Nothing illustrates the lengths to which man will go to satisfy his craving for unusual foods better than the Spanish delicacy percebes, otherwise known as the goose barnacle.

A particularly ugly shellfish native to Galicia and northern Portugal, it has a scaly shell that resembles nothing so much as a recently hatched dinosaur.

The English name comes from the long, webbed foot with which it clings to cliffs, and which is also the secret of its foodie appeal. As the weather deteriorates and the waves strengthen, this foot gets progressively thicker and juicier.

By Christmas, it can be 5cm long and up to 1cm wide; this is the peak time for harvesting. It is also the time when the cliffs and rocks are at their most inaccessible, lashed by Atlantic gales and freezing six-metre waves.

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The barnacle fishermen, known locally as percebeiros, work in teams of two, one holding a line and manning the boat, the other clambering onto the rocks, cutting a couple of handfuls of the molluscs with a special tool and stuffing them into a net bag tied around his waist before swimming back through the pounding surf to his boat.

The whole operation has to be timed to the rhythm of the swells to avoid the men being picked up and slammed back against the sharp edges of the very crop they are attempting to harvest.

It’s dangerous work, and every year there are a few casualties - often overenthusiastic amateurs rather than professionals. The latter are allowed to harvest no more than six kilos a day - far more, it is said, than can actually be achieved. Small wonder, then, that percebes cost about £100 per kilo at market.

Like all true delicacies, they are best cooked simply - by steaming or boiling in seawater to which a few bay leaves have been added. It’s vital that they still have a little bit of rock attached to the foot, to prevent the briny juices from escaping. Traditionally, they stay in the pot no longer than it takes the chef to say the Lord’s Prayer.

Eating a bowl of goose barnacles is a job that requires at least two napkins, one of which is fastened around your neck. You grasp the stem and twist sharply, revealing an inner tube that can then be sucked out. If the mollusc is really fresh, it will squirt juice in both directions as you pierce it, requiring the deployment of the second napkin as you dab at your dining companion’s clothing.

The shell itself contains only the sex organs, and they’re not usually eaten, although anyone adventurous enough to devour a plateful of mollusc feet is unlikely to be put off by its genitalia.

What to drink: a good accompaniment to Spanish seafood is sherry - a light, chilled manzanilla from Sanlucar de Barrameda. Aged briefly beside the sea, it picks up salty notes of ozone and iodine, and is barely any more alcoholic than a new-world chardonnay.

Alternatively, try a crisp Galician albariño. Where to eat them: although percebes are found in Portugal, Morocco and Canada as well as Spain, only the Spanish have made a cult of them. One suspects that this has as much to do with the diners’ admiration for the macho heroism of the fishermen as it does with the actual taste of the dish.

Madrid may be landlocked, but not for nothing is it known as “Spain’s busiest seaport”: you’re just as likely to find percebes here as in Galicia. Ribeira do Miño (Santa Brigida 1; 00 34-91 521 9854; no reservations, closed Monday) is a lively, crowded Galician restaurant specialising in laden platters of seafood, although you may have to queue at busy periods. Or try La Trainera (Calle de Lagasca 60; 91 576 8035; closed Sunday), an upmarket favourite in the chic Serrano district.

When to go: percebes are best enjoyed in winter - the colder water, stronger winds and heavier tides encourage their growth. The peak season is over by March. Insider tip: aficionados prefer their barnacles “de sol”, from a rock that is exposed to both sea and air. Those that grow underwater are known as “de sombra”, or “from the shadow”. In season, you may find percebes served with ham as tapas, or garnishing a seafood soup - a relatively inexpensive way to sample this curious dish.

Travel details: EasyJet (www.easyjet.com ) flies to Madrid from Liverpool, Luton, Bristol, Edinburgh and Gatwick. Or try Ryanair (0871 246 0000, www.ryanair.com ) or British Airways (0844 493 0787, www.ba.com ).

De las Letras Hotel (00 34-91 523 7980, www.hoteldelasletras.com ) still has its original 1917 staircase and mosaics, but otherwise it’s designer groovy, with literary quotes on the walls; doubles start at £115. At the budget end of the hotel scale, Abalu (91 531 4744, www.hotelabalu.com ; doubles from £70), behind the Gran Via, is more stylish than you’ve any right to expect for the price.

Or stay for a few days, live like a local and get an apartment. Interhome (020 8780 6633, www.interhome.co.uk ) has a selection of attractive two-bedroom places, with prices starting at £234 for three nights.

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