Treasure of the month: Three alien land snails
From left to right: Lissachatina fulica, Polygyra cereolus and Gastrocopta sterkiana seen in detail. In reality they differ strongly in size. © LIB, Lauschke
Land snails are generally not famous for getting quickly from A to B. Nevertheless, we humans often unintentionally ensure that some species can travel very long distances and spread in a foreign country. They travel as stowaways on ships, hide in cargoes or are gone with the wind. This month we present three snails that have found a new place to live far away from home.
Our treasures are all immigrants to the West Palaearctic – an area that includes Europe, the Mediterranean and the Middle East. Dr Bernhard Hausdorf, head of the mollusc section at the Museum der Natur Hamburg, compiled these finds in a paper. All three tell their own story about how they made the leap across the oceans to our continent.
Gastrocopta sterkiana was already discovered in Israel in 1972. This species originally comes from North America and was found in the En Gedi Oasis on the western shore of the Dead Sea. In the middle of the 20th century, a botanical garden was established here with plants from all over the world. Within these plants, this species probably made the journey from America to Israel. It is considered harmless to the ecosystem there.
The large Giant African land snail (Lissachatina fulica), on the other hand, is already invasive in many parts of the world – meaning they are both alien and harmful to their new ecosystems. In tropical climates, it can spread explosively. It hasn’t just been discovered in Israel, but also on the Ebro delta in north-eastern Spain. “The climate in the Mediterranean region is too dry, however, so populations can only survive on irrigated land and the species does not threaten natural ecosystems here” Hausdorf estimates.
The actual size of the species becomes clear in direct comparison: while the Giant African land snail fits comfortably in one human hand, Polygyra cereolus is kept in a glass tube and Gastrocopta sterkiana in a pill case. © LIB, Steinkröger
One measure taken by humans to get rid of the undesirable species in the past was the targeted import of predatory snails to exterminate the alien agate snail. Unfortunately, endemic species also fell victim to this strategy and were equally defenceless against their new enemies. Moreover, we make it easier for alien species to establish themselves by destroying the natural living conditions on which native snails depend on.
Polygyra cereolus was first discovered on the Arabian Peninsula. At the time, researchers suspected that it had been carried off by the military operation “Desert Storm”. War machines offer the snails enough protection to travel around the world undetected as stowaways. “In the years that followed, it spread very quickly throughout the Mediterranean, so that it can now be found for example in Egypt, Tunisia, Libya, Spain, Italy and Turkey” says Hausdorf.
“A comparatively very nimble snail species can actively spread about ten metres a year. In contrast, snails can be spread much more quickly passively, for example by birds or very small species, even by the wind, and reach distant places or even islands” Hausdorf summarises. This is how it is possible for land snails to have colonised oceanic islands like Hawaii. Since most land-snail-species are hermaphroditic, just a few animals can facilitate the establishment of a new population. Some species can even fertilise themselves.