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Members of the clam genus Tridacna, giant clams (Tridacna gigas) are the largest extant bivalve mollusks. Among the most endangered clam species, giant clams can be found in flat coral sand or broken corals, in depths of as much as 66 feet.
See the fact file below for more information on the giant clam or alternatively, you can download our 21-page Giant Clam worksheet pack to utilise within the classroom or home environment.
Key Facts & Information
- Young giant clams are hard to identify from other species of Tridacninae. Adults are the only giant clams that cannot close their shells completely. Even when closed, portions of the mantle can still be seen, unlike the very similar Tridacna derasa. However, this can only be distinguished with increasing age and growth. Small gaps continue to remain visible between shells through which retracted brownish-yellow mantle can be spotted.
- Tridacna gigas consists of four or five vertical creases on its shell, a main feature that differentiates it from the similar shell of T. derasa, which displays six or seven vertical folds. As with extreme deposition of coral matrices made of calcium carbonate, the bivalves with zooxanthellae, or single-celled dinoflagellates, may seem to grow huge calcium carbonate shells. The mantle’s edges are composed of symbiotic zooxanthellae that are assumed to make use of carbon dioxide, phosphates, and nitrates given off by the clam.
- The mantle border is lined with hundreds of different eyespots of about 0.5 mm in diameter.
- Each of these has a small chamber with a pupil-like aperture and a base of a hundred or more photoreceptors, which permit the giant clam to react to the unexpected dimming of light in which they withdraw their mantles and slightly close their shells as a way of protecting themselves from potential predators.
- Aside from their reaction to dimming, giant clams also respond to the movement of an object prior to the casting of a shadow. In order for this to occur, an image forming optical method is needed as the reaction is based on the local dimming of a portion of the generated image with respect to the rest. The motion of a dark object caused this sequential dimming, letting enough time for the mantle to be retracted before a potential predator is nearby and casting a shadow.
- The largest identified T. gigas species measured 137 centimeters, and was found around 1817 on the northwestern coast of Sumatra, Indonesia. The weight of this specimen’s two shells was 230 kilograms, implying that the live weight of the clam would have been about 250 kilograms. Currently, these shells are exhibited in a museum in Northern Ireland.
- Another peculiarly large giant clam was discovered in 1958 off the Japanese island of Ishigaki. However, it was not scientifically studied before 1984. The shell’s length was 115 centimeters and the weight of the shells, together with its soft parts, was 333 kilograms. Researchers assumed that the live weight of this giant clam species would be around 340 kilograms.
- Giant clams feed on algae, which act as a supplementary source of nutrients. These plants consist of unicellular algae, whose metabolic elements add to the filter food of clams, resulting in the species’ massive size which is as large as a meter in length, even in coral reef waters with less nutrients. The clams enrich algae in a specialized circulatory system which lets them keep a substantially higher amount of symbionts per unit of volume.
- In small clams – 10 milligrams dry tissue weight – filter feeding releases about 65% of total carbon required for respiration and growth; large clams (10 grams) require only 34% of carbon from this resource.
- Giant clams sexually reproduce and are considered as hermaphrodites, having both egg and sperm. Self-fertilization is impossible, although this trait permits them to mate with any other member of the species, reducing the burden of searching a compatible mate, at the same time doubling the number of offsprings. Compared to other forms of sexual reproduction, hermaphroditism guarantees that new gene combinations can be inherited by other generations.
- Since giant clams are unable to move or wander, they adopt broadcast spawning, emitting sperm and eggs into the water. A spawning induced substance, secreted through a syphonal outlet, assists in the transmission and synchronization of the release of sperm and eggs to guarantee fertilization.
- Discovery of these transmitter substances prompts the giant clam to expand its mantle in the central region and to contract its adductor muscle. Each clam crowds its water chambers and clogs the incurrent syphon. The shell intensely contracts with the adductor’s assistance, so the excurrent chamber’s contents passes through the excurrent syphon. A few contractions containing only water, eggs, and sperm are present inside the excurrent chamber, and then flow through the excurrent syphon into the water.
- Eggs are about 100 micrometers in diameter, and adult giant clams are able to release more than 500 million eggs at a time.
- Spawning takes place within incoming tides near the second (full), third, and fourth (new) quarters of the moon phase, occurring every two or three minutes, which can even reach to 30 minutes to two and a half hours.
- Fertilized eggs can be seen floating in the sea for about 12 hours until a larva, or a trochophore, hatches. The larva begins to develop a calcium carbonate shell. Two days after fertilization, it grows into 160 micrometers, and forms a “foot”, which is used for mobility; it can also swim to locate suitable habitats.
- At around seven days, the clam stays on the ground, although it shifts location frequently within its first few weeks.
- Giant clams are now endangered because of the intensive exploitation of bivalve fishing vessels. Majority of the large adult T. gigas are killed, since they are most likely profitable.
- Giant clams are also eaten in Japan (known as himejako), France, South East Asia, and other Pacific Islands. Some Asian delicacies include the meat from the muscles of clams.
- Giant clam shells are sold as ornaments in the black market.
Giant Clam Worksheets
This is a fantastic bundle which includes everything you need to know about the giant clam across 21 in-depth pages. These are ready-to-use Giant Clam worksheets that are perfect for teaching students about the giant clams (Tridacna gigas) which are the largest extant bivalve mollusks. Among the most endangered clam species, giant clams can be found in flat coral sand or broken corals, in depths of as much as 66 feet.
Complete List Of Included Worksheets
- Giant Clam Facts
- Tell Me About It
- A Giant Clam’s Anatomy
- Fill in the Puzzle
- Tank of Facts
- In a Relationship
- Mollusk Comparison
- Under the Sea
- Let’s Dig In
- Color the Clam
- Giant Clam Reproduction
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Link will appear as Giant Clam Facts & Worksheets: https://kidskonnect.com - KidsKonnect, February 5, 2021
Use With Any Curriculum
These worksheets have been specifically designed for use with any international curriculum. You can use these worksheets as-is, or edit them using Google Slides to make them more specific to your own student ability levels and curriculum standards.