A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders Invasion of the Flatworms Easily hidden in imported plants, some land flatworms are conquering the world. Ronald Sluys robably a million more Ameri- to have invaded France, Europeans succinctly and appropriately described cans have heard the word already had cause for alarm: Another these land flatworms as follows: “ flatworm because of me,” says invasive land flatworm, Arthurdendyus In general form they resemble lit- my colleague Jean-Lou Jus- triangulatus, was known to have reduced tle slugs, but are very much nar- Ptine, who is an expert on these organ- earthworm numbers in the British Isles. rower in proportion, and several isms at the National Museum of Natu- Ancestors of terrestrial flatworms of the species are beautifully co- ral History in Paris, as he wrote in an colonized the land, from the sea or loured with longitudinal stripes. email to me last year. In 2014, Justine freshwater, hundreds of millions of announced the discovery in France of years ago. The current worldwide oc- Darwin took some of the worms a land flatworm originally from New currence of these terrestrial worms is that he had collected in Tasmania on Guinea, Platydemus manokwari, which is a testament to their remarkable evo- board the Beagle and managed to keep considered by the International Union lutionary success in occupying a new them alive for two months. Never shy- for the Conservation of Nature to be niche on land. Almost nothing appears ing away from experiments, he cut one of the 100 worst invasive species. It to eat these flatworms, and they are some animals in half and discovered was the first time that P. manokwari had able to regenerate even when cut in that after 25 days each piece had re- been seen on the European continent. two. A land flatworm also will repro- generated to an almost complete ani- This land flatworm is particularly noto- duce asexually by fragmentation: The mal. About 18 months earlier he had rious because it spreads easily and eats anterior and posterior halves of its collected other species of land flat- native earthworms and snails, thereby body break apart, and the posterior worms from forests along the Chilean affecting local ecosystems. In 2015, Jus- half crawls around headless until it coast. Always a keen observer, he had tine reported finding P. manokwari in grows a new head in about 15 days, noticed that when these worms experi- the United States. Before these two ma- while the front half grows a new tail. ence adverse conditions—for example, jor continental introductions, the New Until recently, this obscure group of when they are handled by humans— Guinea flatworm had been a problem invertebrates had not received much they may very quickly, before one’s on a smattering of Indo-Pacific islands attention, even though a small group of eyes, disintegrate into a slimy mass. but had not reached any mainland, at natural historians, including me, had Little could Darwin have suspected least not to anyone’s knowledge. But begun monitoring them. Justine’s lat- that 180 years later he would have been now they seem to be popping up ev- est discoveries sparked an avalanche of able to collect land flatworms in Eng- erywhere, and they are not the only records of land flatworm introductions land that originated from the Southern species doing so. and have brought attention to the ease Hemisphere. How could these appar- Land flatworms, a group of about 910 with which these organisms spread ently delicate animals have reached the species, typically live in tropical jungles and acclimate to new places. Although Northern Hemisphere—and what can or temperate wooded areas and can sur- the invasion has apparently acceler- we do to stop their continued spread? vive only in moist soil environments. ated and worsened in recent years, our A hint at an answer to the first ques- Many places in the world are being present knowledge is still limited. The tion was presented all the way back invaded by land flatworms that have time is ripe for biologists to unravel the in 1878, still during Darwin’s lifetime, stowed away in the soil of imported or- diversity, ecology, and natural history when a new species of tropical land flat- namental plants. Flatworms may not of land flatworms, so that the environ- worm was found in the hothouses of seem particularly worrisome or threat- mental implications of their spread the botanical gardens at Kew Park in ening, but they are voracious top-level may be understood and curbed. the London borough of Richmond upon predators of soil organisms, and their Thames. The species was aptly named presence can change nutrient cycling, The First Invasive Land Flatworm Bipalium kewense after this site. It was endanger native species, and alter an In our shared fascination with these immediately clear that the botanical ecosystem’s plant community. Indeed, worms, Justine and I have been fol- gardens at Kew represented a curious even before P. manokwari was found lowing in the footsteps of a notable location, because the native distribution predecessor, Charles Darwin. During of other species of Bipalium covers In- his HMS Beagle voyage, Darwin was dia, Southeast Asia, and Madagascar. Ronald Sluys is a senior researcher, specializing in struck by the fact that there is a group In the following years it became clear planarian flatworms, at the Naturalis Biodiversity of free-living flatworms, or planarians, that B. kewense had actually colonized Center in Leiden, the Netherlands. Email: ronald. that actually live on land (most planar- hothouses and similar artificial environ- [email protected]. ians live in freshwater or in the sea). He ments, such as plant nurseries and gar- 288 American Scientist, Volume 104 © 2016 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. MYN/Gil Wizen/Nature Picture Library The first introduced land flatworm discovered, Bipalium kewense, was found in the botanical generalist carnivores that prey on other gardens in Kew Park on the outskirts of London in 1878. Native to Southeast Asia and intro- invertebrate organisms such as earth- duced to England via the horticultural trade, this land flatworm has since spread throughout worms, snails, slugs, insect larvae, and much of the world. several arthropods, including isopods and springtails. Land planarians search den centers, all over the world. But it Kingdom. From 1963 onward observa- for, attack, and capture prey much larger has also established itself outdoors—for tions of other nonnative land flatworm than themselves. For example, Peter Du- example, in gardens in California, Loui- species from the Southern Hemisphere cey of the State University of New York siana, Florida, and the West Indies. have been reported in the British Isles. at Cortland found that in North America To a large extent B. kewense’s colo- Most of these worms probably orig- the introduced flatworm Bipalium ad- nizing success can be explained by the inated in New Zealand or Australia ventitium may attack earthworms that worm’s ability to propagate itself by and reached the United Kingdom by have more than 100 times its body mass. fragmentation, an asexual process that transport of ornamental plants. Al- The worms employ different techniques Darwin had already noticed during his though most introduced flatworms to subdue their prey. Among these are experiments on the Beagle. All species were found in greenhouses early on, by physical force, adhesive mucus, and the of Bipalium are hermaphroditic, having the 1980s their numbers had increased use of an effective digestive secretion both testes and ovaries. But in temper- considerably, and the majority of these over the surface of the live prey—or ate regions B. kewense animals rarely new flatworm species had also estab- into it, using a protrusible pharynx. The develop a reproductive apparatus and lished themselves in outdoor environ- latter is a muscular, often tube-shaped they therefore are unable to sexually ments. One species, the New Zealand structure that can be protruded out of reproduce. However, on rare occasions flatworm, A. triangulatus, even reached a ventral opening about midway down this flatworm has been known to re- the remote Faroe Islands. To date, 15 the flatworm’s lower body surface. This produce sexually as well. species of introduced land flatworms tube leading to the gut can be inserted Almost every year B. kewense is have been found in the British Isles. into the prey, sucking out its contents found in new places—for example, this As these introductions have con- by bits and pieces through peristaltic year they were found on São Miguel tinued, we still know little about the pumping of the pharyngeal muscles. Island in the Azores and on São Tomé myriad of potential ecological effects Recent experimental work by PhD Island in the Gulf of Guinea. It is pre- they could induce. But what we do student Piter Boll, from the Universi- sumed that B. kewense travels the world know does not bode well. dade do Vale do Rio dos Sinos in Brazil, as a result of the horticultural trade, has revealed that there are even land transported in pots of tropical plants. Problematic Eating Habits flatworms that specialize in eating oth- That many Britons are keen horticul- Nonnative flatworms can spread ers. The land flatworm that is being turists is well known.