Their most famous dish may beliver with onions, but the secret to eating well in Venice is to choose dishes that are from neither the countryside nor the ocean, but from the lagoon: eels, duck, small fish such as cuttlefish, the vegetables that grow in the salty soil of the nearby islands and, above all, crabs.

The marshes around the lagoon have been fenced off since Roman times, creating vast tidal basins that are a sort of halfway house between the wild and the farmed.

It’s in these shallow waters, and in particular the area around the island of Murano, where thegran-seole are caught – small lagoon crabs no bigger than your hand.

Granseole are a delicacy whenever you eat them. Twice a year, however, in spring and autumn, something happens to make them even more sought-after: the young male crabs shed their shells in order to grow larger ones.

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During this change – the muta – fishermen net the crabs and keep them in special tanks called vieri until the brief period, sometimes no more than a day, when they are soft enough to eat, skin and all. Then they are known not as granseole, but as moleche. The size and appearance of a large bald spider, they are rushed to the Rialto fish market in giant baskets to be sold.

There are only a few ways of cooking this delicacy. Sometimes they are served in a crisp batter that mimics the hard crust of the absent shell, but the usual way to eat them is col pien – stuffed. However, it is not the cook who does the stuffing, but the crab itself. The live moleche are placed in a bowl containing beaten eggs, salt and grated parmesan: they feast on this mixture for a few hours, then, when they are sluggish from overeating, they are coated with flour, dropped into hot oil and quickly fried – absolute freshness being of the essence.

Served in a bowl, their hairy limbs tangled together, glistening with crystals of salt, they do not look like one of the great dishes of the world, but appearances are deceptive. The texture is as soft as ripe fruit, the taste quite extraordinary – a cross between crab, oyster and lobster, like a fragrant mouthful of warm sea.

Where to eat them: when the mutais happening, almost every fish restaurant in Venice worth its name serves moleche. Two good ones are Alla Madonna, a reliable standby near the Rialto where you may have to queue for a table, and Al Fontego dei Pescaori, owned by a family who also run one of the best fish stalls at the Rialto market.

An even more traditional place would be one of the city’s cramped, dark bacari, or wine bars, such as Alla Vedova, where, standing up, you can wash your snack down with an ombra, a glass of “shade”, so called from the days when the wine-sellers lurked in the corners of St Mark’s Square to keep their wares cool.

What to drink: an uncomplicated white will suit the crabs’ salty flavour and the rich oiliness of the batter. The local wine is Fragolino Bianco, served from unmarked flagons. This used to be, as its name suggests, a slightly sweet wine with strawberry aromas, but because of EU legislation is now more likely to be made with another local grape, tocai. Alternatively, you could splash out on the local masterpiece, La Rocca, a single-vineyard Soave madeby Leonildo Pieropan, which Decanter magazine has designated one of the 10 greatest wines of Italy.

When to go: the exact times of the muta vary from year to year, depending on the warmth of the sea. It’s usually November and April, but has been known to start as early as October and January. Insider tip: for a real Venetian feast, follow your crabs with some wild duck (mazoro), eel stewed with bay leaves (bisato su l’ara) or a rich, creamy risotto. Here in Venice, they serve their risotti all’onda (“like a wave”) – the dish should be liquid enough that it threatens to spill if you tip it from side to side.

- Alla Madonna, Calle della Madonna, San Polo 594; 00 39-041 522 3824; closed Wednesdays.

- Al Fontego dei Pescaori, Calle Priuli, Cannaregio 3726; 041 520 0538; closed Mondays.

- Alla Vedova, Calle del Pistor, Cannaregio 3912; 041-528 5324; Closed Thursdays, dinner only Sundays. No credit cards

Anthony Capella’s latest novel, The Various Flavours of Coffee, will be published next week by Sphere at £6.99

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