If it has to come off, cut the threads with a sharp blade as close to the rock as possible, very carefully, without touching the soft tissue. A clam that has picked up a piece of rock or gravel usually drops it later.

A healthy clam spreads its mantle fully, well past the shell edge, and pulls it in quickly when a shadow passes over it. New white shell shows at the edge. A slow reaction to a shadow is a warning.

A clam that stays closed for long periods is in trouble. Gaping is worse: the shell hangs open with the mantle pulled in or limp, and it often ends in death. A pale or white mantle is bleaching. Mantle edges that look pinched or curled mean something is irritating the clam, and the cause has to be found before anything is done (next section). Cloudy water with clams looking off, while everything else is fine, points to a spawning event (Module 27).

A clam whose mantle edges pull in, curl or look pinched is reacting to something. Work through the causes. Stress: was it moved, or did its light change? Stings: is a coral or anemone within reach? Water: has any number swung? Fish and pests: is something picking at the mantle, or are snails at the shell edge at night? Fix what you find, and the mantle usually relaxes.

If nothing is irritating the clam, disease is left. Pinched mantle disease starts as curling at one spot on the mantle edge, spreads along it, and ends with a clam that barely opens. It passes from clam to clam and is nearly always fatal within a week or two if it isn't treated. Perkinsus is another disease that can show this way (Module 30).

Pinched mantle disease is treated with a freshwater dip. Fill a clean container with RO water, bring it to the tank's temperature, raise its pH to match the tank's with a little buffer such as baking soda, and aerate it. Lift the clam with its rock, never by pulling. Hold it in the dip for about 30 minutes, moving it gently for the first few minutes so the water reaches between the mantle and the shell. Then return it. It stays closed at first and can take a day or two to open, and a few more days to look normal. Some clams need a second dip after a few days of rest, and other clams in the same system may need dipping too. A dip is hard on a clam and some don't survive it, so it's used when disease is the likely cause, not for every pinched mantle.

Pyramid snails are tiny, white and cone-shaped, a few millimeters long. They pierce the mantle and feed on the clam's fluids. A few do little harm, but they breed quickly, and many can kill a clam. The signs are a mantle that doesn't open fully and loses color. They feed at night and hide under the shell by day, so look with a flashlight a few hours after lights out: they line up at the shell edge under the mantle.

To remove them, lift the clam with its rock into a container of system water, pick the snails off, and brush the shell to remove their eggs, keeping the brush away from the soft tissue at the byssal opening. Check again over the following weeks. Some wrasses eat these snails, but they feed by day and miss the ones that are hidden. Every new clam is quarantined and its underside checked for snails and eggs (Module 23). Some fish nip clam mantles, and a clam that's nipped stays closed (Module 24).

Stony corals, clams, snails, coralline algae, some tube worms and urchins all build hard parts from calcium, alkalinity and magnesium taken out of the water (Module 3). They draw on one shared supply. Rock covered in coralline algae takes up alkalinity too, so demand is more than the corals and clams you can count.

When shell builders stop growing, their use of alkalinity slows, so a sudden drop in demand is itself a clue that something is wrong. Shrimp and crabs grow by shedding their shell: an empty, whole shell is probably a molt, not a dead animal.

Start with what the clam shows. No new shell at the edge points to water or food: calcium, alkalinity, or too little phytoplankton and too few nutrients. A mantle that won't open fully points to flow on the mantle, a fish nipping it, or snails. A clam on its side points to the light. A pale mantle points to light or a sudden change.

More than one is often true at once: the calcium has run down and snails are feeding at night; the clam was moved up too fast and a current is hitting its mantle. Check everything, then fix each cause.