Derasa Clam

Geographical Origin and Habitat: Tridacna derasa, commonly known as the Derasa Clam or Smooth Giant Clam, is native to the tropical waters of the Indo-Pacific region. It is widely distributed across the Great Barrier Reef of Australia, Fiji, Tonga, Samoa, New Caledonia, and the Philippines. It inhabits protected outer reef slopes, sandy lagoons, and shallow coral rubble fields at depths from 1 to 20 meters, where clear waters allow intense sunlight penetration for its symbiotic microalgae.

Taxonomy: Belonging to the family Cardiidae (formerly Tridacnidae) and the genus Tridacna, it is known as the "smooth giant clam" due to its shell morphology. Unlike other clams like T. maxima or T. squamosa, its shell is relatively smooth with low, undulating ridges and lack distinct scales (scutes). It is the second-largest species of giant clam in the world, surpassed in size only by the massive Tridacna gigas.

Social Behavior: It is a sessile, benthic organism. Juveniles produce thin byssal threads to anchor themselves to rocks or coral rubble; however, as they grow, they lose this byssal gland, and their sheer weight becomes sufficient to keep them securely in place on sandy or gravel substrates. It shows no social behavior but reacts to shadows or physical contact by rapidly retracting its mantle and closing its valves.

Coloration and Sexual Dimorphism: The colorful, fleshy mantle extended during the day displays spectacular patterns of brown, green, gold, and electric-blue wavy lines. These colors protect the clam from solar radiation and host zooxanthellae. Giant clams are simultaneous hermaphrodites, meaning each individual possesses both male and female reproductive organs, and therefore no sexual dimorphism exists between specimens.

Tank Setup: A marine reef aquarium of at least 300 liters is required to support the long-term growth of this species (which can reach 50-60 cm in size). It should be placed on the sandy substrate rather than high up on the rocks. You can place a small flat rock underneath the sand for the clam to settle on. Ensure it is placed in an area with high-intensity lighting and low to moderate, indirect water flow.

Diet and Feeding: The majority of its nutritional needs are met through photosynthesis by the symbiotic zooxanthellae housed in its mantle tissues. However, it benefits from filter-feeding on phytoplankton, especially when young (under 10 cm in length) as their mantle area is smaller. They also absorb dissolved organic compounds, nitrates, and ammonium directly from the water column.

Water Quality: High chemical stability is paramount: water temperature between 24°C and 26°C, pH range of 8.1 to 8.4, specific gravity between 1.023 and 1.025, and calcium levels maintained between 400 and 450 mg/l. Carbonate hardness (KH) must be kept stable at 8 to 10 dKH and magnesium at 1300-1350 mg/l, as these clams consume substantial amounts of calcium to build their heavy shells.

Compatibility and Tankmates: Reef-safe, but must be protected from nipping tankmates. Avoid housing them with angelfish, butterflyfish, triggerfish, large wrasses, or non-reef-safe crabs and shrimps that can pick at the sensitive mantle tissue. Suitable tankmates include peaceful reef fish such as gobies, tangs, chromis, and non-aggressive corals that do not shade or sting the clam.

Aquarium Breeding: Spawning in home aquariums is extremely difficult and rarely successful. They are broadcast spawners, releasing sperm first and eggs later to prevent self-fertilization, typically triggered by thermal or chemical cues. The external fertilization results in free-swimming planktonic larvae that require highly specialized aquaculture facilities and rotifer feeding to rear successfully.

Risks and Diseases: The primary risk is pinched mantle syndrome or bleaching due to insufficient lighting or sudden temperature shocks. They are also highly vulnerable to parasitic pyramidellid snails, which hide near the clam's foot and feed on its tissue at night. Regular manual inspections of the clam's shell base are required to detect and remove these pests before they kill the clam.