Conquering the world at a snail's pace: Expansion of invasive Mediterranean Tramp Slug

It arrived in West Germany in 1977, presumably as a stowaway with vegetable shipments from Italy, and it was first reported from the new Federal States one year after the fall of the Berlin Wall -- the Mediterranean Tramp Slug Deroceras invadens. As the name implies -- invadere is Latin and means "to invade" -- this slug has a habit of settling in new areas.

"By now, this species of slug can be encountered almost worldwide," explains Dr. Heike Reise, curator at the malacology section of the Senckenberg Museum of Natural History in Görlitz, and she adds, "It is primarily found in gardens or under scattered debris, but also in greenhouses and out in the open nature. They are brought in with imported vegetables, garden supplies or tiles."

Together with her colleague from Görlitz, Dr. John M. C. Hutchinson, and a colleague from the United States Department of Agriculture (USDA-APHIS), the scientist has studied the distribution of these mollusks, which reach a length of about 3 centimeters. "We tried to determine in which countries the slug has already been established, when it first occurred there, and whether its expansion is correlated to specific climate factors," explains Hutchinson, the paper's lead author. To this end, the scientists evaluated the available literature, examined material from museums and other institutes and single-handedly collected specimens.

Originally, this slug is native to the Mediterranean region. The first evidence of immigration came from the British Isles in 1930, and within 10 years, the land slug expanded to Denmark, California, Australia and New Zealand. "Today, Deroceras invadens is found across large parts of Europe, Australia, North and South America, and the slug has also made itself at home on several islands, such as the Azores, Madeira and the Canaries," explains Reise. In the context of the study, which was funded by the Paul Ungerer Foundation, the team of scientists could also document several first-time occurrences, e.g., in Mexico, Costa Rica and Ecuador.

"So far, the species has given Asia and Eastern Europe a wide berth," says Hutchinson, and he adds, "We assume that the cold winters in the eastern countries and the hot, dry climate in central Spain and parts of Australia, Africa and Asia hamper the spread of the slugs." However, they do not necessarily render it impossible; in Breslau, a population of these slugs survived several winters at temperatures as low as minus 22 degrees Celsius. In North America and Egypt, the slugs take advantage of the extensive agricultural irrigation systems. "These new habitats created by humans open new paths for the expansion of the slugs into areas that are normally inhospitable for animals, and they may serve as potential corridors for the settling of suitable, isolated habitats," explains Reise.

The scientists from Görlitz assume that the small slugs will continue to expand to other countries and regions. A comparison of the distribution pattern with climate data allows prognoses regarding potential new occurrences. "In principle, all areas with a temperate climate can serve as potential habitat for these animals," says Hutchinson, and offers the following recommendation: "In large parts of China and Japan, as well as in the southern U.S., it would be beneficial to pay close attention to this potential new immigrant. In some areas, early recognition and rapidly applied counter measures may possibly prevent the establishment of this slug."