Basic biology

The giant clam is a member of the Phylum Mollusca and the Class Bivalvia. These clams have two shells (called “valves”) that are hinged on what is called an umbo. Their flesh or mantle is the mechanism that secretes the clam’s shell. The mantle also houses millions of photosynthetic algae that provide food in the form of sugars to the clam. This symbiotic relationship is the reason these clams become giants. Giant clams are filter feeders; they siphon water using their incurrent siphon and “exhale” water out through their excurrent siphon. They have gills, like many other marine fish and invertebrates, which have a dual purpose: gas exchange and moving filtered food to their mouths. Giant clams reproduce via broadcast spawning, which means their sperm and eggs are released into the water and eggs are fertilized in the water. They are able to produce millions of eggs in a single spawning event. Giant clams mature first as males, then develop gonads with both sperm- and egg-producing cells.

After spawning, fertilized eggs hatch as free-swimming, trochophore larvae, then metamorphose into veliger larvae. When the juvenile clams settle onto the reef, they cement themselves to the substrate using their byssal glands, which is an organ that secretes an adhesive. Giant clams also have a muscular foot, which they use during their juvenile stages to keep themselves facing upward or to move along the ocean floor. However, once they reach a certain size, their valves become too heavy to move. Clams have strong muscles called adductors, which retract and contract to draw water into their mantle or to close their valves shut for protection against predators. The adductor is the part of the clam that is used as food; clams are very popular in Japan and in other places in the Pacific.

Cultural significance

Hima is the CHamoru term for giant clams. Two species of hima, the most important giant clam species known to the CHamoru people, are Tridacna gigas and Tridacna maxima. Based on archaeological findings, these two species were most commonly used for tools such as higam (adzes), guesgues (scrapers), and se’se’ (knives). Giant clams are also believed to have been used as precious jewelry such as the sinahi, a crescent moon pendant. The higam was an especially important tool for the CHamorus, because it was used to build the sakman (canoe), which was essential for traveling to neighboring islands and fishing. The higam may also have been used to carve the Marianas’ symbolic acho’ latte. The guesgues may have been used to remove the scales of a fish, and the se’se’ was used to cut food, rope, or make small carvings.

Species distribution and diversity

Giant clams are found throughout the Indian and Pacific Oceans. The geographic distribution of Tridacna gigas ranges from Indonesia to Fiji. Tridacna maxima are found in coral reefs from the Egyptian Red Sea to the Pitcairn Islands. There are 10 living species of hima in two genera, Tridacna and Hippopus: Tridacna crocea, T. derasa, T. gigas, T. maxima, T. noae, T. rosewateri, T. squamosa, T. tevoroa, Hippopus hippopus, and H. porcellanus.

The species native to Guam are T. gigas, T. maxima, T. squamosa, and Hippopus hippopus. However, Tridacna gigas and Hippopus hippopus in Guam are extinct due to over harvesting. Tridacna derasa was introduced to Guam from Palau, but is rarely sighted. Tridacna maxima is the most common species found in Guam and the Marianas. Tridacna squamosa, which can be confused with T. maxima, is also still found in Guam.

Conservation and economic importance

Around the world, giant clams are sold for use in marine aquaria, as souvenirs (whole clam shells), for consumption (sushi/sashimi, stew, or fried), and for jewelry making (earrings, bracelets, and necklaces). Giant clams on Guam have become a regulated species due to overfishing, and they have been listed as a species of greatest conservation need in the Guam Comprehensive Wildlife Conservation Strategy. The commercial use of wild caught clams is prohibited. However, harvesting giant clams for personal consumption is allowed, but they must be greater than seven inches in length.

To reestablish the locally extinct Tridacna gigas, and to increase declining populations of T. derasa and T. squamosa, the Guam Department of Agriculture imported giant clams from the Micronesian Mariculture Demonstration Center in Palau in the early 1980s. In 1990, more than 700 Tridacna derasa were imported to Guam as part of a market study for seafood consumption. However, due to the high cost of clams, the lack of a seafood sanitation facility, and poaching, the importation of giant clams for consumption and farming in Guam ended in the mid-1990s.

In Guam, jewelry made from giant clams has become increasingly popular. Giant clam necklaces in the shape of latte stones and fishhooks are commonly sold in Guam. However, the sinahi is the most notable piece of jewelry that adorns CHamorus. Unique to CHamoru people of Guam and the Marianas, the sinahi is a symbol of pride and culture.

View figures in Flickr: https://www.flickr.com/photos/guampedia/albums/72157687015727246/

About the author

Francisco Villagomez is from the island of Saipan and was a graduate student at the University of Guam under the advisement of Dr. Frank Camacho. He studied the age-based life history and reproductive biology of Guam’s deep bottom-fish snapper Pristipomoides filamentosus, known locally as ‘Opakapaka’.

Editor’s note: This author was a beginning graduate student in 2017 taking a course in scientific writing at the University of Guam. This article was assigned to provide the student with practice in communicating science to non-scientists. The student chose the topic which is related either to their thesis project or work experience. The instructor in the course was Dr. Laurie Raymundo, a UOG Marine Laboratory faculty member.

For further reading

Guam Department of Agriculture Division of Aquatic and Wildlife Resources. Guam Comprehensive Wildlife Conservation Strategy. Edited by Paul C. Bassler and Celestino F. Aguon. Mangilao: DAWR, 2006.

Larson, Christina. “Shell Trade Pushes Giant Clams to the Brink.” Science 351, no. 6271 (2016): 323-324.

Lucas, John S. “The Biology, Exploitation, and Mariculture of Giant Clams (Tridacnidae).” Reviews in Fisheries Science 2, no. 3 (1994): 181-223.

Moir, Barbara G. “A Review of Tridacnid Ecology and Some Possible Implications for Archaeological Research.” Asian Perspectives 27, no. 1 (1986): 95-121.

Paulay, Gustav. “Marine Bivalvia (Mollusca) of Guam.” Micronesica 35-36, (2003): 218-243.