just how small can a small snail be?

When I was a kid I used to collect shells on the beach – got my Girl Guides 'collectors' badge & everything 🙂 So I really enjoyed reading this post over on Sciblogs NZ. And that in turn reminded me of an article I saw recently on microsnails.

According to that article, 

"Microsnail" is the term for the creatures with shells measuring 5 millimetres or less, sometimes much less (Milius, 2016).

So these are some seriously tiny creatures. According to National Geographic1, microsnails are relatively common (albeit with any given species having a fairly restricted range), but they're just so small that people don't notice them. (NatGeo has an error in the first sentence of that story: I think they meant to say that the snails are a fraction of an inch tall.) However, they are a very diverse group: a 2014 paper on microsnail taxonomy (Jochum et al.) states that snails less than 5mm in length

represent the majority of worldwide tropical land snail diversity. 

The NatGeo story is based on the recent description of 7 new species of microsnail from China, the smallest of which, Angustopila dominikae, could fit in the eye of an ordinary sewing needle: its shell is just 0.86mm long2. Apparently A.dominikae held the mantle of 'smallest land snail in the world' for 5 days, before being knocked off its perch by an even smaller snail from Borneo. 

That there are so many of these tiny species of gastropod shouldn't really come as a surprise: there are more microhabitats available for smaller creatures. (Think, for example, of the tiny eyelash mites that frolic on our faces at night.) But those microhabitats may be limited in extent and that can be a problem (for creatures and those classifying them alike). In the case of the microsnail genus Plectostoma,

many species only occur on a single hill and nowhere else on earth.

And as described in this post on physorg.news

Limestone hills are 'sitting ducks' for mining companies, and many are being quarried away for cement, taking their unique snails  with them to their grave. One species, Plectostoma sciaphilum, is already extinct: its home was turned into concrete around 2003. Similar fates await at least six more species. One of these, P. tenggekensis (named and described in the new paper) occurs only on Bukit Tenggek, which the authors [Liew et al., 2014] forecast to be completely gone by the end of 2014.

Sad to think that these jewel-like creatures may be disappearing from the world even faster than scientists can catalogue them.

Photographs of 17 living Plectostoma species from Liew et al., 2014. Image credit T.-S. Liew.

1 NatGeo has an error in the first sentence of that story: I think they meant to say that the snails are a fraction of an inch tall.

2 Now, if that's their adult size, imagine how tiny the juveniles must be! 

A.Jochum, R.Slapnik, M.Kampschulte, G.Martels, M.Heneka & B.Pall-Gergely (2014) A review of the microgastropod genus Systenostoma Bavay & Dautzenberg, 1908 and a new subterranean species from China (Gastropoda, Pulmonata, Hypselostomatidae). Zookeys 410: 23-40. doi: 10.3897/zookeys.410.7488

T-S Liew, J.J.Vermeulen, M.F.bin Marzuki & M.Schilthuizen (2014) A cybertaxonomic revision of the micro-landsnail genus Plectostoma Adam (Mollusca, Caenogastropoda, Diplommatinidae), from Peninsular Malaysia, Sumatra and Indochina. Zookeys 393: 1-107  doi: 10.3897/zookeys.393.6717

S.Milius (2016) The fine art of hunting microsnails: beauty and sorrow in five millimetres or less. Science 189(2): 4



2 thoughts on “just how small can a small snail be?”

  • herr doktor bimler says:

    I should probably scan and send you Fig. 209 from Wells’ “The Science of Life”, showing “small naked-eye creatures”. The smallest example there — about 1/3 the size of a common flea — is “Smallest land-snail”.

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