Clams are bivalve mollusks found in both saltwater and freshwater systems. They are characterized by a shell composed of two equal-sized halves, or valves, joined by a hinge. Clams use a powerful muscular foot to burrow into the sediment of the sea floor or riverbeds, spending most of their lives partially buried. As filter feeders, they draw in water through a siphon to capture plankton and other suspended organic particles.
The Definitive Answer: Clams in Both Habitats
The term “clam” is broadly applied to any bivalve that burrows into the substrate. While species are found in every major aquatic habitat, the vast majority are marine. Over 14,500 of the estimated 15,000 known bivalve species inhabit the world’s oceans. However, roughly 500 species exist in non-marine habitats like rivers, lakes, and streams.
Marine Clams: The Most Familiar Bivalves
Marine clams are the species most commonly recognized, largely due to their prominence in the commercial seafood industry. These saltwater bivalves inhabit a wide range of marine zones, from shallow intertidal flats and estuaries to the deep abyssal plains. They thrive in sandy or muddy substrates that allow for easy burrowing, which offers protection from predators.
One valuable commercial species is the hard clam, or northern quahog, which ranges from the Gulf of St. Lawrence down to Florida. These are sold under various names depending on their size, such as littlenecks, cherrystones, and chowders. Another well-known species is the soft-shell clam, or steamer, identified by its thin, brittle shell and long, fused siphon, often called the “neck.” Giant clams, such as Tridacna gigas, inhabit tropical coral reefs and can grow to over 54 inches and weigh hundreds of pounds.
Freshwater Mussels and Clams: Hidden Ecological Giants
Freshwater bivalves are often referred to as mussels, belonging primarily to the Unionidae family, though some smaller species are true clams. These organisms play a large ecological role in rivers and lakes, acting as highly efficient natural filters. Each individual can process between 5 to 10 gallons of water daily, removing suspended particles, excess nutrients, and pollutants. This continuous filtration enhances water clarity and quality, benefiting other aquatic life.
Freshwater bivalves are also considered excellent bioindicators because their long lifespans and sensitivity to contamination make them reliable monitors of environmental health. North America is a global hotspot, hosting about 300 types. However, they are among the most imperiled animals, with approximately 70% of species declining due to habitat loss and pollution. Unlike their marine counterparts, many freshwater species require a temporary parasitic stage on a specific host fish to disperse their larvae.