Saturday, October 1, 2011

Parasitic worms have center-leaning latitudes

Perhaps the most infamous puzzle in biogeography is the fact that places closer to the equator tend to have more species than places farther from the equator. Why is this true? 212 years have passed since Alexander von Humboldt first noticed this pattern, called the latitudinal diversity gradient, on his voyage to South America and scientists are still not in complete agreement on what causes it.

Not all living thing follow the latitudinal diversity gradient, either. In a paper published in Global Ecology and Biogeography in September, David Thieltges and colleagues discovered that, across Europe, a particular group of parasitic worms called trematodes actually has more species at moderate latitudes than in the south. Of course, since no part of Europe occurs in the tropics, the fact that the scientists didn’t find the traditional latitudinal gradient for trematode diversity in Europe doesn’t preclude its existence for the entire globe.

Trematodes might be bailing off of the latitudinal bandwagon because they aren’t free-living. These parasites must sequentially infiltrate at least two and often three different animals, (such as from a snail, to a fish which is then eaten by a bird) before they can lay thousands of eggs that will start their life-cycle over again. Because they are so dependent on finding hosts, the number of trematode species is more likely to be affected by how many different animal species are available to be parasitized, than aspects of the external environment that vary with latitude.

This is just what Thieltges and colleagues found when they looked at the diversity of animals that trematodes use as a final host (i.e. where they have sex and make eggs). Regions with higher final host diversity had greater trematode diversity. Surprisingly, it is only the variety in final host that matters; snail diversity does not affect trematode diversity, even though trematodes rely on snails to complete the first part of their life cycle.

So, just what is diverse, for trematodes? How many different kinds of worms are we talking, here? Thieltges’ source of information for his research was the book Limnofauna Europeae, compiled in 1978 by Joachim Illes, which catalogs pretty much all of the known animals that live in freshwater lakes, streams, and ponds in Europe. According to this book, the fewest number of trematode species occur in Iceland, at a whopping 9 species. And as for the most? You can find 322 trematodes out on the East European Plains. If this sounds like a lot, just consider the fact that at least 18,000 species of trematodes make the world their home.

You can find this article at:

Thieltges D.W., Hof C., Dehling D.M., Brändle M., Brandl R. & Poulin R. (2011). Host diversity and latitude drive trematode diversity patterns in the European freshwater fauna, Global Ecology and Biogeography, 20 (5) 675-682. DOI:

1 comment:

  1. It may be difficult to believe, but more than 90% of humans will be infected with parasites within their lifetime! Most of the time we don't even realize that we have them, and until such time that their numbers reach critical levels, we will ignorantly go about our business doing what we have always done.

    Ostaderm V