Giant clams, known scientifically as Tridacna gigas, are the largest living bivalve mollusks, capable of reaching lengths of over four feet and weights exceeding 500 pounds. Unlike most other clam species that rely solely on filtering food from the water, giant clams have developed a unique and surprising feeding strategy. This dual approach to nutrition allows them to thrive in the nutrient-poor waters of coral reefs where they reside.
The Primary Food Source: Symbiotic Algae
Giant clams largely depend on a symbiotic relationship with microscopic photosynthetic algae called zooxanthellae, primarily from the genus Symbiodinium. These algae live within the clam’s mantle tissue, the fleshy, often brightly colored, part of the clam that extends outside its shell. The clam opens its shells wide during the day to expose this mantle tissue to sunlight.
Through photosynthesis, the zooxanthellae convert sunlight into sugars and other organic compounds. A significant portion of these byproducts, estimated to be up to 90% of the carbon fixed by the algae, is then transferred directly to the clam. This internal food production system allows giant clams to grow to their impressive size even in clear, oligotrophic (nutrient-poor) ocean waters. This symbiotic association is integral to the clam’s survival; they cannot live long without the algae or in darkness.
Supplementing the Diet: Filter Feeding
While symbiotic algae provide the majority of their nutrition, giant clams also retain their ancestral ability to filter feed. They draw in seawater through a large inhalant siphon and filter out small particles. This filtered food includes microscopic marine organisms such as phytoplankton and very small zooplankton, typically ranging from 2 to 100 micrometers in size.
This filter-feeding behavior serves as a supplementary method to acquire additional nutrients, particularly nitrogen and phosphorus, which may not be sufficiently supplied by the algae alone. For juvenile clams, filter feeding can contribute a more substantial portion of their carbon needs, sometimes up to 65%, before their symbiotic relationship fully matures. As clams grow larger, their reliance on filter feeding for carbon generally decreases, with photosymbiosis becoming the dominant source, providing 60-100% of their carbon budget in adult individuals.
How the Symbiosis Works
The clam actively cultivates the algae within specialized cells and a unique tubular system located in its mantle tissue. This system allows the clam to maximize the exposure of the algae to sunlight. The clam’s iridescent mantle contains specialized cells called iridocytes that can filter light, directing beneficial wavelengths deeper into the tissues to fuel algal photosynthesis while reflecting harmful wavelengths.
The clam absorbs or digests the organic compounds produced by the algae. In return, the clam provides the algae with a protected environment and a steady supply of carbon dioxide and metabolic waste products, such as nitrogen and phosphorus, which act as fertilizers. This mutual exchange highlights the deep interdependence between the clam and its algal partners, enabling both to thrive.