Why freshwater crayfish don’t need milk for healthy bones

Two curious looking ‘stones’ collected from a dam near Lake Yindarlgooda out in the Goldfields were brought into the Museum by inquisitive members of the public. These mushroom shaped stones are made of calcium carbonate, the main mineral in human bones, coral skeletons and the shells of oysters and abalone.

As it turns out these particular ‘stones’ have actually come from inside the stomach of a freshwater crayfish and are called gastroliths (literally stomach stones). Not to be confused with the gizzard stones found in birds which are used to help grind and digest food, crayfish gastroliths actually 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. The calcium provides strength to the exoskeleton so that it can support the animal’s body, give the claws their pinching power and to protect it from predators. 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.

Gastroliths have long been used by people: they were made into adornments and the remains of necklaces made of gastroliths have been found associated with Indigenous skeletal remains. Gastroliths were even used in early dentistry to fill holes in teeth.

Gastroliths have been used in traditional medicine for their absorbent and antacid properties as well as treating a number of disorders including the plague, a cure for syphilis, bladder stones (recommended by Martin Luther and others!), convulsions and vomiting of blood. While this may sound like old wives tales, modern pharmaceutical companies are actively researching the use of gastroliths to treat osteoporosis related conditions.