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Sep 15, 2008 03:32 PM

Why is it dangerous to eat shellfish that don't open?

I know you aren't supposed to eat shellfish that don't open during cooking, but why? What's the reason behind this? I always wondered.


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  1. If they don't open, they're dead already, and there's no way of knowing for how long. Rotten mussels or clams are not something you want to ingest. You want those guys still bouncy and healthy until you kill them - sorry, but it's true. Same with lobsters, same with shrimp if they've still got their heads on.

    22 Replies
      1. re: Kosmonaut

        No it doesn't. It's a myth. See below.

      2. re: Will Owen

        Why is it the same for shrimp that still have their heads? I buy shrimp with heads on all the time that are dead and haven't had any issue (yet).

        1. re: Miss Needle

          Ok. To answer my own question, I found this article that says:

          "...head-on shrimp deteriorate rapidly. An enzyme in the head of the shrimp travels down the tail and begins to eat away at the flesh as soon as the shrimp dies...quick-frozen head-on shrimp that are thawed carefully can be very good. We suggest buying head-on shrimp frozen whenever possible"

          As I buy my head-on shrimp in Chinese markets (haven't found any other place that has them), that's probably why I haven't had any issues as the turnover is pretty quick and their source is the frozen blocks. I think I'll just buy a frozen block of head-on shrimp in the future as an extra precaution. Thanks Will Owen for mentioning the shrimp thing!

          1. re: Miss Needle

            On the coast of Maine, head and good suckin' roe and best of all a buck a pound in season. These are northern shrimp, small, but sweet.

            1. re: Miss Needle

              I've always heard that if you can't buy them live, the best head-on alternative is the one you mention - frozen in a block of ice. Patience is not one of my virtues and our freezer holds too many ice cream and snack options for our kids, so a block of ice sealing in my crustaceans is usually not an option. I have learned my lesson though - made the mistake of buying jumbo U12 shrimp tails for a special dinner once - looked great in the case but after cutting and cleaning the tails, the meat, particularly along the back ridge was mushy and mealy - obviously victims of active enzymes. Went out to dinner instead...

              1. re: bulavinaka

                my nephew has frozen grouper for me in a block of ice; it worked well, and there was no freezer burn.

                1. re: alkapal

                  Grouper is not a shell fish. This is a thread about why it's dangerous to eat shellfish that didn't open. Somebody posted a comment that there is a similar rule for shrimp (a different kind of shellfish).

                  I don't think anyone would argue that putting scaled fish on ice is a good idea.

                  1. re: rubinow

                    rubinow, i was born and raised on the gulf coast in florida. i know grouper is not a shellish, please! i'm sorry, but i didn't realize you are moderating this thread. my post was prompted by earlier posters' observations. my POINT about the grouper WAS a tangential point about seafood IN ICE. geesh!

                    and, for the people who read, i was referring to grouper *filets* in ice, (caught and cleaned by my nephew) which i brought up on the plane in my carry-on when i returned to d.c. i prepared it caribbean style, and it was delicious (remarkably, with no discernable alteration in flavor or beautiful texture, despite its icy sojourn).

                    1. re: alkapal

                      I knew where you were coming from on that grouper response... thanks!

                      1. re: alkapal

                        Wow. Didn't claim to be a moderator.

                        Is there any other fish that packs well in ice that we should know about?

                        You're right: your comment is tangential. I know you were born and raised on the gulf coast in Florida and everything, but most fish is shipped in ice. It's actually the preferred method of doing it.

                        What shipping a grouper in ice has to do with shrimp with heads going bad is anyone's guess.

                        1. re: rubinow

                          I think the well-respected alkapal was referring to seafood holding up extremely well in a block of ice, as opposed to just, say, ice chips or cubes. Unless you know that the shrimp in the "fresh" fish case has been very recently defrosted, buying some is a gamble. It can be purchased frozen in a block of ice. All Chinese markets carry it, as well as many conventional markets as well. I hope this cancels out the confusion that I started...

                          1. re: bulavinaka

                            thank you, bulavinaka, for your compliment and understanding!
                            indeed, it is *remarkable* how the icily-ensconced fish holds up!

                            1. re: bulavinaka

                              Wooly mammoths also hold up well in a block of ice.

                            2. re: rubinow

                              I once froze some squirrels that I had shot in a block of ice, after I had skinned and cleaned them.

                              1. re: redfish62

                                But why would you shoot a squirrel in a block of ice? Wasn't it frozen to death already?

                      2. re: Miss Needle

                        Our local Harris Teeter market frequently carries head-on shrimp, & one thing I've found is that they not only have a short shelf life raw, but an even shorter one cooked. While I always eat leftover cooked seafood within 48 hours, the leftover cooked head-on shrimp stank to high heavens just the following day, so I learned my lesson there. Definitely peel them & de-head them completely before fridging them for later use. Ugh.

