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Farming snails

© Esther Wullschleger

Fungus farming has long been known from some insects such as leaf-cutting ants. Visitors of the Southern American tropics most probably get to see these extraordinary ants. Quite conspicuous, each individual worker ant transports a little piece of foliage or other plant part into the nest, so that there is a row of seemingly moving little green plant parts that finally disappear in a hole in the ground.

The leaf-cutting ants use this plant material to cultivate a colony of fungi in the nest. The cultivated fungi, well cared for by the ants, are finally used as food by the farming ants. Since last year, we know that some snails are almost as refined when it is about food cultivation. US researchers have found a snail species with similar abilities to increase the availability of certain fungi which they consume. This snail, however, is not that highly specialized as the leaf-cutting ants, and the fungi grow without it as well.

The marine shore snail Littoraria irrorata lives in beds of Spartina marsh grass along the shores of the Northern American part of the Atlantic coast, where it grazes live grass and feeds on dead, decaying grass. However, the purpose of grazing the fresh grass is primarily not to eat healthy grass tissue. Rather, the snail opens up the grass tissue, so that it is more vulnerable to fungus infections. Additionally increasing infection probability, the snails place their droppings right over the plant wound. It remains unclear though whether the snails do this on purpose, or whether the droppings are incidentally concentrating wherever the snails spend most of their time while feeding.

The spores of the fungus are abundant in the Spartina marsh grass beds, so the researchers found infections also if plant tissue was artificially opened up with a razor blade. However, if there were Littoraria snails that placed their droppings into the plant wound, infection risk was getting about twice as high, the scientists found. Also, young Littoraria snails which have been fed on intact grass leafs only grew much more slowly. Some of them even starved. This all suggests that the snails thrive especially on the fungus that causes infections on the plant.

Much to the inconvenience for the plant, the snails' activities do also increase spread of "their" fungus. This brings advantage to the fungus itself, which is why the ecological relationship between Littoraria snails and the fungus is considered mutually beneficial, in other words symbiotic. Removal experiments of the US scientists showed that, on the other hand, the presence of the fungus farming snail suppresses growth of Spartina marsh grass.

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