Tridacna Clam Care Guide: Derasa, Squamosa and Maxima
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The short answer: Tridacna clams are photosynthetic animals for an established, stable reef tank; derasa and squamosa are the hardier choices, and maxima needs more light. All of them want strong reef lighting, moderate flow and steady alkalinity and calcium, because they build their shells from them. Place a clam on the sand bed or a stable, flat surface where it cannot fall, never balanced on rockwork. Clams get most of their food from light, and every new clam should be checked for pyramidal snails, a common pest that feeds on them.
Quick facts
Derasa (Tridacna derasa), squamosa (Tridacna squamosa) and maxima (Tridacna maxima, including clams sold as Red Sea maxima) are the giant clams most often kept by reefers. Care advice for clams varies, so the table below lists conservative, widely agreed general husbandry points.
| Derasa and squamosa | Maxima | |
|---|---|---|
| Care level | Intermediate; among the hardier giant clams | Intermediate to advanced; more light-demanding |
| Lighting | High | High to very high |
| PAR guidance | No single agreed number; strong reef lighting | No single agreed number; the strongest light in the tank |
| Flow | Moderate | Moderate |
| Placement | Sand bed, or a stable flat surface on the sand bed; never on rock where it can fall | Sand bed or a secure, flat surface in strong light; never on rock where it can fall |
| Behavior | Anchors with byssal threads (large derasa often rest on their weight instead); mantle retracts when a shadow passes | Attaches with byssal threads; mantle retracts when a shadow passes |
| Compatibility | Peaceful; keep away from coral stings; prey for pyramidal snails | Peaceful; keep away from coral stings; prey for pyramidal snails |
| Feeding | Mostly photosynthetic; small clams may benefit from fine phytoplankton | Mostly photosynthetic; small clams may benefit from fine phytoplankton |
Light and placement
Add a clam only to an established tank with stable parameters. Giant clams host symbiotic algae in the colorful mantle and depend on strong light to feed them. Derasa and squamosa tolerate somewhat lower placement than maxima, which most reefers keep in the strongest light the tank offers.
Placement matters as much as light. A clam should sit upright, hinge side down, with the opening in the shell near the hinge (where the byssal threads come out) resting against the surface. The safest spots are the sand bed or a flat, stable piece of rock or shell set on the sand bed. Do not balance a clam on rockwork: a clam that falls can be injured, land upside down or end up against corals. A clam set on sand may shift at first; smaller clams then anchor themselves with byssal threads; squamosa usually stays attached as it grows, while larger derasa often rely mostly on their weight.
Acclimate gradually to stronger light. A common approach is to start a new clam lower in the tank or under reduced intensity, then raise it or increase intensity over days to weeks once the mantle extends fully each day. Once a clam has attached and is doing well, avoid moving it; pulling a clam free can tear its byssal threads and the tissue behind them. If it must be moved, cut the threads carefully rather than pulling.
Flow
Clams do best in moderate flow: enough to keep detritus from settling on the mantle and around the shell, without blasting the mantle so hard that it stays retracted. Indirect, varied flow works better than a single strong jet aimed at the clam.
Feeding
Tridacna clams are mostly photosynthetic and get most of their energy from light. They also filter fine particles from the water. Smaller clams are commonly said to benefit more from filter feeding, so some reefers add fine phytoplankton to the tank. A healthy adult clam in a stocked reef usually does not need target feeding. Do not overfeed; excess food raises nutrients and feeds nuisance algae.
Water parameters
Clams build their shells from calcium and carbonate, so steady alkalinity, calcium and magnesium matter. Keep general reef ranges:
- Temperature: about 76–80°F
- Salinity: about 1.025–1.026 SG
- Alkalinity: about 7–11 dKH
- Calcium: about 400–450 ppm
- Magnesium: about 1250–1400 ppm
- Nitrate and phosphate: low but not zero
Stability matters more than a perfect number. Avoid fast swings in alkalinity or salinity, and never use copper-based medications in a tank with clams or other invertebrates.
Neighbors and spacing
- Corals: clams do not sting, but corals sting clams. Keep sweeper tentacles, wandering anemones and fast-growing corals from reaching the mantle, and leave room for the clam to open fully.
- Fish: many angelfish and butterflyfish are known to pick at clam mantles. Research any fish before adding it to a tank with clams.
- Pyramidal snails: these small, white to cream snails feed on clams, usually at night, around the edge of the shell and the byssal opening. They are one of the most common clam pests.
- Shading: a coral that grows over a clam can block its light. Keep the area above the clam open.
Common problems
- Tiny white snails on the shell, or small jelly-like egg masses. Usual cause: pyramidal snails. Reefers inspect new clams before adding them, check the shell edge and underside with a flashlight after lights out, and remove snails and egg masses by hand with tweezers or a soft brush. Repeat checks are needed, because eggs keep hatching.
- Mantle stays partly retracted. Usual cause: a recent move, too much flow, pests, fish picking, or a change in light or water chemistry. Reefers check for snails and nipping fish, test parameters, and keep the clam in one spot.
- Mantle turns pale or white. Usual cause: bleaching from light shock, temperature swings or poor water quality. Reefers stabilize conditions and avoid sudden jumps in light intensity.
- Mantle looks pinched or shriveled at the edges. This is often called pinched mantle. The cause is not well understood; reefers isolate the clam if possible, look for pests, and check water quality.
- Shell gaping open with no reaction to shadow or touch. Usual cause: a clam that is dying or dead. Remove it promptly, because a dead clam can foul the water quickly, and test water.
- Clam falls or tips over. Usual cause: placement on rockwork or an uneven surface. Set it upright on the sand bed or a flat, stable surface.
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FAQ
What is the easiest giant clam for a reef tank?
Derasa and squamosa are widely considered the hardier giant clams. Maxima needs stronger light and is usually recommended for more experienced reefers.
Where should I put a clam in my tank?
On the sand bed or a flat, stable surface where it cannot fall, upright with the hinge side down, under strong light. Do not balance it on rockwork.
Do clams need to be fed?
Mostly no. They are photosynthetic. Small clams may benefit from fine phytoplankton added to the tank.
How do I check a clam for pyramidal snails?
After lights out, look closely at the shell edge, the underside and the byssal opening with a flashlight. Remove any small white snails and jelly-like egg masses by hand, and repeat over the following weeks.
Why does my clam close when I walk by?
Clams react to shadows by pulling in the mantle. A quick reaction is a normal, healthy response. A clam that stops reacting needs a closer look.
Can I keep a clam with fish and corals?
Yes, in many reef tanks. Avoid fish known to pick at clams, such as many angelfish and butterflyfish, and keep corals from stinging or shading the clam.