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Holey Snails!

Holes in rocks on Cardoness Beach, near Gatehouse of Fleet, SW Scotland

Q: I have taken several photos of rocks on Cardoness Beach, near Gatehouse of Fleet, SW Scotland. These rocks have a series of holes, mostly in straight lines.

I have been told that snails excrete a chemical substance which reacts with the rock resulting in a small hole. The sea water then pound into the rock & over many years the holes grow to form these holes.

I find all this rather far fetched, any information will be most welcome.

From Penny Thornton (May 2010)

Reply by Dr Ted Nield

I am not sure if those holes are what they are alleged to be, but various bivalves (not “snails”) do bore into things, including solid rock and wood.

The wood-borer is called Teredo, and is commonly known as the shipworm. It is a specialised wood borer with a reduced shell. The rock borer, which has a specially armoured shell that it uses to crunch the rock after its digestive juices have weakened it, is called Pholas. These holes clearly follow the bedding lineation of the rock, which suggests that they may be following a compositional difference. Pholas prefers carbonate-rich substrates so this, if it is indeed due to bivalve-boring, may be showing us horizons that are richer in calcium carbonate.

Alternatively this could be no more than differential chemical erosion of such horizons. The only way to be sure of biological agency is to find shells of dead Pholas spp. in place, or another organism of the same species nearby.