Clams are bivalve mollusks, characterized by soft bodies encased within a two-part, hinged shell. While the idea of a clam “swimming” like a fish is a common misconception, most clams do not move through water in this manner. These aquatic invertebrates primarily engage in deliberate locomotion, often interacting with the seafloor. This article explores how clams move, why they move, and highlights bivalves that can truly swim.
How Clams Typically Move
Most clams primarily use a muscular foot to navigate their environment, particularly for burrowing into sand or mud. This foot is a versatile structure that extends into the sediment, acting as a probe. Once extended, the clam inflates the tip of its foot with blood, creating a bulbous anchor that secures it firmly within the substrate.
With the foot anchored, the clam contracts its muscles, pulling its shell downward into the sediment. Simultaneously, the clam contracts its shell valves inward, narrowing its body and reducing friction with surrounding particles. This coordinated action allows the clam to wedge itself deeper into the ground. After pulling itself deeper, the clam relaxes its foot, extends it to a new position, and repeats the cycle. This process enables clams to bury themselves for protection and stability, typically moving slowly through the substrate.
Why Clams Move
Clams move for several reasons, tied to their survival and reproductive success in their aquatic habitats. A primary motivation is to find suitable substrate for protection. By burying themselves, clams avoid predation from marine animals. This burrowing also helps them regulate body temperature and avoid desiccation, especially in intertidal zones where they can be exposed during low tide.
Movement also plays a role in foraging, as clams are filter feeders that strain microscopic organisms, like plankton, from the water. While largely stationary once settled, limited movements can help them access areas with richer food sources. In some species, such as the hard clam, horizontal movement across the sediment surface occurs, particularly during spawning periods. This “walking” behavior is thought to help aggregate mature individuals, facilitating greater fertilization success.
Bivalves That Can “Swim”
While most clams are burrowers, certain bivalves have evolved distinct methods of propulsion that allow them to “swim” through the water. The most well-known example is the scallop, which uses a unique form of jet propulsion. Scallops rapidly clap their two hinged shells together, forcefully expelling water from between the valves.
This rapid expulsion of water generates thrust, propelling the scallop in the opposite direction. This swimming is not sustained over long distances but serves as an effective escape mechanism from predators like starfish. The powerful adductor muscle, responsible for closing the shells, is exceptionally developed in scallops, enabling the force needed for this jet propulsion. While scallops are the most proficient, some other bivalves, such as file shells, also exhibit limited swimming capabilities using similar jet propulsion.