Tiny snails survive digestion by birds

Faeces containing snail shells (c) Shinichiro Wada Excreted snails can survive

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Snails are able to survive intact after being eaten by birds, according to scientists.

Japanese white-eyes on the island of Hahajima, Japan feast on tiny land snails.

Researchers found that 15% of the snails eaten survived digestion and were found alive in the birds' droppings.

This evidence suggests that bird predation could be a key factor in how snail populations spread.

Japanese white eye (c) Shinichiro Wada The Japanese white-eye or mejiro is widespread in Japan but considered an invasive species in Hawaii

It is well known that plant seeds are dispersed by birds that eat fruit.

But in findings published in the Journal of Biogeography, researchers from Tohoku University, Japan investigated whether invertebrates could also spread in this way.

Previous research has shown that pond snails can survive being eaten by fish but the same was not known for land snails.

Studies of the diets of birds on the island of Hahajima identified the Japanese white-eye's preference for the tiny land snail Tornatellides boeningi.

In the lab scientists fed the birds with the snails to find out whether any survived the digestive process.

"We were surprised that a high rate, about 15 percent, of snails were still alive after passing through the gut of [the] birds," explained researcher Shinichiro Wada.

Start Quote

This is the first study showing that birds can indeed transport a substantial [number of] micro land snails in their gut alive”

End Quote Shinichiro Wada Tohoku University, Japan

They also studied the genetic differences of T. boeningi populations found across the island and discovered considerable variation.

Rather than only mating with nearby snails, these results suggested that different populations made contact despite their geographical isolation.

"Biogeography of wingless terrestrial invertebrates, in particular snails, is often faced with mysterious long distance dispersal patterns that can only be explained by hand waving arguments involving birds' feet or guts or cyclones," said Mr Wada.

"This is the first study showing that birds can indeed transport a substantial [number of] micro land snails in their gut alive."

One snail in particular helped researchers identify how numerous snails could travel over distances via bird droppings.

"One of the snails fed to the bird gave birth to juveniles just after passing through the gut," Mr Wada told the BBC.

The main factor allowing the snails to survive being eaten is their small size, according to the scientists.

Tornatellides boeningi (c) Shinichiro Wada T. boeningi is the most common species of snail on Hahajima

At an average of 2.5mm the micro snails fared much better than larger species in previous studies whose shells were severely damaged when eaten by birds.

Mr Wada and his colleagues said further study is required to find out whether the tiny snails have other adaptations that allow them to survive.

Hahajima lies 1000km south of Tokyo in the Bonin Islands archipelago, known as the Ogasawara Group in Japan.

The islands were recently added to the UNESCO World Heritage List "for the wealth of their ecosystems which reflect a wide range of evolutionary processes".

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