Small giant clam  (Tridacna maxima)

The small giant clam is classified as Least Concern (LC) on the IUCN Red List (1) and is listed on Appendix II of CITES (3).

At less than a third of the size of the true giant clam (Tridacna gigas), the small giant clam (Tridacna maxima) deserves its name. As an adult, it has a large shell that adheres to a rock by its byssus – a tuft of long, tough filaments that protrude from a hole next to the hinge of the shell. When open, the bright blue, green or brown mantle is exposed and obscures the edges of the shell with its prominent and distinctively furrowed edges. The small giant clam is a bivalve mollusc, referring to the two valves on the mantle. These siphon water through the body to extract oxygen from the water using the gills, and to feed on algae (4). The attractive colours of the small giant clam are the result of pigment cells, which have a crystalline structure inside. These are thought to protect the clam from the effects of intense sunlight, or bundle light to enhance photosynthesis, the energy-producing process carried out by the tiny algae living within (4) (5).

The small giant clam has the widest range of all giant clam species. It is found in the oceans surrounding east Africa, India, China, Australia, Southeast Asia and the islands of the Pacific (1).

Found living on the surface of reefs or sand, or partly embedded in coral (2), the small giant clam occupies well lit areas, due to its symbiotic relationship with photosynthetic algae, which require sunlight for energy production (4).

A sessile mollusc, the small giant clam attaches itself to rocks or dead coral and siphons water through its body, filtering it for phytoplankton, as well as extracting oxygen with its gills. However, it does not need to filter-feed as much as other clams since it obtains most of the nutrients it requires from tiny photosynthetic algae known as zooxanthellae (4).

Beginning life as a tiny fertilised egg, the small giant clam hatches within 12 hours, becoming a free-swimming larva. This larva then develops into another, more developed, larva which is capable of filter-feeding. At the third larval stage, a foot develops, allowing the larva to alternately swim and rest on the substrate. After eight to ten days, the larva metamorphoses into a juvenile clam, at which point it can acquire zooxanthellae and function symbiotically (4). The juvenile matures into a male clam after two or three years, becoming a hermaphrodite when larger (at around 15 centimetres in length) (4) (5). Reproduction is stimulated by the lunar cycle, the time of day, and the presence of other eggs and sperm in the water. Hermaphroditic clams will release sperm first, followed by eggs (4).

The small giant clam is collected in small numbers for the shell trade, and it is also eaten (2). In Asia, the muscle of the small giant clam is considered a delicacy, and the Bedouins around the Red Sea also eat this marine invertebrate (5).

It has been shown that numbers in protected regions are considerably higher than numbers outside these areas. Research into clam farming, as well as into the ecology, growth rates and reproductive behaviour of the small giant clam is necessary to understand the conservation needs of this species. The Tongan Government gives a limit for the minimum size that can be harvested (2).

View information on this species at the UNEP World Conservation Monitoring Centre.

To learn more about a Whitley Award-winning conservation project for this species, click here.

For further information on giant clams and their cultivation: 

• Ellis, S. (1998) Spawning and early larval rearing of giant clams (Bivalvia: Tridacnidae). Center for Tropical and Subtropical Aquaculture, 130: 1–55. Available online at:
  http://www.ctsa.org/upload/publication/CTSA_130631672860873095404.pdf