TankBud

Derasa clam

Tridacna derasa

Also known as Southern Giant Clam, Smooth Giant Clam

SaltwaterClam

Derasa clam (Tridacna derasa)
Stan Shebs · CC BY-SA 3.0

Care at a glance

Temperature
75–81°F
24–27°C
pH
8.1–8.4
Adult size
19.7 in
50 cm
Minimum tank
40 gal
150 L
Temperament
Peaceful
Social behavior
Solitary
Swim level
Bottom
Diet
Photosynthetic
Difficulty
Beginner
Breeding
Hard
Rarity
Very common
Reef safe
Reef safe
Family
Cardiidae (Tridacninae)
Origin
Indo-West Pacific, from Australia's Great Barrier Reef and Papua New Guinea through Indonesia and the Philippines to Fiji, Tonga, and Palau

Overview

The Derasa clam is a large, peaceful giant clam for a reef tank. It is considered the best starter giant clam.

Did you know

A well-fed Derasa clam can nearly triple its shell length in a single year. Captive-bred stock from giant clam farms in Palau and elsewhere in the Pacific now supplies most of the derasa in the aquarium trade. That is a big part of why it stays affordable, even though the whole family is protected.

Caring for the Derasa clam

The Derasa clam is the most forgiving clam to light. It tolerates moderate reef lighting that would leave a maxima or crocea pale. It adjusts faster than most Tridacna, but still drip acclimate a new arrival. Start it under lower light for a week or two before stepping up to full intensity.

Give it an open sandbed rather than rock, in a tank of 40 gal (150 L) or more. Adults can shift position slightly on their own. Leave room to grow, since it becomes one of the largest clams commonly kept.

Keep the water at 75 to 81°F (24 to 27°C) with a pH of 8.1 to 8.4. Hold calcium, alkalinity and magnesium stable and elevated: calcium 400 to 440 ppm, alkalinity 8 to 11 dKH and magnesium 1280 to 1350 ppm. It is one of the faster-growing clams and pulls hard on all three to keep up.

It hosts photosynthetic zooxanthellae in its mantle tissue and draws on them for energy and the material to build shell. Clams under 10 cm do better with a phytoplankton or greenwater supplement a few times a week, while larger clams need no extra feeding. It is otherwise low-maintenance, but it is still vulnerable to mantle-nipping wrasses, crabs, shrimp and pyramid snails, so keep an eye out.

Good to know

The Derasa clam, Tridacna derasa, is the smooth giant clam of the family Cardiidae, subfamily Tridacninae. It is named for its thick shell with shallow, rounded folds, instead of the sharp ribs or scutes seen on squamosa or crocea. Along with Tridacna gigas, it was one of the first two giant clam species listed under CITES Appendix II, in 1983. The protection was extended to the entire family two years later, a response to how quickly both large species were being fished out.

In the wild

The Derasa clam lives on the outer edges of reef slopes across the Indo-West Pacific. It usually lives deeper than other Tridacna species, at around 4 to 10 meters. Wave energy is strong there, but the light is a little less intense. Juveniles anchor briefly with byssal threads, but adults lose that attachment and rest unattached on sand or coral rubble under their own weight.

Common questions

What size tank does a Derasa clam need?

A Derasa clam needs at least 40 gal (150 L).

What temperature and pH does a Derasa clam need?

A Derasa clam is comfortable at 75–81°F (24–27°C) with a pH of 8.1–8.4.

What does a Derasa clam eat?

A Derasa clam draws on the photosynthetic zooxanthellae in its mantle for energy and for the material to build its shell. Clams under 10 cm do better with a phytoplankton or greenwater supplement a few times a week, and larger clams need no extra feeding.

How big does a Derasa clam get?

An adult Derasa clam reaches about 19.7 in (50 cm).

Related species