On a nearly-sunny
January afternoon, I and a group of Wild Art kids stumbled upon
something we'd never seen before: About twenty little round
calciferous half-spheres, deposited a few meters away from a creek. Not exactly uniform but almost, the little things were
approximately 8mm 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 meter 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 of Ecoleaders, who are
extremely knowledgeable about freshwater ecosystems. In addition to being some of the kindest people I know, they are also my neighbours, so 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 our 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 Biodiversity 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 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?
