Giant Clam

Tridacna gigas is the largest living bivalve mollusk, reaching up to 120 cm (47 in) across and over 200 kg (440 lb), with an average wild lifespan exceeding 100 years. Its shell has four or five distinct vertical folds, differentiating it from Tridacna derasa's six or seven. Adults cannot fully close their shells; small gaps always remain, revealing the retracted brownish-yellow mantle tissue. Its mantle is fleshy, extending beyond shell edges and densely packed with symbiotic zooxanthellae. Hundreds of eyespots, each about 0.5 mm in diameter, dot the mantle border, containing a pupil-like aperture and over a hundred photoreceptors.

It inhabits shallow coral reefs, found in flat coral sand or broken coral at depths of up to 20 m (66 ft). Though a filter-feeder, 65-70% of its nutrition comes from symbiotic unicellular algae (zooxanthellae) cultivated within a specialized circulatory system. During the day, the clam extends its mantle to expose these algae to sunlight for photosynthesis. Its eyespots enable rapid mantle retraction and partial shell closure in response to sudden light dimming, a defense against potential predators. It also reacts to object movement before a shadow is cast and adjusts mantle orientation with changes in light direction. Tridacna gigas is an endangered species, with populations experiencing rapid declines across its range.

Tridacna gigas is a hermaphrodite, producing both eggs and sperm, though self-fertilization does not occur. Reproduction involves broadcast spawning, releasing up to 500 million eggs (100 micrometres/0.0039 in diameter) and sperm. A Spawning Induced Substance (SIS), detected by chemoreceptors near the incurrent syphon, synchronizes this release, triggering mantle swelling, adductor muscle contraction, and vigorous shell contractions to expel gametes. Spawning coincides with incoming tides, typically near the second, third, and fourth quarters of the moon phase, with intense periods (30 minutes to 2.5 hours) at 2-3 minute intervals. Fertilized eggs float for about 12 hours before hatching into trochophore larvae; these measure 160 micrometres (0.0063 in) after two days and soon develop a foot for locomotion. Larvae are planktonic, consuming plankton and capturing free-floating zooxanthellae. Settlement occurs at about one week of age, though juveniles may relocate frequently. They are considered juvenile once 20 cm (8 in) long, with laboratory-reared clams showing growth rates of 12 cm (4.7 in) per year.