Crayfish Gastroliths

It’s the time of the year when it used to become legal to catch and sell Swedish crayfish (since 1994 there is no limit), and so the grocery stores sell Turkish and Chinese crayfish for a few weeks. The traditional way to eat them is to boil them with dill, salt and a little sugar, and serve them with toast, strong cheese, beer and akvavit. I don’t drink but I love shellfish, so crayfish time is always a treat for me. My wife, being refreshingly unorthodox about traditional Swedish customs, and indeed about all traditional customs thanks to a Maoist childhood, served crayfish with smoked shrimp, aïoli and boiled potatoes last night.

There’s a fun detail about these animals: sometimes you find a pair of little white buttons in their heads. These are known as kräftstenar in Swedish, “crayfish stones”, and gastroliths in English. (The same word is also used for actual stones eaten by crocodiles, birds and other dinosaurs to help digest their food.) As Andrew Hosie of the Western Australian Museum explains:

… crayfish gastroliths … represent a remarkable physiological process to conserve calcium.

Much like people require calcium for strong and healthy bones, so too does a freshwater crayfish to maintain its armour. … As crayfish (indeed all crustaceans) grow bigger, they must periodically shed the exoskeleton and form a new one. To start a new exoskeleton from scratch would require large amounts of new calcium.

The hormones that drive moulting (referred to as ecdysis) trigger calcium carbonate to be removed from the exoskeleton and starts forming a pair of these gastroliths in the stomach. After the crayfish has moulted, the gastroliths are reabsorbed and used in the strengthening of the new exoskeleton. Only freshwater crustaceans form gastroliths because unlike seawater, freshwater has very little dissolved calcium salts, so in an effort to retain calcium, crayfish form these little gastroliths, or even eat the old exoskeleton.

Check out the strange story of what my friend Eddie unexpectedly caught in his crayfish trap.


  1. #1 GregH
    August 24, 2014

    Very interesting. Why is it no longer legal to buy Swedish crayfish?

  2. #2 Martin R
    August 24, 2014

    Sorry, poor phrasing on my part. Since 1994 there is no longer any limit on when you can.

  3. #3 Birger Johansson
    August 24, 2014

    Crayfish consist (in my experience) of 80% shell and 15-20% meat. Like surströmming you have to invest far too much time in separating edible stuff from gunk.
    — — —
    Unfortunately the native Swedish variant of crayfish is endangered since another variant -contaminated by parasites- has been introduced.

  4. #4 Birger Johansson
    August 24, 2014

    (OT) “Europe’s oldest village sought under Greek bay” (Update)
    The village/source of the artifacts found is believed to lie beneath the current coastline.

  5. #5 Birger Johansson
    August 25, 2014

    Do not eat Dr. Zoidberg!
    — — —
    Incidentally, the new Laundry novel is out. Yayy!

  6. #6 Eric Lund
    August 25, 2014

    Crayfish consist (in my experience) of 80% shell and 15-20% meat.

    I have never had crayfish, but I have eaten a related species, lobster. (Perhaps they are even the same species, much as Swedish elk are the same species as North American moose.) It’s the same story with lobster: mostly shell with a little bit of meat. Crab, a somewhat more distant relative, can be the same way, depending on species (but other crab species are soft-shelled and therefore have a higher proportion of meat).

    I’ve never been especially fond of lobster, but that may have something to do with how it’s prepared in this country–the US, especially the New England region, absorbed some of the worst habits of British cooking. (We make up for it the same way the British have: by incorporating techniques from other cuisines, in our case brought over by immigrants.) I’ll sometimes eat crab. The only times I have ever had lobster have been in group settings where somebody else chose the menu.

  7. #7 Birger Johansson
    August 25, 2014

    (OT) Skeptic alert: Confusion about issues in crusade against GM organisms:
    — — —
    BTW Our conservatives in Svithjod* are positive to bringing in more refugees from Syria and elsewhere (while being open about potentially high short-term costs).
    I don’t like the conservatives but in this issue they stand out from their brethren in Britain and USA in a positive way.
    * this is how Snorre Sturlasson spelled it.

  8. #8 Martin R
    August 25, 2014

    We have lobster (hummer), sea scampi (havskräfta) and freshwater crayfish (kräfta) in Swedish waters.

    Svi-thiod = Swedish people. Cf. theodisc, “of the people” –> Deutsch.

  9. #9 Birger Johansson
    August 25, 2014

    Damn! My ignorance is exposed for all to see.
    We have a song for rock lobsters, but crayfish and sea scampi have nothing. Discrimination!
    — — —
    John, FYI: “Kate Bush’s most stylish moments – in pictures”

  10. #10 John Massey
    August 26, 2014

    Thanks, Birger. The Babooshka outfit was just outlandishly provocative, complete with multiple bladed weapons.

    Currently very busy working on two engineering projects (creating land for much needed housing) and proposals (i.e. bids) for five others. I turned 65 a month ago, and I don’t recall a period of my life when I have worked harder or more productively. The real irritation is that you have to persuade people to let you work. (“Oh but surely you would be happier being retired and doing what would like to do?” I am already doing what I like to do.)

  11. #11 Thinker
    August 26, 2014

    Birger @9: “We have a song for rock lobsters, but crayfish and sea scampi have nothing. Discrimination!”

    I feel your pain — here’s something to alleviate it, sung to the tune of “Clementine”:

    Oh my darling, little crayfish, oh my darling little bit,
    Thou art meant and was forever to be drowned in aquavit!

    In the lakes and in the rivers that go flowing to the sea
    there are scores of little crayfish growing large enough for me.

    Oh my darling…

  12. #12 Martin R
    August 26, 2014

    Thinker! Good to see you around the blog!

  13. #13 Birger Johansson
    August 26, 2014

    John, today K B starts a new set of concerts in Britain, the first such in decades… will be interesting to hear what she is up to.
    Angleton in the Laundry did not have to worry about enforced retirement, except assasination. Even then, he had the option of getting summoned into a new “meat suit”.
    Martin, Lidl provides new brands of goat cheese, if you want to try it. (Since I eat neither that or crayfish I don’t know if they mix)

  14. #14 Birger Johansson
    August 26, 2014

    (OT) “Ancient shipwreck off Malta leaves 700 BC cargo”
    This could be a window into the earliest iron age, when the Greeks still viewed the Phoenicians as trading partners instead of rivals, shortly after the Phoenicians started colonizing the West and just as the Greeks started their spread around the mediterranean.

  15. #15 Birger Johansson
    August 26, 2014

    (OT) Discworld Diary 2015: We R Igors: First and Last Aid
    I saw this one is available now.

  16. #16 John Massey
    August 28, 2014

    I’ll be pulling an all-nighter tonight. People who work as civil engineering consultants working on proposals know exactly what I’m taking about. Caffeine and sedatives help; the caffeine to keep you awake, and the sedatives to prevent you from panicking.

  17. #17 B
    Prince George, BC, Canada
    September 15, 2014

    If there is no longer any limit on the amount of crayfish that one may catch, why do the stores sell imported crayfish at this time? Wouldn’t times when local crayfish are not available be when you would want to sell imported crayfish?

  18. #18 Martin R
    September 16, 2014

    Huge price difference.

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