One clue that there is more life at Kwilákm’s beaches than meets the eye is the litter of dead clam shells.
These shells, many bleached white to reveal their composition of calcium carbonate, are often concentrated in tide line debris on a beach, or scattered across the muddier sections of beach. But where do all these clams live?
Clams are members of a family of invertebrates (animals without an internal skeleton) called bivalves that live within two shells connected by a hinge and pulled together by muscles attached to the shells.
Ancient clams date back in the geological record to the Cambrian Era 500 million years ago. Clams have a powerful muscular foot used for burrowing and live in or on sandy or muddy bottoms below shallow water.
Burrowing clams differ from other bivalves such as oysters and mussels that live on and attach themselves to hard surfaces.
Clams live just beneath the sand or mud surface, where they are protected from wave action and many predators.
Clams are filter feeders that collect food particles from seawater. This food is largely bits of plants, bacteria, and algae that occur on surfaces or float in seawater. They do this by drawing in and expelling seawater through two tubes, the siphons or neck of the clam.
These tubes can reach to the sand surface, and are used both to breathe as well as eat. The sand flats just offshore Sandy Beach are pockmarked with volcanoes that are the tops of many clam siphons.
Water is drawn into the clam by the movement of millions of hairs (cilia) on gills. Other hairs on the gills filter food from the passing water and transport the food to the mouth of the clam.
Most of Kwilákm’s clam shells are from shallow burrowing species that live within the top metre of sediment. Clams live underground to avoid predatory sea stars, snails, crabs, otter, mink, seals, and marine birds. They have two shells (valves) that are small, round to oval, with both shells of equal size. Some surface-burrowing species, such as cockles, have raised radial ribs on their shells that strengthen the shell against predators and damage.
Kwilákm may have deeper-burrowing species such as razor clams, but their shells are rarely seen. They burrow to greater depths for greater protection, and use longer siphons to reach the surface. Their shells are more elongate, smooth, and flattened to allow more rapid movement through the sediments.
Clams move with a blade-like muscular foot that is pointed for digging. The foot digs with a back and forth movement, creating a burrow. During digging the foot is extended; it grabs the burrow wall by expanding its foot, allowing it to grab and pull the rest of the animal downwards. At the same time, the clam quickly closes its shell, forcing water into the burrow and fluidizing the sediment, making movement through it easier. Both foot and siphons can be withdrawn inside the shells for protection.
It is interesting to compare the shells of different clams, as well as with oysters, to understand their differing defensive strategies against predators.
Oysters have very thick shells, often with sharp spikes to deter predators. Oysters stand their ground and use armament to protect themselves. Clam don’t stand their ground, but rather hide in the sand or mud below the surface. The shallowest burrowing clams, the cockles, are most vulnerable to predators that dig them up. To protect themselves, they have thick rounded and strengthened shells with deep ridges that allow the two shells to lock together when closed. Cockles need this protection because their round shape means that they are slow diggers. Deeper burrowing clams have more streamlined shells that allow them to dig faster to escape predators, and so their shells can be thinner. Examples are butter clams and littleneck clams.
Clam shells grow at the edges, though increases in thickness take place everywhere. Clams reproduce by dispersing both eggs and sperm into seawater. Fertilized eggs develop into larvae that swim briefly and disperse widely before settling permanently on the bottom. Because of their sedentary lives, clams don’t need to draw much oxygen from water to live. During a low tide when their host sediment is dry and without water, clams can almost shut down their need for oxygen.