Crayfish Gastroliths
Canadian quarter is 23.88 mm in diameter.
I can’t remember when, but a couple of years ago, I think it was a co-op student at work, or maybe a new instructor, came over and asked me what some little thing was that they had found. It was small, flat yet round and slightly depressed on one side, like a danish or a donut whose hole didn’t make it all the way through. I examined it and took a couple of photos, and told them what I thought. “Probably a seed of some kind”, I said, but I never did figure out what fruit that seed came from. Turns out it wasn’t a seed after all but something a little bit stranger.
You likely guessed it by the title of this post, that what I once thought was a strange seed is actually a gastrolith. I was going to entitle this post “Gastroliths” but it turns out that there are a couple of different types of gastroliths. Really gastrolith is a very simple term translating from the Greek to mean “stomach stone”. So any stone which ends up in the stomach of a bird, reptile, or some mammals might be called a gastrolith. But for Crayfish it’s a little different. They don’t just eat some rocks to help with digestion as a bird or some mammals might. Instead, they produce the stones.
Crayfish have exoskeletons which are hard, rigid impermeable cuticles which are secreted by a layer of living tissue called the hypodermis (the “underskin”) which is strengthened by calcium salts. This exoskeleton armour has similar functions to our skeletons in that it provides support for internal organs and creates a framework for the attachment of muscles. For Crayfish, and many other invertebrates, this is on the outside, hence the prefix exo- which comes from Greek meaning outer, outside, or external.
Crayfish moult this exoskeleton from 6-11 times in the first year of life, and then the number of moults goes down to 4-5 in the following year. They moult because the hardened or “sclerotized” exoskeleton can’t grow with the Crayfish, they’ve got to leave it behind and grow a new exoskeleton. When the Crayfish are ready to moult they become less active and get ready to crawl out of their own “skins”. But this poses a problem. Crayfish imbue their armour with a lot of calcium to make it harder and tougher. Some Crayfish, like the ones who live in the ocean, have a lot of access to calcium in the form of shellfish, or other lifeforms with shells made of calcium carbonate, but the freshwater Crayfish have a harder time of finding all the calcium required to sclerotize their exoskeleton. They can’t just throw that old exoskeleton away.
Crayfish gastroliths, dirty as we found them, Saugeen River, 18.07.2021
Cleaned gastroliths, months later, 22.12.2021
So, life found a way. Instead of just ditching all of the calcium they reabsorb it from the exoskeleton and transfer it into the gastroliths, which are these rigid cookies made of interwoven fibre-like strands of chitin within which the calcium is deposited. Imagine a hardened candy shell of a M&M, but where the shell is made of the same stuff that Ant and Spider bodies are made of. Now fill that shell in with calcium and you are starting to get the idea of a gastrolith. It’s like a little purse full of calcium which they will then use to fortify the new exoskeleton.
These gastroliths are made by the gastrolith disks, specialized patches of tissue on the stomach wall. They are made from the calcium absorbed from the exoskeleton they are about to moult. Once moulted the gastroliths release the calcium they have stored, calcifying and hardening the new exoskeleton. How do they do this releasing exactly? I still don’t know. There isn’t a lot of research that I have found… yet.
From Crayfish of Ontario document created by Ontario Nature and the Toronto Zoo
D.F. Travis wrote a paper in 1963 entitled "Structural features of mineralization from tissue to macromolecular levels of organization in the decapod Crustacea”, that calcium carbonate is the chief mineral component of gastroliths, making up 76 % of the mineral in Orconectes virilis (now Faxonius virilis) which is commonly known as Virile Crayfish, or Northern Crayfish.
Personally, the only time I have found a gastrolith has been in scat. Not Crayfish scat, but instead Northern River Otter (Lontra canadensis), Mink (Neogale vison), and Raccoon (Procyon lotor) scat. River Otters eat a lot of Crayfish and they will sometimes get one, or four, while the crayfish are in the midst of moulting (remember that the Crayfish tend to slow down and are less active before moulting, so this might be an opportune time for a River Otter, Mink or Raccoon to come along and snatch them up). I guess these mammals don’t digest the gastroliths and they just come out in the scat, only to be found later by weird tracker types excited by poop.
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