                    2. re: Will Owen

                      That's a great explanation, thanks! So I take it the fact that they open during cooking is their adrenaline/fear response to being cooked.

                      1. re: Chew on That

                        I think it's more like the act of dying rather than fear, which means they weren't already dead when you applied heat. But I don't know that for certain. Not sure if shellfish have an adrenaline response or not.

                      2. re: Will Owen

                        Glad I read replies, before posting, because you said everything that I was going to add, just well before I hit this thread.


                        1. re: Will Owen

                          This is a myth. See article cited below by a couple posters.

                        2. Ummm....I think it's because their dead. The shell opens as the mollusk is giving up the ghost, so if it doesn't open, they were dead for who knows how long? I just made most of this up, but there may be a pearl of truth in this oyster!!!

                          1 Reply
                          1. re: adamshoe

                            If you cook the shellfish in a semi-dark room, and backlight the pot, and are very quiet, you can see the little mollusk souls escaping.

                            OK, so I WAS making that one up, but you're correct on the reason to NOT eat a cooked bi-valve, that does not open.


                          2. The original comment has been removed
                            1. Shellfish should be cooked alive. And you know that they were alive if they open after cooking. Dead ones stay closed. Cooking and eating shellfish that were dead is dangerous because once dead I beleive that they breakdown in a manner that makes eating them harmful.

                              1. I can understand a tenacious mollusk that held up through a short cooking interval and fought off the intruders... but how does a dead one stay clamped shut?

                                11 Replies
                                1. re: Veggo

                                  I'm not sure why that it is, but from what I understand, it's a clear indication that the clam or mussel is dead and not to be eaten.

                                  1. re: Veggo

                                    maybe the hinges become brittle and harden

                                    1. re: dumpycactus

                                      I've heard that they open while cooking because while they are still alive, they oven their shell to try to cool themselves off. If they die of something else, they don't open up.

                                      1. re: gjd131

                                        Great story, but biologically preposterous. A dead clam opens up, period. The hinge is always trying to open the shell, and it's only the action of the adductor muscles (muscle, singular, in the case of scallops) that keeps them closed. When the animal dies, the muscle dies, and the shell opens.

                                        1. re: FlyFish

                                          That still doesn't explain why a dead one would stay closed even after cooking, going by what you said, they should have already been opened before you started cooking. Plus, I didn't say that I was an expert on the subject, its just something I heard on Alton Brown or some other show.

                                          1. re: gjd131

                                            They are dead! Do you move when dead?

                                            1. re: gjd131

                                              That's assuming that (1) a clam that is dead before cooking would be closed, and (2) a previously dead clam, after cooking, would remain closed. Both assumptions are incorrect.

                                              1. re: FlyFish

                                                You've never put all closed clams in the pot and then had one or two that never open? That was the whole point of the original topic. Anyway, here's an answer I found, they basically say that the shock of being boiled alive is what does it.


                                                1. re: gjd131

                                                  The "google anwers" response contains several errors and is itself internally contradictory. Paramyosin is not unique to clams, or even to molluscs, though it certainly is a major component of clam adductor muscles. It allows the adductor muscle to maintain tension at a very low expenditure of energy, a concept in invertebrate physiology knows as the "catch mechanism." "Very low" does not equate to zero, and a dead clam is unable to provide energy to its muscles, and they in turn lose tension - in addition, of course, the muscle proteins are denaturing due to the elevated temperatures of cooking.

                                                  The point is that all closed clams are maintaining that closure by the catch mechanism involving paramyosin - the idea that a closed clam that's somehow previously disturbed uses its paramyosin to maintain closure but one that you sneak up on doesn't is simply scientifically untrue. Even assuming for the sake of argument that disturbed clams are the ones using paramyosin, surely being placed in boiling water (they don't die right away, after all) would be sufficient disturbance for them to die in what the author refers to as a "contracted" state, so under that logic none of them should open when cooked. I'll give him the benefit of the doubt and assume that he had his tongue firmly planted in cheek when he refers to a clam experiencing "panic and shock," but as you seem to have taken that part literally - we can argue over the concept of whether a clam experiences "shock" - in the clinical sense it probably does undergo some short of shock. But I can assure you that the nervous system of a bivalve is unable to formulate an emotion such as panic (or anything else). And obviously, if it did, its response would be to "clam up" not gape open.

                                            2. re: FlyFish

                                              Say what you will, but I don't buy them opened and I don't pry them open to eat them if they don't open by themselves during cooking.

                                              I was banging around on oysters I had just harvested and found that they opened slightly which made it easier to get the knife into them and pry them open.

                                        2. re: Veggo

                                          It is due to rigor mortis. The bi-valve dies, and its muscle contracts in death, holding the shells shut.