Sold by size rather than species on the east coast, and the size decides the use — littlenecks for steaming whole, cherrystones for the grill, big quahogs for chowder because they're too tough to eat any other way. The one universal step is purging the sand.
Hard-shell clams on the US east coast are all Mercenaria mercenaria at different ages: littlenecks are the youngest and smallest at seven to ten per pound, topnecks next, cherrystones larger, and quahogs or chowder clams the oldest and toughest. Soft-shell clams — steamers — are a different species with a thin brittle shell and a protruding siphon that can't fully retract, which is why they're never fully closed and why the old tap-test doesn't apply to them. Manila clams, the small ridged ones common on the west coast, are sweeter and cook fastest. All of them filter-feed by drawing water through a siphon, which is how they end up full of sand.
Clams live buried in sand and mud and pump water constantly, so their gut and mantle cavity hold grit. Purging works because a live clam continues to filter: given clean salted water it will expel sand and take in clean water over an hour or two. Fresh water kills them and makes it worse. Salinity matters — around 35 grams of salt per litre approximates seawater, and tap-water salinity that's too low stresses them into clamping shut rather than filtering. Cooking is a matter of the adductor releasing: heat denatures the muscle holding the shells together and it lets go, which is why an open shell reads as done. That takes only a few minutes, and clams held past that point shrink and toughen fast.
The clams release their own liquor as they open, so they create the broth they cook in — no stock needed.
HowA little wine, garlic and butter in a wide pot, clams in, lid on, high heat five to eight minutes. Remove each one as it opens rather than waiting for the last, since the early ones overcook while you wait. Strain the liquor through cloth to catch grit.
Direct heat opens them fast and concentrates the liquor rather than diluting it, and the shells char slightly.
HowStraight onto a hot grill or into a 500°F oven, four to six minutes until they pop open. A spoonful of garlic butter into each as it opens.
Quahogs are too tough to eat whole and are the traditional chowder clam precisely because chopping solves that and their liquor is the strongest.
HowSteam open, chop the meat, and strain and keep every drop of liquor as the base. Add the chopped meat back at the very end — simmering it into the pot turns it to rubber.
Littlenecks and topnecks are sweet raw and cheaper than oysters, with a firmer, more mineral bite.
HowWell chilled, shucked with a clam knife — thinner and sharper-edged than an oyster knife — with lemon and horseradish or a mignonette.
Sand is the failure that ruins an otherwise good pot, and it can't be fixed after cooking — once the shells open, any grit inside disperses into the broth. Purging means an hour or two in cold salted water at roughly seawater strength, around 35g of salt per litre, in a bowl where the clams sit on a rack or elevated so the expelled sand falls below them rather than getting re-filtered. Fresh water does not work; it kills them and they stop filtering. The second error is repeating the standard advice indiscriminately. The rules are: a live hard-shell clam is closed or closes when tapped, and one that stays open is dead and gets discarded. But soft-shell steamers physically cannot close, because their siphon protrudes — judging them by that rule means throwing away the whole bag. For steamers, the test is whether the siphon retracts when touched.
Buy live, in mesh rather than sealed plastic, from somewhere with turnover. Shells should be intact and unchipped and the bag shouldn't smell of anything but the sea. Store in the fridge under a damp cloth, never in water and never sealed airtight. Size selection matters more than with most shellfish: littlenecks for steaming and pasta, cherrystones for grilling and stuffing, quahogs only for chowder. Manila clams are excellent and cook in half the time. Frozen and canned clams are legitimate for chowder and inferior for anything else. A pound per person for a main, half that as a starter.
Doneness: Not a temperature cut — done is when the shell opens, which takes four to eight minutes. Pull each as it opens. A clam that hasn't opened after ten minutes was likely dead going in; a stubbornly closed one can occasionally be a strong adductor, so prying it and checking that it smells clean is reasonable, but when in doubt discard it.
Mostly yes, and the reason is that an unopened shell usually means the clam was dead before cooking, which means it may have been degrading for a while. Occasionally a live clam has an unusually strong adductor and stays shut through cooking. If you want to check, pry it and smell — a clean sea smell is fine, anything sour or ammoniac is not. Given the cost of getting it wrong, discarding is the sensible default.
One to two hours in cold salt water at roughly 35g per litre is enough for most farmed clams, which are often purged commercially already. Wild-dug clams may want three to four hours or a change of water. Longer than about four hours starts to stress them. Put them on a rack or in a colander suspended inside the bowl so the sand they expel doesn't sit where they can filter it back in — that detail matters more than the duration.
Same species at different ages, and the age determines everything. Littlenecks are youngest, most tender, and the ones to steam or eat raw. Cherrystones are bigger and slightly chewier — good grilled or stuffed. Quahogs are old, large and genuinely tough, which is why they get chopped for chowder rather than served whole. Buying quahogs for a pasta dish is the common mistake.