Glenda McCreadie of Alexandra asks :-

While walking around the edge of a Central Otago dam, when the water was very low, I found about 10 white, chalky, hemispherical, button-like objects about 14mm in diameter. What are they likely to be?

Anthony Harris, an invertebrate zoologist at the Otago Museum responded.

The enclosed objects were gastroliths from the freshwater crayfish Paranephrops zealandicus.

Crayfish sometimes have a pair of gastroliths, made of calcium carbonate, on either side of the cardiac chamber, in the cephalothorax. Gastroliths are solid, hemispherical objects that are resistant to weathering and survive long after all other parts of freshwater crayfish have decayed.

Gastroliths are only present for a short time (about 40 days) before the crayfish moults and function as a repository of calcium for incorporation in the new exoskeleton after moulting. They also store calcium as it is removed from the old exoskeleton, so that it becomes softer, enabling it to be shed more readily. After the crayfish moults, the gastroliths fall into its stomach, where they can be reincorporated into the new outer shell, or cuticle.

New Zealand has two freshwater crayfish species, of which Paranephrops planifrons occurs in the North Island and northern South Island, while the larger P. zealandicus occurs in the east and south of the South Island and on Stewart Island. Both species are common in many lowland streams and rivers.

Fish, including trout and perch, and birds, such as shags, often take larger freshwater crayfish after moulting, when the exoskeleton is soft. Consequently, the gastroliths sent in to the Otago Museum could have come from freshwater crayfish that had been eaten by predators.