The geoduck clam (Panopea generosa) is a marine bivalve mollusk native to the Pacific Northwest and is the largest burrowing clam in the world. It possesses an elongated, fleshy siphon that extends well beyond its shell. Known for its remarkable longevity, often living over a century, the geoduck sustains its massive body and long life entirely through a process of water filtration.
Primary Food Sources
The geoduck is a herbivorous suspension feeder, consuming microscopic organisms suspended in the water column above the seafloor. Its diet is primarily composed of phytoplankton, which are tiny, photosynthetic organisms that form the base of the marine food web.
The clam also consumes various other small particles collectively referred to as seston. This includes diatoms, zooplankton, and fine organic detritus, which is non-living particulate matter. The geoduck is a non-selective particle feeder, drawing in whatever small particles are present in the water it pumps.
Since the geoduck remains stationary, its energy intake is governed by the quality and volume of water it filters. This constant, low-energy food source allows the clam to maintain its large size while remaining buried deep within the sediment.
Filter Feeding Mechanics
The geoduck’s method of eating is entirely dependent on its muscular, fused siphon, which acts as a dual-purpose intake and exhaust snorkel. The siphon is divided internally into the incurrent siphon and the excurrent siphon, allowing for a continuous, one-way flow of water necessary for both respiration and feeding.
To feed, the clam draws water down through the incurrent siphon, which extends up through the sand or mud to the water column. The water is then passed over a pair of large, specialized gills inside the shell cavity. These gills are lined with cilia, tiny hair-like structures that beat rhythmically to create a current and trap food particles in a layer of mucus.
Once trapped, the particles are transported along the gills to the labial palps, fleshy appendages near the mouth. The palps sort the particles, rejecting larger or undesirable matter as pseudofeces, which is expelled. Accepted food is then directed into the mouth for digestion, and the remaining filtered water is expelled through the excurrent siphon.
Diet, Growth, and Longevity
The geoduck’s long lifespan and growth pattern are linked to its stationary, filter-feeding existence. By feeding on a perpetually available, low-energy diet, the clam maintains a slow metabolic rate. This reduced energy expenditure contributes to its longevity, with some individuals living for 179 years.
Growth occurs rapidly in the first 10 to 12 years of life. This initial growth is necessary for the clam to burrow deep enough to escape most predators. After this phase, growth slows dramatically, and the clam invests its continuous energy intake primarily into maintenance and reproduction. Scientists determine a geoduck’s age using shell growth rings, similar to tree rings.
As a non-selective filter feeder, the geoduck is dependent on the cleanliness of its marine environment. The clam removes nutrients and organic matter from the water, which can contribute to improving local water quality. However, this feeding strategy also means the clam can bioaccumulate harmful substances, such as toxins produced during harmful algal blooms, directly from its food source.