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Driller killer: the solution … ?

Those who’ve been in contact fall into two camps: those who think the holes were probably caused by another mollusc, and those who think they were caused by a beetle. But tellingly, all the mollusc experts so far think that beetles of some sort must be responsible, and two have noted that they’ve seen holes like this in terrestrial snail shells in the Mediterranean.

As Richard Wright and others have kindly pointed out, there’s a 2004 book on “Natural enemies of terrestrial molluscs” with a detailed chapter on Coleoptera as predators of terrestrial gastropods by Bill Symondson of Cardiff University. Bill’s response must therefore be the most authoritative that I’ve received but is still a guess: “I would guess that these are the exit holes of drilid beetles, that parasitise many species of snails”.

The great thing about Bill’s response is that it has turned the mystery on its head – it’s not about what’s trying to get in but what’s trying to get out! In support of the parasite theory: (1) there are 5 separate holes – why would a predator need more than one?, (2) the holes are quite evenly spaced as they might be if 5 parasites had partitioned up the snail between them, and (3) one of the holes is right by the mouth aperture so there’s no way it was the easiest route of attack to get IN to the shell.

So, given that Drilus flavescens is the only drilid on the British list and it has been recorded not too far away to the south in Berkshire and Oxfordshire (map here), can we add it to the Buckinghamshire list? Maybe.

Andrew Duff dug up another interesting lead here. This person has lots of Garden Snail Cornu aspersum (formerly Helix aspersa) shells in their garden with one or two neat circular 4mm holes in them. Nobody on the wildaboutbritain forum seems to know what’s caused them but whatever it is must be the same beast that drilled the Grangelands Helix pomatia. However, the garden in question is in Lincolnshire! I could believe that Drilus flavescens exists at Grangelands – a superb calcareous grassland just north of the known range. But I find it harder to believe that Drilus exists in a garden in Lincolnshire!

For a definitive answer, Julia’s going to try and breed the parasites out in 2010.

Meanwhile, thanks to all those who’ve been in touch and if anyone has further suggestions, I’d be glad to hear them.


3 Comments

  1. Bill says:

    Mark,Deep in one of the threads I saw this link:Natural Enemies of Terrestrial Molluscs – Google Book SearchAnd the article that takes you to made a pretty convincing argument for it being another mollusc called: Aegopinella nitidula. Not sure if you found or considered that as a possibility.Bill

  2. Mark Telfer says:

    Thanks Bill,I really need to keep an open mind about the hole maker. The book says: "Mordan (1977) found that predation on snails by A. nitidula almost invariably conforms to a characteristic behaviour. The initial attack is made through the shell aperture of the prey, and results inmuch of the head-foot being eaten. The remaining visceral mass is then reached by radulation of an irregularly shaped hole in the shell of the prey. The hole is typically situated about three-quarters of the way back from the aperture on the umbilical surface of the outermost whorl, although occasionally radulation of the prey shell occurred at the apex. Mordan observed that A. nitidula required between 1 and 2 h under laboratory conditions to penetrate through the shell and consume theremaining animal tissues".Aegopinella nitidula is a very common snail so it seems odd that these holes are so rarely observed.Mark

  3. jason says:

    I somtimes find snail shells in my garden with holes such as these i once dissolved a snail to extract the tongue and found a larvae which was by then cleared by the solution so i made a slide and it looks like a beetle larvae.

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