Common Names: Giant Clam, Gigas Clam
Colour: The mantle of this species is usually golden brown, yellow or green, and has numerous iridescent blue or green spots on its surface, particularly around the edges. In larger specimens the blue spots become so numerous that the whole mantle may appear bluish or purplish. Numerous pale or clear spots, especially
Figure 12.2 Tridacna gigas
Upper and lateral view of shell Below: Map with geographical distribution of the clam. After Lucas 1988.
Note: Although the map indicates the distribution of T. gigas includes Fiji, Dr. Bruce Carlson informs us that based on his many years of observation throughout Fiji, he doubts T. gigas ever naturally occured there (though it has been "re-introduced"). He searched but found no living T. gigas nor fossil shells in Fiji that could support the notion that T. gigas had become extinct there.
in the center portion of the mantle, are characteristic of this species. These windows may serve a light focusing function, or merely allowT more light through (see chapter 4).
Distinguishing Characteristics: Adults have large, thick, heavy shell, without scutes. Juveniles may have sparse scutes or tubular projections near the umbo. The shells of juvenile T. gigas and T. derasa are very similar, though the ribs are slightly more pronounced in T. gigas. In large specimens, when viewed from above, top margins of the shell have 4-5 large, inward facing, triangular projections. There are no tentacles on the incurrent aperture (Lucas, 1988; pers. obs.). Max. Length: 1.5 m (4 ft.).
Similar Species: Tridacna crocea and T. derasa can be confused
A large Tridacna gigas on the reef.The dark mantle with mustard coloured blotches and many tiny blue rings is typical of large adults. Note the gills visible through the incurrent siphon. Jan VanBuuren.
A very large T, gigas photographed in New Guinea. For comparison, the diver is about 1.8 m (6 ft.) tall. The bumpy mantle of this clam is reminiscent of Tridacna tevoroa. A. Storace.
with young T. gigas. Besides the differences in colour pattern, T. gigas has more pronounced ribs without scutes. The triangular inward projections on the shell margins become apparent as T. gigas becomes larger. Possible hybrid crosses of T. gigas and T. derasa have been offered for sale. The appearance and colouring is divided between the two species, and there are tentacles on the incurrent aperture, as in T. derasa. Some of these "hybrids" have beautiful teardrop shaped clear windows along the edge of die mantle.
Natural Habitat: The Giant Clam is the largest bivalve in the world. Hunting for food and over-collection as a novelty item have greatly reduced its numbers over much of its range, so much so that it is listed as threatened by the International Union for the Conservation of Nature (Crawford and Nash, 1986). It is commonly
found in shallow lagoon areas or on reef flats, embedded in the substrate. Some specimens are so heavily over-grown with sponges, corals and algae that their shape is barely recognizable. As the clam grows, it loses its byssus gland and relies on its size and weight to hold it in place on the reef. In protected areas, like the Great Barrier Reef, this species can reach densities of up to 30 clams/hectare (Crawford and Nash, 1986).
Aquarium Care: As mentioned above, T. gigas has been the focus of commercial clam propagation projects for many years, recently providing small specimens for the aquarium industry. Therefore all specimens sold in the aquarium trade are aquaculturally grown. These clams have proven to be extremely easy to care for and, like T. derasa, can grow very rapidly in the aquarium when provided with adequate light and calcium levels, and can easily outgrow the home aquarium. This is the fastest growing tridacnid clam.
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