The mystery by the creek: Solved!

On a nearly sunny January afternoon, a group of Wild Art kids and I stumbled upon something we’d never seen before: About 20 little round, calciferous half-spheres, deposited a few metres away from Killarney Creek. 

Not exactly uniform but almost, the little things were approximately eight millimetres in diameter, and seemed similar to sand dollar skeletons. They looked a little like covered buttons. 

However, when we broke one open, the inside appeared to be solid, comprised of pinkish calcium carbonate. As for other clues in the area, the half-spheres were found on a bit of pristine forest floor, surrounded by needles and cones, about a metre or so above the flood-level of the creek. The only other item of note in the area was the claw of a signal crayfish. We puzzled about it for quite a while, and took a few home to research. 

The most obvious thing to do was to consult Sue Ellen Fast and Will Husby, who are extremely knowledgeable about freshwater ecosystems, particularly Killarney Creek, which they live very close to. 

I took some of the little half-spheres by their house and Will had a good look. Will has easily cleared up some previous Wild Art mysteries, such as the identification of our local signal crayfish, freshwater sponges and freshwater fingernail clams. However, on examining these little half-spheres, he was stumped.

So off to the local Facebook forum, where I could easily post a photo of our mysterious find, and get some responses. I also personally emailed the photo to a few other knowledgeable locals and the curator of marine invertebrates at the Beaty Museum. 

Understandably, most people looking at the photo thought they looked like mushrooms or seeds. 

However Sue Ellen had done a vinegar test and confirmed they were indeed calcium carbonate, so mushrooms or seeds seemed out of the question. 

Other suggestions ranged from urchins to discarded candies or drugs, fossilized berries, concretions from garbage left in the park, or tiny geodes. We all see through the lens of our own experience.

And then, unexpectedly, the answer appeared in my email. Will had come through, after all, having followed a hunch, based on my finding of a crayfish claw nearby. 

What we have found are gastroliths! 

Will says, “They are found in freshwater crayfish (and) are part of a system for conserving calcium used in making their exoskeletons.” 

He speculates that they were part of an otter’s poop, which was left on the creek bank before being eroded by rain and leaving only the gastroliths behind. 

Andrew Hosie of the Western Australian Museum explains on his blog (1) that “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.” 

He also tells us that “pharmaceutical companies are actively researching the use of gastroliths to treat osteoporosis-related conditions.” 

Isn’t it wonderful how one mysterious discovery can bring people together and open our minds?

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