How do I buy and keep live shellfish at home?

How to judge mussels, clams, oysters and live crabs at the counter, how to keep them alive in a domestic fridge, and how long you actually have before they are no longer worth cooking.

Difficulty: beginner
Time: 10 min
Read: 4 min
Safety: caution

Short answer

Buy from a busy counter, in a net or open bag rather than sealed water, and ask when they came in. At home, tip them into a bowl, cover with a damp cloth, and put them in the coldest part of the fridge — never sealed in plastic and never sitting in fresh water. Cook mussels and clams within a day, oysters within a few days.

Live shellfish is the one thing on the fish counter that is still an animal when you buy it, and everything about handling it follows from that. It needs to be cold, damp and able to breathe, and it dies quickly if any of those fail. Nothing you do at home improves it — you are only slowing the rate at which it gets worse.

Safety

Live bivalves are the highest-risk thing most people bring home from a fish counter, and the risk is almost entirely set before you own them — by which water they grew in and how cold they were kept. Buy from a supplier who can tell you where they came from, keep them cold and able to breathe, and cook them the same day or the next.

What you'll need

A bowl and a damp tea towel or newspaper
Fridge space in the coldest part
Coarse salt, for purging clams (optional)

Step by step

Buy where the turnover is high.Live shellfish sells or it dies. A busy counter that gets deliveries several times a week is worth more than any amount of inspection at a quiet one. Ask what day it came in — a counter that cannot answer is a counter to walk away from.
Check how it is being held.Mussels and clams should be cold, damp and loose — in a net, on ice, or in an open box. Shellfish sitting in a sealed bag of its own water, or in a pool of fresh water, is suffocating. Oysters should be stored cup-side down so they keep their liquor.
Look and smell before you buy.The whole batch should smell of clean sea and nothing else. Shells should be mostly closed, wet and heavy for their size. A tray with a lot of gaping, dry or broken shells has been sitting.
Get it home cold.A cool bag with an ice block, and the shellfish stop on the way home rather than the first. An hour warm in a car boot costs you more shelf life than a day in the fridge.
Unpack it as soon as you are in.Tip it out of the bag into a bowl or colander set over a plate, so any liquid drains away rather than drowning the ones underneath. Pull off any obviously broken shells now.
Cover with a damp cloth and refrigerate.A damp tea towel or damp newspaper over the top, in the coldest part of the fridge, usually the bottom shelf at the back. Damp keeps them from drying out. Covering with cling film or a lid does not — it seals them in.
Never store them in fresh water.Tap water kills marine bivalves within an hour or two. If you want to purge clams, do it in cold water salted to roughly sea strength — about 30g of salt per litre — and for no more than a couple of hours, then lift them out rather than pouring.
Cook within the window and check again at the pan.Mussels and clams: the day of purchase, at a push the next. Oysters: several days, sometimes a week, if they are kept cold and cup-down. Live crab and lobster: cook the day you buy. Sort again immediately before cooking, because the batch will have changed.

Tips

Debeard mussels at the last moment. Pulling the byssal threads out injures the animal, so a debearded mussel has a shorter life than one you left alone.
Scrub mussels under cold running water rather than in a filled bowl, so grit goes down the drain instead of settling back onto them.
Live crab and lobster should be lively rather than limp. A crab that does not react and a lobster with a hanging tail have been out of the water too long.
If you cannot cook them in time, cook them and then keep the cooked meat rather than letting the live animal deteriorate. Cooked mussel meat keeps a day in the fridge and freezes acceptably.
Frozen half-shell mussels and frozen cooked prawns are perfectly respectable and far safer than live shellfish held too long. Use them when the timing does not work.

Common mistakes

Sealing live shellfish in a plastic bag or airtight box — They need oxygen. Sealed, they suffocate within hours and you have converted a live animal into a spoiling one without noticing.
Storing them in tap water so they 'stay fresh' — Fresh water kills them. If you want them in water it must be salted to roughly sea strength, and even then only briefly.
Buying live shellfish at the start of a long shop — The clock is thermal. Anything above fridge temperature shortens their life sharply, and a warm hour is not recovered by chilling afterwards.

Questions people ask

How can I tell whether a mussel or clam is still alive?

A closed one is alive. An open one should close, slowly, if you tap it or squeeze the shells together — give it a full minute, because cold shellfish is sluggish. One that stays limply open, or a shell that is broken, is out. Bear in mind that a recently dead one can still look and behave convincingly, which is why the storage matters more than the test.

Do I need to purge mussels?

Rope-grown mussels, which is nearly all UK farmed mussels, have very little grit and need only a scrub. Wild or dredged mussels and most clams benefit from an hour or two in cold salted water. Adding flour or oatmeal to the water is folklore and does nothing useful.

Can I freeze live shellfish?

Not in the shell while alive — it kills them slowly and the texture suffers badly. Cook them first, pick the meat, and freeze that with a little of the cooking liquor. It is a compromise, but a usable one.