Colour: Tridacna derasa generally have either a striped pattern of wravy lines or a spotted pattern consisting of various combinations of orange, yellow, black, blue and white. Some specimens can also have brilliant blue or green lines.
Distinguishing Characteristics: Heavy, very plain shell; absence of strong ribbing and scutes; incurrent aperture has pronounced tentacles; loses byssus gland as it grows; very small, narrow byssus gland opening; most have tiny scutes and some have sparser but larger ones and; hinge is usually longer than half the shell length (Lucas, 1988; pers. obs.). Max. Length: 50 cm (20 in.).
Tridacna derasa. J.C. Delbeek.
Similar Species: Tridacna gigas can be confused with T. derasa, however, T. gigas has a different mantle colouration and the top edses of it's shell have triangular projections extending inwards. The mantle of T. derasa also extends further over the shell.
An adult Tridacna derasa in its natural habitat on the Great Barrier Reef, Australia. In the Coral Sea T. derasa are often vivid blue. Elsewhere they are typically shades of golden brown and green with a blue margin on the mantle. J. Sprung.
Natural Habitat: Tridacna derasa are common in oceanic environments, particularly in the 4 to 10 m (12 to 33 ft.) range of outer reef edges (Crawford and Nash, 1986). This species loses it's byssus gland fairly early and is often found lying freely on the substrate in lagoons (Yonge, 1975). This species is greatly sought after as a food item and has been hunted extensively throughout its natural range. As a result they are listed as threatened by the International Union for the Conservation of Nature. In protected areas such as the Great Barrier Reef, they can be found in densities of up to 30 clams/hectare (Crawford and Nash, 1986). This species was one of the first, along with T. gigas, to be commercially bred. As a result, specimens sold in the aquarium trade today are the product of aquaculture projects and are not wild-caught.
Aquarium Care: This species is the most widely available and hardy of the tridacnid clams. They can be placed almost anywhere in the aquarium and do well under a variety of light intensities. Of course, the more light you can supply, the faster they will grow. These clams can grow extremely quickly in the aquarium and it is not unusual for 6 cm (2.5 in.) individuals to double or triple their size in less than a year, provided they are given abundant calcium (>400 mg/L). It is not unusual for this clam to develop scutes when grown in the aquarium, possibly as a result of the artificial light regime and the effect on expansion of the mantle. Occasional specimens do form scutes clearly
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