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It's not the most beautiful dinner to look at and it has a very odd name... but in China the geoduck is an expensive delicacy, so on North America's Pacific coast the race is on to farm them and cash in.

The first thing people notice is the shape - it's not nicknamed the King Clam for nothing.

Its long, probing siphon bulges out of its shell and burrows through the sand - sucking in sea water, and squirting it out again (minus vital nutrients) from its tip.

Even its name sounds peculiar when pronounced correctly: "gooey-duck".

Media playback is unsupported on your device Media caption How do you farm geoduck? (Video by David Botti)

It is the largest burrowing clam in the world and can weigh up to 16lb (7.25kg), but it has another claim to fame.

It tastes delicious.

So delicious in fact that diners at high-end seafood restaurants in China will pay up to $300 (£200) for a fresh geoduck imported live from Canada or the US.

Its delicate texture and exotic looks are prized by connoisseurs - who consider the "elephant trunk clam", as it is known in China, to be an aphrodisiac.

"You really feel a mouthful of the Pacific... it's slippery, it's very tender, very sweet," said diners interviewed by the BBC in Beijing.

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Geoduck - the vital statistics

The name "geoduck" is thought to come from the Nisqually (Native American) word "gweduc," meaning "dig deep"

The bivalve grows in the tidal flats of Oregon, Washington State, British Columbia and Alaska, on beaches with deep sandy substrate

It takes six years for a farmed geoduck to reach maturity but they can live up to 160 years- as measured by the rings on their shells

The siphon can grow to 1m (3ft) or more in length

The clams may be served sauteed, as chowder, sushi or sashimi, but are most often blanched in broth

Back in the US, surprisingly few Americans have ever heard of geoduck, let alone tasted this delicacy that inhabits their shores.

You won't find many on dinner tables. Instead - more than 90% of harvested clams are flown straight to China and Hong Kong. It's a lucrative business but demand outstrips supply, leaving seafood firms struggling to keep up.

Media playback is unsupported on your device Media caption Singapore diners react to geoduck on their plates

Bill Dewey, whose firm Taylor Shellfish has been farming oysters since 1890 on the tidelands of Puget Sound in Washington State, has spent the last 25 years attempting to build a geoduck business.

"Geoduck is our newest species and it took us a long time - about a decade, just to get to a point where we weren't throwing money away," says Dewey.

"It's still not a cookbook today but it's profitable. And we're starting to sell more in the US too. But the Chinese demand drives the price - and that market is willing to pay a lot."

The landed value of geoduck is $10 (£6.40) per 1lb but it sells for $150 (£96) per 1lb in Chinese restaurants. In London, one prestigious Chinese dining establishment quoted £200 ($312) for a whole 2lb (1kg) geoduck.

Why do the Chinese love it so much?

"Part of it is the phallic appearance - it's a sexual-looking beast and there's a draw to that," says Dewey.

"But it's also quite a status symbol to be served geoduck."

In the US the clam has also achieved a certain cult status - especially in Washington state, where some have embraced the bizarre bivalve as a kind of talisman. Take for example "Speedy", the official mascot of the Evergreen State College.

Their battle song is: "Go, Geoducks go. Stretch your necks when the tide is low. Siphon high, squirt it out. Swivel all about; let it all hang out."

For families on the coast going out digging for shellfish in the summer has been "part of our local heritage" says Dewey.

"It's a family outing. You go out at low tide with a shovel and try to find a geoduck," The trademark spurt from its siphon is a giveaway, he says.

"But even then it takes hours to dig him out - it's a major excavation. If you're lucky you get one and you take it home and make good fritters."

There are signs that the geoduck's appeal could be spreading beyond the North-West, however.

"It is getting a little more popular, more mainstream," says Dewey.

He produces 700,000lb (318,000kg) of farmed geoduck every year, he says, and sells about half domestically, especially to the Asian markets that can be found in many US cities.

The clam has been getting some publicity recently, which probably helps.

Celebrity chefs have included the clam in their recipe books and on TV cookery programmes. "We did segments on shows like Bizarre Foods and Dirty Jobs," laughs Dewey.

And it is being seen more often in seafood restaurants too.

"I used not to sell it - but now it sells itself," says chef Taichi Kitamura, owner of Tamura Sushi Kappo, in Seattle.

"Even the tourists want it. They see it on the Food Network (TV) and because of its impressive, somewhat sexy look, they want to try it. We charge $10 a pair of sushi, or $20 for an order of sashimi.

"But the price needs to come down for it to become really popular. Supply needs to catch up with demand."

Kitamura says the flavour of geoduck is among the "cleanest-tasting" of seafoods.

"There's nothing fishy about it. The texture is crunchy and chewy - though there are tender parts if you cook it right," he says.

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Why China loves the 'elephant trunk clam'

The BBC's Fuchsia Dunlop, author and Chinese food expert:

It has that texture Chinese people adore - that slightly crisp rubberiness, but also tenderness if it's cooked properly.

Westerners have an aversion to slippery, springy, slimy, grisly foods, but to the Chinese they're attractive - because they have this very developed sense of texture. Take the sea cucumber - it looks like a yucky slug to Western eyes but in China it's a delicacy.

Wild seafood is very prestigious in China. If you can afford to take people out for a special dinner you go to a restaurant with tanks of live shellfish.

In the West if we want to impress or schmooze someone we might order an expensive wine. But in China it's food that you have to spend money on.

Traditionally it would be bird's nest or shark's fin, but geoduck is also expensive - and it's impressive. It's huge. To eat a rare and exotic shellfish from the pure waters of Canada is highly desirable.

Westerners often look down on the Chinese: "Oh they eat such weird things" - as if they are desperate. But actually it's a sign of a very discerning food culture - a love of the exotic.

Cooking the creature is a delicate process, agrees Dewey. The siphon feels "leathery" when it first comes out of the water, he says.

"But when you prepare it, you dip it in boiling water to release the outer skin. It's like a three-foot long condom when it comes off.

"You're left with this smooth, clean, white piece of flesh. In the sushi bar they'll slice it up. But there are lots of recipes - like chowder, or ceviche [raw fish cured in citrus juice] - one of my favourites."

If geoduck aquaculture really takes off in the bay, it could help transform the coastal economy, he says. At the moment it is hard to get a licence, but Dewey insists the business is sustainable.

"We're not mining the ocean - we are producing the babies in our hatchery," he says.

Perhaps the talk of babies reminds him of something else.

"I'll tell you a funny story," he says. "You know my wife and I got married out on the tidelands. Our aisle was made of netting - and instead of rose petals we had our guests scattering baby clams."

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