The geoduck clam, pronounced “gooey-duck,” is a Pacific Northwest marine bivalve known as the world’s largest burrowing clam. Its most distinguishing feature is an incredibly long, muscular extension that resembles a neck. It is this immense, trunk-like appendage that is responsible for the behavior of “squirting” water, often observed in tidal flats. The geoduck’s shell is relatively small, but the overall clam body is much longer. The animal’s inability to fully retract its soft parts into its protective shell makes it unique among clams.
The Siphon Structure
The impressive appendage protruding from the geoduck’s shell is a massive, fused siphon. This structure can extend up to a meter (over three feet) in length, allowing the clam to remain safely buried deep within the sand or mud. The siphon is composed of tough, muscular tissue and is essentially a double-barreled tube.
The two channels, the incurrent and excurrent siphons, are completely fused along their length. At the tip, two separate openings serve as the intake and exhaust ports for the clam’s constant water flow. The siphon’s strong muscular walls and the surrounding mantle cavity control the movement of water, drawing it in and pushing it out with considerable force.
Water Movement for Life Support
The geoduck’s squirting action is a continuous necessity for survival, driven by the need to maintain fundamental life processes. The primary function of the incurrent siphon is to draw in seawater containing microscopic food particles and dissolved oxygen. Once inside the mantle cavity, the water passes over the gills, which are responsible for gas exchange and filtering.
The gills extract oxygen for respiration and simultaneously trap phytoplankton, which forms the clam’s diet. Cilia, or small hair-like structures, move the filtered food particles toward the mouth. The filtered water, now depleted of oxygen and nutrients, is then collected in the suprabranchial chamber.
This spent water, along with metabolic waste products and feces, is then forcefully expelled through the excurrent siphon. This constant, one-way flow of water allows the sedentary geoduck to feed, breathe, and eliminate waste without moving its shell. The sheer volume of water processed by the clam often results in the visible expulsion or “squirt” seen at the surface.
Squirting as a Survival Strategy
While the constant flow is for life support, sudden “squirting” is often a behavioral response to the environment. When the clam is disturbed or senses a threat from a predator, it uses powerful muscles to rapidly contract the siphon. This sudden contraction forcefully expels a large volume of water from the mantle cavity and siphon tip.
The rapid expulsion of water is a defense mechanism that allows the clam to quickly pull its soft siphon down and deeper into its burrow. This shields the siphon from predators such as sea otters and certain species of dogfish. The geoduck is a deep burrower, establishing its shell at a “refuge depth” of two to three feet below the surface, so a quick retraction is an effective survival strategy.
The forceful expulsion of water also serves the practical purpose of maintaining the clam’s burrow opening, particularly on intertidal beaches. By jetting water, the geoduck clears away accumulated sediment, ensuring the siphon’s openings remain unobstructed and connected to the water column. This action is frequently observed at low tide when the clam must keep its lifeline clear of collapsing sand and mud.