Gigas clam

Care at a glance

Overview

The Gigas clam is the true giant clam, a peaceful, reef-safe animal for the sand bed of a very large reef tank. It suits a reef keeper with a little experience.

Caring for the Gigas clam

A Gigas clam needs only moderate to fairly high, stable light and moderate flow. Its day-to-day care is closer to a derasa or squamosa than to the pickier maxima or crocea. Place it on open sand with real room around it.

Even a juvenile a few inches across is on its way to becoming a two to four foot animal within a couple of decades. Start from at least 198 gal (750 L), and plan around the multi-hundred-gallon footprint it will eventually take up, not its size on arrival. Before buying one, ask whether your tank can still hold it comfortably ten or twenty years from now.

Keep calcium at 400 to 440 ppm, alkalinity at 8 to 11 dKH and magnesium at 1280 to 1350 ppm, all elevated and steady. Those levels support what is, size for size, some of the fastest shell growth of any clam in the hobby. Run the tank at 75 to 81°F (24 to 27°C) with a pH of 8.1 to 8.4.

The Gigas clam gets the bulk of its energy and shell-building material from photosynthetic zooxanthellae throughout its mantle. Specimens under about 8 cm should also get a phytoplankton or greenwater supplement several times a week. Larger clams need none.

Keep an eye out for mantle-nipping wrasses, crabs, shrimp and pyramid snails, as with any Tridacna. Acclimate a new gigas the same careful way as any other clam, floating it to match temperature and drip acclimating slowly.

Good to know

Tridacna gigas is the true giant clam and the largest living bivalve mollusk, in the family Cardiidae, subfamily Tridacninae. It can live well over a hundred years and keeps growing steadily for most of that time. Along with Tridacna derasa, it was one of the first two giant clam species listed under CITES Appendix II, in 1983. That protection was extended to every species in the family two years later.

In the wild

The Gigas clam lives on shallow reef flats and lagoons across the Indo-Pacific, from Myanmar and the Andaman Sea east to Kiribati. It has been fished to local extinction across much of its former range. Australia's Great Barrier Reef and the aquaculture centers in Palau are now among its strongholds. Juveniles briefly byssal-attach, but an adult is far too heavy to need any attachment and simply rests on sand or rubble under its own weight.

Common questions

Related species

• 

  Derasa clam

  Tridacna derasa

  19.7 in (50 cm) · 40 gal (150 L)

  19.7 in (50 cm)40 gal (150 L)75–81°F (24–27°C)

• 

  Squamosa clam

  Tridacna squamosa

  15.7 in (40 cm) · 40 gal (150 L)

  15.7 in (40 cm)40 gal (150 L)75–81°F (24–27°C)

• 

  Maxima clam

  Tridacna maxima

  13.8 in (35 cm) · 40 gal (150 L)

  13.8 in (35 cm)40 gal (150 L)75–81°F (24–27°C)

• 

  Crocea clam

  Tridacna crocea

  5.9 in (15 cm) · 30 gal (115 L)

  5.9 in (15 cm)30 gal (115 L)75–81°F (24–27°C)

• 

  Horse hoof clam

  Hippopus hippopus

  11.8 in (30 cm) · 40 gal (150 L)

  11.8 in (30 cm)40 gal (150 L)75–81°F (24–27°C)