Who Goes There? Scrape marks on rocks.
Update: 4 pm PDF October 26
Looks very likely that these tracks have been made by a limpet species. Further information under UPDATES below.
Here’s an unsolved mystery, that led to another unsolved mystery, and I suspect there will be more related mysteries to come. 🙂
It began with the photo shown below with an ID request by Marcie Callewart John and Stephen Lindsay. Stephen had taken the photo of the underside of this rock in Stewardson Inlet in Clayoquot Sound, SW Vancouver Island, British Columbia.
They wrote: “We were wondering about your thoughts on these markings. . . . Limpet or chiton feeding marks? Or egg attachment marks?”
I knew that the markings were from the radula of a grazing marine mollusc but not WHICH mollusc.
Marine and terrestrial snails and slugs (including nudibranchs), limpets and chitons all have incredibly strong “rasping tongues” covered with teeth-like structures called radula. In moon snails and some species of whelk, the radula are strong enough to drill holes into shells so that they can feed on whatever mollusc relative lives inside the shell.
In the grazing molluscs, it is the tongue studded with radula that enables them to scrape algae off rocks.
ICT scan of limpet radula from The Scientist Magazine.
I thought the radular scraping were more likely from a marine snail than a limpet or chiton BUT needed expertise bigger than my own to solve this “who done it”. Thankfully, I could reach out to Rick Harbo, author of Whelks to Whales.
Rick confirmed these were radular markings but did not recognize which marine snail made them. He had a mystery of his own. See below.
This led to input from Dr. Douglas Eernisse, professor of Biology at the University fo California, Santa Cruz. He did not recognize the specific tracks in Stephen’s or Rick’s photos. He shared the image below showing OTHER radular tracks but with a big difference. His photo showed the marine snail species making the tracks. See the Black Turban snails having dinner? To give you an idea of scale, maximum size of Black Turbans is 4.5 cm.
So out into the world this blog goes in the hopes of engagement and interest and maybe even that someone has documented similar tracks as those in Stephen’s and Rick’s photos with the grazers caught in the act.
I hope it makes you smile too to reflect on how we humans still have so many mysteries to solve. Just peering under a rock or any algae covered surface could lead to another mystery, leaving you wondering “Who goes there?!”
Schematic to give a sense of how the radula are positioned in gastropods (represented by the black zigzag line). I am emphasizing here that both marine and terrestrial snail and slug-like animals have radula. Source of image: Wikipedia.
UPDATES
Information shared by Jason Knight points solidly toward a limpet species being who made these tracks.
- The screen grabs below are from Dale Fort’s blog with the same image also being found on the website of the Field Studies Council in the United Kingdom for the Common Limpet (Patella vulgata). We do not have this species in British Columbia but the similarity in the pattern certainly supports that a species of limpet made the tracks in Stephens’ image.
2) The following screen grab is from Smithsonian Ocean, photo by Helen Scales. The species of limpet is not specified.
Extra: A fascinating study from 2015 found that limpets (generally) are the “bulldozers of the seashore”. The study found that their “teeth” (radula) are made of the strongest biological material ever tested (and these teeth are less than a millimetre long)! The strength is the equivalent of one string of spaghetti holding up 1,500 kgs. From Professor Steven Hawkins, of the University of Southampton. “The reason limpet teeth are so hard is that when they’re feeding, they actually excavate rock. In fact, if you look at their faecal pellets they actually look like little concrete blocks – because by the time it’s gone through their gut it’s hardened.” (Barber et al).
Related TMD blog:
Sources:
- Barber, Asa & Lu, Dun & Pugno, Nicola. (2015). Extreme strength observed in limpet teeth. Journal of the Royal Society, Interface / the Royal Society. 12. 10.1098/rsif.2014.1326.
- Dale Fort Blog 11
- Weaver, James & Wang, Qianqian & Miserez, Ali & Tantuccio, Anthony & Stromberg, Ryan & Bozhilov, Krassimir & Maxwell, Peter & Nay, Richard & Heier, Shinobu & DiMasi, Elaine & Kisailus, David. (2010). Analysis of an ultra hard magnetic biomineral in chiton radular teeth. Materials Today – MATER TODAY. 13. 42-52. 10.1016/S1369-7021(10)70016-X.
More tracks made by gastropods
Terrestrial:
Tracks made by a species of garden slug, Richmond British Columbia. Photo: George Holm.
Tracks made by a Banded Garden Snail, Cepaea nemoralis in Queensborough, New Westminster, British Columbia Photo: George Holm.
The Marine Detective 
7 Responses to “Who Goes There? Scrape marks on rocks.”
Interesting!! During our dive the other day I was trying to follow tracks in the sand…they almost looked ‘bird like’… next time will get a picture!!
Fabulous my friend. ANOTHER mystery!
So informative, as always. I learned about this when I moved back to the Sunshine Coast and put in a greenhouse, which promptly got covered in a thin skim of algae (as things do here). Then I noticed these patterns on the poly panels and had no idea! Quite amazing, actually. Thanks so much for sharing your wonder of nature with us…
A wonderful mystery! Next up you will be creating a field guide to mollusc tracks! 🙂
It is neat to see how different they are. I will be on the lookout next time I am on a rocky shore.
Smiling Michael. Intelligence from the UK has helped point the tracks in the first image to some species of limpet.
What a beautiful article. I used to see track marks on our elevated canoe stored in our backyard. Our son said they were made by snails although I couldn’t comprehend how they would have climbed up to the algae covered bottom of the suspended canoe. Now I am starting to have a fabulous understanding of these sorts of creatures and how brilliantly they have evolved and of course how similar we all are. I love your posts. Just wonderful. Jane Heavyside
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How I appreciate this feedback Jane and knowing how the content connects with you. Thank you so much.