Shells with holes made by boring bivalves

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P1180409aBlog1 Group of Flat Oyster shells (Ostrea edulis Linnaeus) with burrows made by boring bivalved molluscs (1) 

In an earlier posting I talked about pebbles with holes made by sea creatures and focused primarily on the large burrows made by boring bivalved molluscs, like piddocks, in rocks. However,  evidence of rock-boring bivalves is also found in some very thick seashells which, to the infesting organisms, seem much the same as a soft stone. In the UK, the only native species of mollusc with shells that can potentially grow very thick like this is the European Flat Oyster (Ostrea edulis L.).

The photographs show the damage caused by boring bivalves in thick, old oyster shells from Gower beaches. It is not possible to tell definitely in each instance whether the oysters were infested while still alive, or whether the empty shell has been used after death. Damage like this has been recorded in ancient Roman oyster shells, for example, from archaeological excavations in Dorchester in Dorset. This is an indication that living oysters can be infested in this way since the shells here were food remains. The most likely species responsible for this infestation damage is the Flask Shell, Gatrochaena dubia (Pennant) – this likes to bore into organic carbonates as well as sandstone and limestone..

P1180411aBlog2 Infestation damage caused by boring bivalved molluscs in a Flat Oyster shell (Ostrea edulis Linnaeus) (2) 

In some of the shells illustrated the burrows totally penetrate the shell. If this had happened while the shell was still occupied, then the animal would have been affected: it would have tried to lay down new layers of the type of shell that lines the internal cavity in order to protect itself – or it would have perished.

What would the Flask Shells be doing in the oyster shell? Basically, they would be using the shell (or rock) as a secure shelter. There is no deliberate aim to kill or damage the organism within. Flask Shells and other rock-borers enter a shell or rock crevice when they are small. The hole, burrow or tunnel is enlarged to accommodate them as they grow bigger. In fact, it may never be possible for the bivalve to leave the burrow because the entrance is much smaller than the animal. Empty shells are frequently found within the burrows. When this happens, it is possible to make a firm identification of the organism. When shells are absent from the burrow, a certain amount of educated guesswork (based on habitat preferences and geographical distribution) is required to suggest an identification.

P1180452aBlog3 Close-up of two confluent burrows made by boring bivalved molluscs in a thick Flat Oyster shell (3)

How do rock-boring Flask Shells, Piddocks and other species create the burrows? These bivalves all have specially adapted shells, sometimes very tough and sometimes fragile depending on the hardness of the substrate they prefer to burrow into. There are often sharp ridges or spines on the outer surface of the shells. These bivalves scrape away the rock or shell they occupy in an ingenious way. They all have two exceptionally long and powerful fleshy siphons (tubes) for sucking water and food particles inwards and for expelling water and waste outwards. The siphons are often so large that they cannot be retracted into the cavity enclosed by the two hinged shells. They lie along the tunnel and connect the animal to the outer watery world. The animals use hydrostatic pressure via these siphons to force their shells outwards and grind away the stone or shell in which they burrow.  

P1180421aBlog4 Very infestation-damaged thick Flat Oyster left valve shell with a large burrow caused by a boring bivalved mollusc (4)

The large diameter burrows, often about a centimetre across, provide windows to observe the internal structure of the shells. You can see that the shell is made up of a sequence of layers. These layers alternate between a narrow, more resilient shell type and a thicker, softer shell form. This reflects the complex microscopic make up of the shell where different types of shell occupy different positions in the morphology, and serve various functions. Peering closely into one of these rock-borer tunnels reveals a fascinating landscape resembling the strata of the Grand Canyon.

P1180425aBlog5 Close-up of large tunnel made by a burrowing bivalved mollusc species in a thick Flat Oyster shell (5) 

P1180426aBlog6 Two large round entrances to burrows made by boring bivalve molluscs in an old Flat Oyster shell (6) 


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14 Replies to “Shells with holes made by boring bivalves”

  1. Jessica, I have never once thought about how holes get in shells. Ever. You have broadened our world today! Mine, anyway. And it does look like the Grand Canyon down in the holes, a miniature canyon. I love the way your detailed eye looks at the natural world.


  2. I just get curious about why things are the way they are. Then I find myself, like Alice down the rabbit hole, in a whole new and wonderful miniature world – that is Alice as in Alice in Wonderland by Lewis Carroll.


  3. The natural story is fascinating in itself, but I can’t help thinking of all the metaphors it could generate, too. Poems and stories and whole epic novels lie waiting on the beach.


  4. I think, with my scientific background, I am more literal than literary. On the other hand, you, Pamela (and Kathy, and Amy Lynn) are the creative one/s with a gifted way with words to whom the beach would provide endless inspiration.


  5. Oh Jessica, the rest of us would be writing those epics about the wee little folk who live inside those shells (Mr. and Mrs. Flat Oyster and their crazy relative the Bivalves) and how one of the children gnawed his way out because his sister punched him in the head…

    We need you to keep us real, Jessica. Please don’t stop. 🙂


  6. There is plenty of scope in the world for every kind of view point…and even I once desperately wanted to believe in fairies (but through a process of experimentation concluded they didn’t exist)!


  7. Dear Jessica,
    I am visiting a town on the Northern California coast called Crescent City. There is a stretch of beach here called “Pebble Beach” because pebbles wash ashore. I keep finding large pebbles with holes bored into them and have been wondering how they got there. So I went searching the web and here you are!!

    Thank you so much for publishing your blog. It was so helpful. Love your clear writing that even a non-scientist can understand!

    Very sincerrely,
    Patricia St. Pierre


  8. Thank you, Patricia. I am so pleased you found the information that you needed on my blog. I really appreciate your feedback. Enjoy your stay in Crescent City. I wish I could visit Pebble Beach, too.


  9. Where did you pick up your oyster shells please? Your pic 3 has what seems likely to be the siphonal tube (characteristic figure 8 shape with calcareous liner) of Rocellaria dubia. I reckon the wee beast is very common. We see it all over the place subtidally in Dorset and Devon, but records from elsewhere are very scarce. Lovely pics bringing out the beauty of the shells…

    Liked by 1 person

  10. Hello, Nick. Thank you for the information. I am fairly certain that I picked up this oyster shell on the beach at Rhossili in Gower, South Wales.


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