"Can you imagine a market like this for a city of only 70,000 inhabitants?" Marcella Hazan asked us early one day as we walked together through Venice's mercato del pesce al minuto, or retail fish market, on the Campo de la Beccarie, near the Rialto bridge. We knew what she meant—but the truth is that we could imagine it very well, since this city was Venice, where seafood in great variety and of the highest quality is an obsession, a religion, a birthright.

The first thing any honest Venetian cook or food lover will tell you, in fact, is that this cooking is very difficult to reproduce in other places because it depends so very much on that particular menagerie of scaled and tentacled and hard-shelled sea creatures on display at the market five mornings a week. The mercato, also called La Pescaria, is not huge; thirty or so fishmongers and their assistants stand behind tables glistening with fish guts and brine beneath a canvas canopy supported by colonnades in the Venetian neo-Gothic style. The present structure dates only from the early 20th century, but seafood has been sold at the Rialto for at least 500 years. A legend in decorative metalwork over a nearby gate reads: PISCIS PRIMUM A CAPITE FOETET—"Fish Starts Rotting from the Head". In a city-state whose leader served at the pleasure of a powerful governing council, this old saw once had political as well as piscatorial resonance.

Rotting fish is not something you're likely to encounter at the mercato del pesce. The array of seafood varies from season to season, of course, and even from day to day. But unless the fish is identified as frozen (there is a bit of this) or obviously dried or salted, you may be sure that everything sold here is as fresh as it can be. "The only things you don't want to eat very fresh," advises Gigi Naccari, a third-generation fishmonger who is one of the best-known and respected sellers at the market, "are the cephalopods—squid, cuttlefish, octopus—which are always better after two or three days."

At Naccari's stand and at those of his neighbors one morning, this is some—just some—of what we see: clams, including common vongole (whose local names include bibarasse, bevarasse, and pevarasse, and which are littlenecks, often harvested from the lagoon), vongole veraci (twin-horned true clams, caparossoli or caparozzoli in Venetian), and vongole filippine (manila clams, artificially seeded in the lagoon decades ago and now prolific here); cappe sante, or scallops, sold both in and out of the shell; schie, minuscule sweet gray shrimp from the lagoon, still jumping in their boxes; another kind of larger sweet-fleshed shrimp, called mazzancolle; zotoleti, the tiniest of cuttlefish; several sizes of scampi, which (American-Italian restaurant usage notwithstanding) aren't garlic-cloaked rubber shrimp, but delicate saltwater crayfish; exquisite little purple-red octopi known as moscardini and their larger cousins, which the Venetians call folpetti; small mussels (peoci in Venetian), sometimes born in the lagoon; sprawling coral-red grancevole (spider crabs); the elongated, light-skinned turbot called soaso; two kinds of mullet: triglia di scoglio (caught among the rocks) and triglia di fango, or barbone (from the sandy bottom); anguille (known locally as bisati), long, dark gray eels and a great traditional favorite grilled or fried; pescecane, or dogfish, a kind of small shark, prized for the flavor it lends to fish soups; passarini, literally "sparrows"—little rounded sole-like fish, best fried; and pagamei, bony little gobies, usually boiled with seasonings, then passed through a food mill and stirred into risotto.

Not everything in the market is fished in local waters—the fishing grounds of the northern Adriatic are closed annually for 40 days between July and September to allow stocks to replenish, so during that period everything comes from elsewhere (except for the fish, including the celebrated branzini, or sea bass, that are farmed in the area)—but what does come from the lagoon usually sells for a premium. Local seafood is labeled nostrano, "ours," a standard term all over Italy—but somehow here it seems literally proprietary. Somehow here it seems to define a whole cuisine.

In repeated visits to the fish market, we get to know Gigi Naccari, and start talking to him about the myriad ways the impeccable products he sells become great meals—both in home kitchens and at top Venetian restaurants like Fiaschetteria Toscana, whose seafood he supplies. "What do you eat at home?" we ask one day—fishing, as it were. In response, as we'd hoped, he invites us to join him for lunch one Sunday in June.

Naccari was born near the mercato del pesce and lived in Venice for many years, he tells us, starting to work at his father's stand when he was 14. Today he and his wife, Marisa, live in an attractive house on the edge of farmland in San Liberale, a mainland hamlet near Venice's airport—where they plan to offer us "just a typical weekend meal." It turns out to be a dazzling seafood feast.

With the Naccaris and their good friends Bruno Zancan and Carla Meneghin—he is also a fishmonger at the market, with the distinguished look and wit of a popular college professor—we sip prosecco on the awning-shaded terrace and nibble perfect little scampi in mayonnaise. We've barely finished when Naccari disappears and returns with a platter of small spiny lobsters and heaps of spider-crab meat—both simply boiled and brought to the table, we're surprised to see, with olive oil and vinegar on the side. "We don't use lemon juice on lobster or crab," he explains. "We don't think it improves the flavor." Following his lead, we drizzle olive oil over both, followed by a few careful drops of vinegar and plenty of salt and pepper. The acidity of the vinegar offsets the sweetness of the seafood and the earthy unctuousness of the oil; we've discovered a new little trick of flavor.

Marisa brings out a bowlful of moscardini that are only three inches long, floating in their darkened cooking water. "This octopus comes from Caorle, in the lagoon, and it's the best in the world," Naccari says, with a confidence that discourages doubt. "It's also the most expensive," Zancan adds. Naccari shrugs—as if to say, Well what did you expect?—spoons two octopi onto each plate, and instructs us: "Put one in your mouth whole and eat it plain, then cut the other into a few pieces and season it with oil and vinegar. That way you'll have two experiences." We do as we're told. The whole octopus is an explosion of flavor, sweet, salty, almost herbaceous; the pieces seem both more subtle in flavor and more complex; we love both, and above all love the way in which one dish has become two, so simply.

Next comes marinated fresh anchovies, deliciously oily, with Naccari's interpretation of the classic Venetian sarde in saor: Instead of frying the sardines, then marinating them in a vinegar-based solution, he puts them raw into an earthenware pot with onions, tomatoes, pine nuts, raisins, white wine, and olive oil and cooks them briefly, covered, on the stove top; then he lets the fish drink in the flavors of its sauce. Over steaming bowls of brodetto, an immensely flavorful but delicate version of the traditional Adriatic fish soup, Naccari talks about retiring late this year. "What will you do?" we ask. "Nothing!" he exclaims, happily. "I've earned it! I've worked there since the '40s!"

We finish our repast with a dramatic- looking and deeply flavorful dish of cuttlefish in its own ink with white polenta, and one of Carla's specialties, huge butterflied shrimp gratineed with bread crumbs. "Do you always eat like this?" we have to ask, amazed. "We don't eat this many things every day, of course," replies Marisa, "but we eat these things." She smiles and repeats, as if pronouncing some ancient Venetian mantra: "We eat these things."