Land planarians, members of the family Geoplanidae, are elongated predatory flatworms that glide across soil and leaf litter in search of prey. More than 1,000 species are currently known to science, and the true diversity is almost certainly far higher because the species-rich subtropical and tropical regions remain undersampled. Several planarians have successfully colonized areas far outside their natural ranges, and in Europe at least 30 non-native species have been found in heated glasshouses of botanical gardens and plant nurseries or in outdoor environments, including urbanized areas, gardens, and agricultural fields. The horticultural trade is considered the primary pathway for their introduction worldwide. The commercial transport of potted plants allows these flatworms to spread easily as contaminants, hitchhiking in soil, under pots, and among roots. Other means of introduction and spread may include the movement of topsoil, agricultural produce, silage bales, and farmyard manure.

What makes land planarians such formidable invaders is a combination of reproductive and defensive traits. They are hermaphrodites capable of sperm storage, and they produce egg capsules that typically contain between one and fifteen offspring, with the number varying among populations of the same species depending on food availability. Some species can also reproduce asexually by fragmentation, which can be their primary means of reproduction. Perhaps most importantly, land planarians appear to have almost no natural enemies. Their gland cells secrete a distasteful mucous secretion that repels most potential predators. Only a handful of natural enemies have been reported, including some birds, shrews, carabid and staphylinid beetles, a predatory fungus gnat, a carnivorous snail, and other planarians, but the flatworms are generally not preferred prey. Parasites and pathogens of land planarians are almost unknown, although recent research has demonstrated the presence of two species of Microsporidia in land planarians in Northern Ireland.

The ecological stakes are considerable because all land planarians are predators of ground- and soil-dwelling invertebrates, including gastropods, earthworms, and arthropods such as isopods and spiders. Earthworms are considered beneficial soil organisms because they play a major role in soil aeration, drainage, and nutrient recycling, while gastropods contribute to plant decomposition, nutrient recycling, and soil formation. Predation on these creatures may therefore affect soil properties, with cascading effects on the food web. Four land planarians, Arthurdendyus triangulatus, Bipalium kewense, Obama nungara, and Platydemus manokwari, are listed as invasive alien species of Union Concern under European Regulation 1143/2014, which obliges Member States to draft action plans to prevent unintentional introductions. The New Zealand flatworm, A. triangulatus, probably feeds exclusively on earthworms and has established populations in the United Kingdom, Ireland, and the Faroe Islands. The New Guinea flatworm, P. manokwari, recognized as one of the world’s worst invasive alien species, is a voracious predator of gastropods and a major threat to endemic partulid snails on several Pacific islands. It is also a host and transmission vector for the rat lungworm Angiostrongylus cantonensis, which can infect humans and cause meningitis.

For the Belgian review, the research team compiled an annotated checklist and distribution maps using data collected by the authors and citizen scientists through the iNaturalist and Waarnemingen.be/Observations.be platforms, including observations gathered as part of the FlatwormWatch citizen science project launched in May 2024. The nine species recorded in Belgium are Anisorhynchodemus cf. signata, Bipalium kewense, Caenoplana coerulea, Caenoplana variegata, Diversibipalium multilineatum, Dolichoplana striata, Marionfyfea adventor, Obama nungara, and Parakontikia ventrolineata. Obama nungara is by far the most widespread, reported from 63 localities that mainly include private gardens, courtyards, plant nurseries, and garden centers. The species was first found in the country in June 2017 and is probably native to Brazil and Argentina. It feeds on a variety of invertebrates, including gastropods, earthworms, and other planarians, and can attain remarkably high densities in gardens, with cumulative abundances of up to 1,585 individuals per month recorded in French studies. Its populations can rebound after dry periods, and its opportunistic feeding behavior, ability to shrink to survive starvation, and capacity to produce egg capsules after several weeks in isolation add to its invasive potential.

The climate-matching analysis used open data from the Global Biodiversity Information Facility and Köppen–Geiger climate maps to assess how similar each species’ current climatic distribution is to the Belgian climate, which is situated almost entirely within the temperate Cfb zone. Under current conditions and projections for 2041–2070, climate-matching percentages were highest for Arthurdendyus triangulatus at 99.9 percent, Marionfyfea adventor at 95.8 percent, Caenoplana variegata at 74.5 percent, and Obama nungara at 68.5 percent. Platydemus manokwari showed a mere 0.2 percent match, and Dolichoplana striata just 1.9 percent. Interestingly, the projections for 2071–2099 revealed a divergence among emission scenarios. Under high and very high emissions, climate-matching percentages decreased for A. triangulatus, C. variegata, M. adventor, and O. nungara, whereas they increased for B. kewense, D. multilineatum, D. striata, and P. manokwari. Under the very high emissions scenario, the match for D. multilineatum rose to 75.4 percent and for B. kewense to 58.7 percent, suggesting that continued warming could open Belgium to tropical species currently restricted to glasshouses.

