Crayfish Gastroliths

Kraeftstenar

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.