15 May 2013


One of the longest-living animals, which is also the world's biggest clam, could have important stories to tell about Earth's climate history.
A crate of geoducks for sale at a seafood market. Credit: iStockphoto

A crate of geoducks for sale at a seafood market. Credit: iStockphoto

WHEN IT COMES to longevity in the animal kingdom, tortoises are usually the first to come to mind. Yet with specimens able to live to 168 years old, if there ever was a competition for who could live the longest, the geoduck (pronounced ‘gooey duck’) would give many species a run for their money. Additionally, having an average weight of 0.5-1.5 kg when mature, they can lay claim to being one of the largest species of clams.

The mollusc is native to the northwest coast of the United States and west coast of Canada. Their unusual name comes from a Native American word meaning ‘dig deep’ which is likely a reference to the practice used to harvest them.

So to what does the geoduck owe the pleasure of its long lifespan? According to Claudia Hand, research biologist at the Pacific Biological Station in Canada, being hidden from most predators is a great advantage.

“A geoduck’s long life is made possible due to low stress, or wear and tear, because they live buried in a metre of soft soil, well away from most predators,” Hand says. Geoducks can bury themselves up to one metre deep and have a siphon to match, which they use to feed on plankton through filtration. Reaching one metre in length, their siphon is on average four times longer than the shell and cannot be retracted.

Unfortunately for many, such an advantage doesn’t exist during their youth. Geoducks are ‘dioecious’, meaning they have separate sexes. Spawning typically occurs from late winter to early summer. Triggered by the ideal temperature conditions and other cues, the males release sperm, which, in turn stimulates female geoducks to release their eggs, which can number in the millions each year. With fertilisation taking place in the water column, success depends on many environmental factors, leaving the larvae very vulnerable.

“Their larvae, which are like plankton, suffer high mortalities through predation, and currents can carry them far out to sea,” explains Hand. “Depending on tidal currents and prevailing winds, usually around the two-month mark, if the surviving larvae find themselves in a suitable location of near soft shoreline sediments, they will settle. At this time, they are vulnerable to predators like crabs and sea stars until they dig deep enough. This type of hit-or-miss recruitment is sometimes referred to as sweepstakes recruitment because, when all the variables line up, there can be huge success.”

From its inception in the 1970s, the commercial harvesting industry for this clam has grown considerably. Hatchery production numbers in South Puget Sound (in the U.S. state of Washington) range between 2.5 million and 3.5 million annually, and the increasing demand from Asia (where they can be sold as much as $66 per kilogram) has helped foster a market estimated at around $80 million per year in Washington, USA and British Columbia, Canada.

However, they may yet serve more than our stomachs. Like trees, geoducks have annual growth rings, deposited on their shells every winter. Prior studies of shell growth in another mollusc called a bivalve show a strong correlation to temperature, so there is potential for the animals as climate proxies. With tree growth primarily responsive to conditions on land, and coral reefs limited to tropical regions, geoducks could prove to be very useful in helping bridge the gaps in Earth’s climate history and if their potential lifespan is anything to go by, they could have quite a story to tell.


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