The Harmonia+ risk assessment, a standardized protocol that combines scores from introduction, establishment, spread, and impact modules into invasion, environmental, and overall risk scores, confirmed the hierarchy of concern. Obama nungara was the only species to receive a very high environmental risk score, resulting from a very high invasion risk score and a medium environmental impact score, and its overall risk was classified as medium. Diversibipalium multilineatum, a Japanese species found only once in Belgium, in a private garden in Keerbergen in September 2024, also received a high environmental risk score and a medium overall risk classification. This large flatworm feeds on earthworms and appears to be expanding its range northward in Europe, contributing to uncertainty about its climatic tolerance. All other species received low overall risk scores. The three other outdoor species, C. variegata, M. adventor, and P. ventrolineata, were given medium environmental risk scores resulting from very high invasion risk scores and very low environmental impact scores. Caenoplana variegata feeds mostly on isopods, and photographic evidence from Belgian citizen scientists suggests that Marionfyfea adventor feeds on small gastropods, including snails that could potentially include the Annex II listed Vertigo species protected under the EU Habitats Directive.

Four species, Anisorhynchodemus cf. signata, Bipalium kewense, Caenoplana coerulea, and Dolichoplana striata, have so far been observed exclusively in heated glasshouses in Belgium, where tropical temperatures and high humidity sustain them. Bipalium kewense, the hammerhead flatworm first described from Kew Gardens in the United Kingdom but probably originating from Southeast Asia, has actually been present in Belgium for more than a century, with the earliest records dating to December 1906 at the Ghent University Botanical Garden. It now occurs in at least 78 countries worldwide, most likely through the transport of potted plants. Caenoplana coerulea, native to eastern Australia, received a very high invasion risk score partly because it has been recorded outdoors near the Belgian border in France, and nearly half of its global occurrences were recorded in climate zones present in Belgium. The review also assessed two species of Union Concern that do not yet occur in Belgium. Arthurdendyus triangulatus received a medium environmental risk score, with the Belgian climate highly suitable according to both the climate-matching analysis and earlier CLIMEX predictions, since conditions resemble those of Britain and Ireland where the species is firmly established. Platydemus manokwari received a high environmental risk score because of its high environmental impact, although Belgian climatic conditions are probably suboptimal for this year-round wet and humid species.

Management options for land planarians remain frustratingly limited. There are no known biological control methods, and no pesticides available for land planarians are free of nontarget effects on earthworms. A combination of removal trapping, removal of refuges, hot-water treatment, scorching, and turning the soil may be somewhat suitable as eradication or control measures once an invasive land planarian has been introduced, but the effectiveness of these measures depends on the extent of the infestation and the habitat complexity of the area. Because a substantial proportion of a land planarian population can remain hidden in the soil, it is uncertain whether eradication or population-control measures will ever be effective. The authors propose a two-tier approach to prevent new introductions: blacklisting selected high-impact invasive flatworm species to which restrictions, surveillance, early-warning, and rapid-response obligations under the EU IAS Regulation apply, and conducting generic biosecurity inspections of live plants by plant health authorities.

The role of citizen science emerges as a central pillar of the Belgian effort. Most land planarian observations were reported by citizen scientists on the Waarnemingen.be/Observations.be data platform, which includes records collected through the FlatwormWatch project. The project encourages gardeners, naturalists, terrarium hobbyists, and visitors to botanical gardens to look for land planarians in hiding places in gardens and glasshouses and to check potted plants, providing identification tools, search guidance, courses, and feedback through a newsletter. Several new locations and species have been reported since the project began, and the resulting data underpin this review. Continued monitoring is important because the nine species reported here may underestimate the actual number of non-native planarians in Belgium, and additional species may be introduced. More species have been found in neighboring France and the Netherlands, and several non-native land planarians already in Belgium may still be in the early stages of their invasion and may expand their ranges. The review provides a basis for developing suitable detection and monitoring methods, as well as rapid-response strategies for high-risk non-native land planarians before the quiet invasion beneath our feet becomes impossible to reverse.

Subject of Research: Non-native land planarians ( Platyhelminthes , Tricladida , Geoplanidae ) in Belgium: A review and risk assessment

Article Title: Non-native land planarians ( Platyhelminthes , Tricladida , Geoplanidae ) in Belgium: A review and risk assessment

Article References: Van den Neucker, T., Soors, J., Jacobs, A., Reniers, J., Delva, S., Kmentová, N., Vanhove, M. P., D’hondt, B., Scheers, K., Bronne, L., Ewers, C., Abraham, L., Groom, Q., Halfmaerten, D., Neyrinck, S., Breugelmans, K., Backeljau, T., Justine, J.-L., Gastineau, R., … Adriaens, T. (2026). Non-native land planarians (Platyhelminthes, Tricladida, Geoplanidae) in Belgium: A review and risk assessment. NeoBiota, 108, 305-334. https://doi.org/10.3897/neobiota.108.190976

Image Credits: AI Generated

DOI: 10.3897/neobiota.108.190976

Keywords: Non-native, land, planarians, Platyhelminthes, Tricladida, Geoplanidae, Belgium, review, risk, assessment, scientific research