Derasa Clam

The derasa clam is distinguished from other Tridacna clams by its shell, which lacks scutes and is relatively smooth; this is why it is called the "smooth giant clam" in English. The shell is thick and heavy, with only low, broad ribs. Its mantle extends well beyond the shell and is often decorated with wavy blue, brown and gold lines; there are also intense blue forms from selected breeding lines.

It is considered the hardiest of the Tridacna clams under aquarium conditions, and almost all specimens on the market are farm-raised. This hardiness comes at a price: it is one of the largest Tridacna clams, reaching 50–60 cm in the wild, and it grows quickly under good conditions. A derasa can outgrow a small aquarium within a few years; this should be considered when buying one.

Legal note: All Tridacna species are listed in CITES Appendix II; their international trade requires a permit. Only documented, farm-raised specimens should be bought.

In the Western Pacific, from northern Australia and the Great Barrier Reef to the Philippines, Melanesia and western Polynesia, it lives on the open, sandy or rubble bottoms of outer reefs and lagoons. Adults do not attach to rock; they rest on the bottom under their own weight.

• Volume: Small specimens can start in 150–200 liters, but at least 300 liters should be planned for adult size.
• Water: Salinity specific gravity 1.024–1.026, pH 8.1–8.4, temperature 24–27°C; calcium 400–450 mg/L, alkalinity 7–9 dKH, magnesium 1250–1350 mg/L, all stable.
• Light: Moderate to strong lighting. It is placed on the sand; small specimens with thin shells can burn under sudden strong light, so they are first acclimated in a lower, shaded spot.
• Flow: Moderate, random flow.
• Reef compatibility: Reef-safe; as it grows it may shade the corals around it or be stung by them, so space should be left between them.
• Acclimation and medication: It is acclimated by the drip method for at least 1 hour and carried under water; copper-based medications are lethal.

It gets most of its nutrition from the symbiotic algae in its mantle; it also benefits from fine particles by filtering the water. Small specimens (under 10 cm) benefit from phytoplankton given a few times a week. Very nutrient-poor water slows growth in the long run.

Sedentary; it quickly retracts its mantle when a shadow falls over it. Once settled in its spot, it should not be moved frequently.

Good companions: Clownfish, tangs, gobies, cardinalfish, reef-safe fish; cleaner shrimp, snails.

Avoid: Angelfish and butterflyfish, some wrasses, triggerfish and pufferfish; large crabs that climb onto the mantle.

Kept alone, or in large aquariums together with other Tridacna clams, placed far enough apart that the mantles do not touch.

It matures first as a male, then produces both eggs and sperm. On farms it is produced through controlled spawning and larval rearing; it is not bred in home aquariums.

• Parasitic snails: Small pyramidellid snails on the shell edge feed on the clam; they should be checked for and removed at night.
• Retracted mantle: A mantle that does not fully open for days, or a shell that stays gaping, is a sign of a serious problem.
• Light burn: A sudden, strong change in lighting causes fading and burns on the mantle.

It is a hermaphrodite; there is no external distinction between the sexes.