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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. 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.


                                      3. Sometimes they are "mudders", just glued together w/ stinky clam flat mud. They are heavy and do not open!

                                        5 Replies
                                        1. re: Passadumkeg

                                          I think that's it exactly. The instructions are usually "discard any that don't open" - that obviously implies that you're not going to eat them, but the point is that the shells are filled with mud, not that the clam/mussel was dead and spoiled before you cooked it. Because of the way the hinge works, bivalves open when dead, either from natural causes or from cooking. It's much more important to discard any that gape open, and won't close when disturbed, BEFORE you cook them because that indicates a dead one that could have started to spoil.

                                          1. re: FlyFish

                                            Yes, whenever I prepare mussels during the debearding process I ensure that all the mussels are alive by tapping the shell or running cold water over them and seeing if the shell closes completely. Using they are just a tad open still.

                                            1. re: Chinon00

                                              I pick them 200 yards from our house and in the pot 15 min. later. The kids are sick of muele mariniere, but not in spicy Dijon mustard cream sauce. Good on the grill too.

                                              1. re: Passadumkeg

                                                Can I come over for dinner? I'll gladly help harvest, or do it all. In AZ, mussels are a real crap shoot. Almost literally.

                                          2. re: Passadumkeg

                                            This happened to me the other night. We cooked a clam sauce and a couple that didn't open found their way to the finished dish. When we finally opened the shells, all of the mud came out into our dinner. Not good.

                                          3. I always thought this rule was strange because people eat raw clams and oysters all the time -- and since they are shucked, no one knows if they would have opened after cooking.

                                            2 Replies
                                            1. re: sbp

                                              People eat raw clams and oysters...and sometimes get sick. And that is why I will no longer eat oysters on the half shell at hotel buffets, no matter how upscale the hotel.

                                              And I agree with FlyFish and Chinon00. Tapping the mussels before cooking is most important.

                                              1. re: sbp

                                                I eat Horse mussels on the half shell., but blue mussels are too much work for too little.

                                              2. I thought that it was only the second half of the rule. dispose of any open clams before you cook them, then get rid of any that dont open when you cook. That way you most likely got rid of all the dead ones.

                                                2 Replies
                                                1. re: KaimukiMan

                                                  That was our household rule when we went clamdigging in SoCal. I never got sick and I'm still here. :)

                                                  1. I can't imagine anyone wanting to pry open a shellfish to eat it if it didn't open during the cooking process. Though I haven't ever done it, I would think they would smell off.

                                                    2 Replies
                                                    1. re: Scargod

                                                      I cooked mussels for the first time today, fresh and not frozen like I usually do and shortly after purchase I saw a few were open so I discarded (about 3 or so) but then I learned that if you pinch them a bit between your finger and thumb and they close means they're alive, so I did that to all of the ones that were open when I was ready to cook them and it was true, they closed, also made quite a bit of noise. After scrubbing, rinsing and putting them in the pot to steam some some of them didn't open, though all of them were closed when they went in.. I automatically assumed they were bad but I was curious and wondered if I was supposed to pry them open but thank God I resisted the temptation! Something just seemed "wrong" about them so they were left alone and went into the garbage.

                                                      1. We used to love this little Italian place nearby, and we would always get really great meals and great service. One night my husband ordered zuppa de pesce- and was served a gorgeous bowl with about 7 unopened mussels. He LOVES mussels and was bummed out that all of the ones he was served he couldn't eat. Our sweet waitress came over to check on us and saw the bread plate full of the mussels and asked if she could take that. He told her sure, that he couldn't eat them because they hadn't opened. We continued eating and she reappeared with a steaming bowl - mussels with more sauce on them. "Here you go, the cook just cracked them all open for you-enjoy!"
                                                        We just looked at each other-grew up NEVER eating unopened shellfish because we both were always told they make you very sick. He left them there.
                                                        **we don't go there anymore because they changed owners and the food is terrible now

                                                        1. I know this post is really late, but I just now saw the thread. I steamed some clams tonight for the first time and most opened. I thought the last 4 were dead but then I read that some clams might be a little slower than others and to try a few more minutes. It took an additional 5 minutes, but they did open. I was wondering though... Can you steam them so long that even the dead ones will pop open or were they just little fat ones that needed the extra time? Thank you for any help on this. I had to cook them tonight but they will be eaten tomorrow evening.

                                                          1 Reply
                                                          1. re: Denekie

                                                            Some clams DO take longer than others to open, & I've gladly eaten those without any adverse reactions. Others might disagree, so I think it's pretty much up to you.

                                                          2. Tap shellfish on counter before cooking, discard any that don't close (they are dead). Cook until the shellfish reach a safe temperature (145 F), at which point they will open. Not all of the shellfish in a pot will reach this temperature at the same time, so remove the open ones and continue to cook the others, removing them as they open. If you have a few that refuse to open, they are probably full of mud, so toss them.

                                                            1 Reply
                                                            1. re: pikawicca

                                                              Thank you very much Pikawicca :-)

                                                            2. I have worked in the shellfish business for years and now run my own shellfish company. I have eaten lots of unopened mussels and here is why http://www.abc.net.au/science/article...

                                                              There are lots of myths around shellfish that are no longer relevant due to the harvesting/cleaning and purifying methods see here


                                                              1. Regarding mussels, it's a myth, an old wives' tale. See this article from the Australian Broadcasting Commission, for specific advice:


                                                                Summary: Dangerous mussels smell bad. The most dangerous will be among those whose shells open early, before they've had time to cook away pathogens. Mussels shells opening or closing have nothing to do with a mussel's safe edibility.

                                                                1. It's a myth..common misconception.. It's not if they don't open but if they're ALREADY open when you buy them which indicates that they are dead.. Think about it this way.. A clam must be shucked when eaten raw.. How is one to know if these raw ones that must be pried open would open or not when they are cooked..you wouldn't because it doesn't matter, they're alive and fresh hence having to shuck them.. It's simple .. Already open= dead... Tightly closed= alive

                                                                  1 Reply
                                                                  1. re: Clugst36

                                                                    I also love the inherent contradiction in the standard advice about cooking shellfish. First check to see if the shell is closed or open. If open, shellfish is dead so discard and do not cook. After cooking, if shellfish does not open, it was dead before cooking so discard as only shellfish that were alive before cooking open their shells upon dying. Apparently the originally dead mussel/clam was able to hold its shell as tightly closed as a live one before and after cooking unlike the other dead ones which couldn't keep the shells closed because they were dead. Huh? This is advice propagated by people who have not spent a lot of time on the water collecting shellfish. You're more likely to get ill from live shellfish that have been improperly harvested. For example, after a heavy rain I wait at least a few days before I go and collect mussels/oysters/clams. The runoff clouds the water with all sorts of crap that they will just filter out of the water. I don't think the commercial harvesters in my area can always wait to do that.

                                                                    This canard ranks up there with don't go swimming for an hour after swimming otherwise you will cramp up and drown.