12 July 2005

Zoogenetes harpa: the snail that rode the continents

Zoogenetes harpa is a small land snail; many of them easily fit into a bottle cap. Thomas Say described this species in 1824 based on the shells he had collected a year earlier during his 2nd western expedition under the command of Major Long. That expedition visited Ohio, Indiana, the “Northwest Territory” (Wisconsin and Minnesota), Michigan and New York.

Zoogenetes harpa, found only in the northern hemisphere, has an intriguing distribution pattern. The orange colored patches in the map below show the ranges of this species across 3 continents forming a roughly circular pattern around the North Pole1. How can we explain this pattern in the distribution of an animal that cannot swim or fly? Could a floating bottle cap have carried a few lucky snails across the treacherous Arctic Ocean from one continent to another?

Background map was created at OMC.

Fantasy aside, the only meaningful answer comes from the consideration of the positions of the continents about 190 million years ago during the early Jurassic period. That was when the continents that were later to become North America, Europe and Asia were still connected. Clearly, Zoogenetes harpa first appeared sometime before then, dispersed across the then connected land masses and was subsequently carried on the drifting continents to the places where it lives today.

Map is from Scotese.

What is also intriguing is that the shell morphologies of the populations on North America and Europe don’t appear to have changed since the Jurassic. It would be instructive to compare their genes.

1. The limits of the ranges indicated on the map are approximate, since I don't know the exact boundaries, especially in Asia. I have compiled the distribution data from Pilsbry, 1948; Likharev & Rammel'meier, 1952; Kerney & Cameron, 1979.


Tim Pearce said...

While I agree that vicariance (e.g., being carried by continental drift) readily explains the existence of major groups on separate continents, I expect that dispersal explains the widespread circum-boreal distribution of Z. harpa. It seems unlikely that the populations on the different continents would maintain such similar shells over 190 million years. Instead, I suspect that dispersal, for example by wind or by riding in the feathers of birds, does a better job of explaining the distribution. I agree that one way to address the vicariance versus dispersal debate would be to examine molecules such as DNA. Vicariance would predict that all individuals on a particular continent would be more similar to each other than to individuals on other continents. Dispersal would predict that individuals on adjacent parts of different continents (e.g., either side of the Bering Strait) would be most similar. Let me know what you find out!


But if they are carried by wind or birds, why don't we find them elsewhere, for example in the southern hemisphere? There is some other factor that is restricting harpa's range (climate? competitive exclusion?), which I didn't want to get into.

The populations in NA & Europe could be genetically different, but conchologically similar.

ümit said...

Columella aspera, although somewhat ubiquitous, also shows a boreal distribution may possibly have jumped 4000 km away to C Africa see (http://www.conchsoc.org/2index.htm?row2col1=2abs37_3.htm)
If my paleo-knowledge is valid to you, it says that till 50 million y.a. there was a continent formed by Eurasia and N America together, also several land bridges from -both E and W ways (after and before the split), for instance many mammals like (ancestral) llamas, sheeps etc. came following the Bering route after this split (50 m. years).

Besides i have read in Zhadin i think, which i dont have with me now, that it occurs in Caucasia and Kopetdag (?)which mean a very ancşent type of (scattered) distribution enforced by main ice ages. I suspect it from Alps, N-Iran, and Taurus Mts if that is the case and writing this i saw the range on Alps was marked on the map.

In little pupilloideans, passive transport is highly available and the origins of these is often impossible to find. For şnstance Vallonia pulchella (and several close species) has a similar range with Z. harpa, but to the south especially, the role of man in distribution is highly questionably.

Distributional type of Z. harpa is reliably boreal as in Vallonia Vertigo etc. But i think the fascinating speed of dispersal in samll creatures, partially aquatic or semi-aquatic ones in molluscan examples, will dispute some general concepts of biogerography in future. See the examples of Physella acuta (incl. cubensis and heterostropha) and Pisidium spp. seeming to be "bird-way" travellers

ümit said...

Sorry, i meant Licharev and Rammelmeier (for Zhadin).


A connection between Alaska & Asia, the Bering Land Bridge, was present not 50 My ago, but about 18,000 y ago during the last glacial maximum.

ümit said...

The continent was Laurasia.(http://www.minerva.unito.it/SIS/Modelli%20terresti/Cretacico.gif) The Holarctis remained intact for a very long period indeed.
I wonder if (temporary) land bridges acted comparably efficient as continental movements for terrestrial snails known to spread with rather passive ways. At least for aquatic (+ semi-aquatic??) molluscs it seems so, seeing the radiation of Lymnaeids in Pacific isles.
Despite not being the main idea of the topic, didnt Bering bridge open repeatedly?

Note: There is a single record of Z. harpa from Caucasia but not from other southern mountains

Carl Christensen said...

I'm with Tim Pearce and the dispersalists on this one. Take a look at the land snail fauna of the Hawaiian Islands to see how easily small snails (e.g., pupillids, succineids, and non-achatinelline achatinellids) can cross water gaps.

PS: Hello to Aydan and Tim; I've fled DC and am back to lawyering in Hawaii.