Calcareous tubes are one of the most common types of encrustation on shells, stones, and flotsam at the seashore. These are made by small marine worms of the Serpulidae family. Unlike the the Polydora type marine worms mentioned in earlier posts, which create burrows and tunnels in the actual matrix of shells and stones, adult worms such as Pomatoceros triqueter and Hydroides norvegica cement the chalky tubes they occupy onto the surface of the substrate.
Pomatoceros is one of the types of organism frequently responsible for worm tubes on seashore objects in the UK. The tube has a distinct almost triangular cross-section. Along the length of the tube is a clear sharp ridge or keel. There is a single central one in Pomatoceros triqueter (Linnaeus) but in P. lamarcki (Quatrefages) there are two vestigial longitudinal ridges laterally placed on each side of the median keel. The tubes can be found as single and scattered specimens, or as tangled knots and heaps with the tubes in various orientations.
The Pomatoceros tubes zig-zag across the surface of the shell. From a distance it looks as if someone has been scribbling with a white marker. The vague resemblance of the tubes to Gothic writing script has given them the common name of “German Writing” .
Another type of distinctly-shaped chalky tube is made by the worm Hydroides norvegica (Gunnerus) or other species in that genus. [I have found these tubes less frequently than Pomatoceros and only have the photograph above to illustrate them in this posting]. These tubes are circular in cross-section (contrasting with the triangular shape in Pomatoceros). There is no keel and the tube is un-ornamented except for the visible growth lines. The tubes are irregularly coiled in one plane or occur in masses of tubes lying parallel to each other and with the openings all facing the same direction.
The photographs in this posting show a variety of these calcareous tubes on European Flat Oyster shells – Ostrea edulis Linnaeus – from a selection of UK locations. The ones in the top picture are on archaeological shells. Some of the recent or modern specimens (shown below) were found on the seashore and others were dredged fresh and alive from natural deep water oyster beds.
For more information about European Flat Oyster shells in Jessica’s Nature Blog, search in the OYSTER VARIATIONS category.
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How do the worms make the casts on the shells ? Have they eaten sand ? To extrude out of their bottoms ? Or tiny bits of shell ?
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Now that is an interesting question, Jane. I had not thought of it before, but my first thought was that this worm does not extrude anything from its anus to create the shell. That was impossible. I looked it up on Wikipedia and it says there that the larvae “settle on the substrate and build a temporary delicate, semi-transparent tube formed of mucus and calcareous matter.[4] This is later hardened by a secretion of calcium carbonate from the collar and grows at the rate of 1.5 millimetres per month.[2] Although it may superficially give the appearance of being formed in bands, this is caused by spurts in growth interspersed with quiescent periods.[4][5]”. The references to which the article refers follow in the text, but I have so far been unable to check them out to see what is exactly meant by the collar. I’ll see if I can find out more details elsewhere.
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I have just traced an old journal article about the formation of the tube in the keel worm Pomatoceros triqueter (now called Spirobranchia triqueter) which you might like to read at https://plymsea.ac.uk/id/eprint/1930/1/Tube_formation_by_Pomatoceros_triqueter_(Polychaeta).pdf.
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Thank you so much for your help.
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How do I send you a photo of my shell? I am intrigued as it has so much worm cast.
Thankyou.
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You can send a photo of the shell to my e-mail winderjssc@aol.com and I will comment if I can. It would be interesting to know where you found it.
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