Hammerhead Flatworms and Other Land Planaria of Eastern North America

Classification

Common names: land planaria, terrestrial flatworm, hammerhead flatworms (Bipalium species only)

Order: Tricladida (planaria, free-living flatwoms)

Family: Geoplanidae (land planaria, terrestrial flatworms)

General Information

Description

Land planaria, which are also called terrestrial flatworms (and some of which are called hammerhead worms), are flattened, small to large worm-like animals that live in moist, terrestrial environments. Species in North America are typically yellow or orange with dark stripes to dark brown or black. Elsewhere in the world, land planaria can be brightly colored, including reds, pinks, greens, and blues, as well as having stripes and other striking color patterns. Some species have distinctive half-moon or crescent-shaped heads, while others have pointed heads.

Distribution

Land planaria are most common and diverse in tropical regions worldwide but are also found in temperate areas. At least fifteen species occur in eastern North America, 2–4 of which are native, with the rest being introduced from elsewhere.  Nine species have been found or are likely to occur outdoors in Pennsylvania; other speices might be found in heated, indoor spaces such as greenhouses.

Life History

While some terrestrial flatworms have adapted to live in drier environments, most, including the species present in North America, require moist habitats. Such habitats include under rocks, logs, and other debris on the ground, in plant pots and mulch, under bushes and dense shrubs that help retain moisture in the soil, and along stream banks. On rainy days or cool, damp mornings, land planarians may be found in areas that are typically too dry to inhabit, such as along sidewalks.

Land planaria are predatory on soft-bodied invertebrates, including earthworms and other annelid worms, molluscs such as snails and slugs, isopods, and insects. Some species are generalist predators that feed on any soft-bodied organism, while others have a narrower range of prey (e.g., worm specialists or mollusc specialists). Terrestrial flatworms capture prey by engulfing it with mucous and their bodies. Two species (Bipalium adventitum and B. kewense) have been shown to possess tetrodotoxin, a powerful neurotoxin better known from blue-ringed octopuses, newts, and pufferfish, which may be used to subdue prey in addition to mucous.

Terrestrial flatworms are hermaphrodites, which means that every individual has both male and female reproductive organs. This makes sexual reproduction easier as each individual worm only needs to find another individual to mate rather than males and females needing to find the opposite sex. Land planarians can also reproduce through a procession called "binary fission" where an individual flatworm spontaneously breaks in half. Each half regrows then the missing half – the head regrows the tail and the tail regrows a head – in about a week, resulting in two flatworms instead of one. Both of these reproductive strategies mean it is very easy for land planaria to become established in new areas because it only takes one or two individuals to start a new population.

Medical Importance

None. Although some social media posts and memes focus on the fact that land planeria produce tetrodotoxin (see Life History above), flatworms are incapable of injecting the venom into humans or pets.

Land planaria mucous is distasteful to animals and very few things eat them. There are no documented instances of pets or children being sickened by ingesting flatworms or humans being sickened by handling flatworms. However, caution should be used when handling flatworms as they are obviously chemically defended and may be irritating to the eyes or mouth. If land planaria are handled, you should wear gloves or wash your hands immediately afterward.

Environmental Impact

The impact of non-native land planarians depends on the species, their prey preferences, and where they are introduced. Some have very obvious negative impacts – New Guinea flatworms have directly caused the extinction of many snail species on various islands in the Pacific – while others have little, or at least undetermined impacts.

Three species of commonly encountered hammerhead flatworms feed exclusively (or nearly so) on earthworms, which is a cause of concern for many people. Earthworms are good for the soil after all!

However, all (or nearly all) of the earthworms encountered in urban and suburban areas in the Northeast are species that have been introduced from Europe (primarily), Asia, and elsewhere. Species that are native to the region often require forested settings and many are found under logs or the bark of fallen logs, so can’t survive in lawns, gardens, and other highly disturbed areas.  Indeed, a recent study found that the earthworm fauna of Baltimore, Maryland is similar to that of Budapest, Hungary and lacked species native to North America entirely. Even in "undisturbed" areas, such as 100-year-old secondary forests, non-native earthworms may predominate or completely exclude native earthworms, depending on how extensively the area was logged and/or farmed when the primary forest was removed initially.

So the impact of terrestrial flatworms in Pennsylvania and surrounding areas is difficult to assess. In urban and suburban areas, they are feeding exclusively (or nearly exclusively) on non-native earthworms that shouldn't be here to begin with. In disturbed areas where invasive jumping worms (Amynathis spp.) have been introduced, which are often considered worse for gardens and planting areas because they degrade the soil, land planarians may even be considered to be beneficial biocontrol agents. Terrestrial flatworms do not seem to have invaded less disturbed habitats in North America, but even such places there are many areas where introduced, non-native earthworms predominate. It's not clear if planarians will preferentially feed on native or non-native earthworms where they do co-occur, so difficult to know the impact terrestrial flatworms will have on native earthworms.

Control

Management of land planaria in the landscape is difficult to impossible as many species are well established and there are few scalable control recommendations available.

Individual flatworms can be killed by placing them in alcohol or into a bag and freezing them. When handling flatworms, gloves should be worn or the hands washed after – the risk from prolonged exposure to chemicals in land planaria mucous is likely low but unknown, so caution should be used regardless. Salt can be applied to individual planaria if you want to avoid touching them altogether. If salt is used, use caution when applying it around plants to avoid harming vegetation.

Do not cut up land planaria to control them. Terrestrial flatworms can regenerate from fragments (see Life History above), so cutting them into pieces just creates more planaria and exacerbates the problem.

If land planaria are discovered in plant pots, especially recently purchased plants, they should be preserved in ethanol and submitted to your state department of agriculture as there are many potentially invasive species that are not yet established or are only partially or patchily established in North America.

Species Accounts and Identification

Hammerhead Flatworms

Hammerhead flatworms (Bipalium and Diversibipalium species) are the most frequently encountered and recognizable terrestrial flatworms in Pennsylvania due to the large size of some species, unique coloration (yellow with dark stripes), and crescent-shaped heads, which give them their common name. Five species occur in eastern North America, all of which may be found in Pennsylvania.

Wandering Broadhead Planarian (Bipalium adventitium)

Medium-sized species, up to 3.1 inches (80 mm) in length. Head is fan-shaped rather than crescent-shaped, so does not recurve at the tips as in other Bipalium species in North America. Body color is light tan or yellow brown with a single median stripe. The median stripe is dark brown and does not reach the head in most specimens. There is no dark band or collar behind the head.

Bipalium adventitium is likely native to Asia like other Bipalium species, although its native range is not currently known. The species was first discovered in California in 1943 and is now know from Western and Eastern North America, especially the northeastern United States.

This species feeds primarily on earthworms but will also consume slugs and snails in smaller quantities. They reproduce primarily through sexual reproduction and laying eggs with a minority of individuals (3–5% in one study) experiencing binary fission.

Shovel-headed Garden Worm (Bipalium kewense)

Large species, up to 12 inches (300 mm) in length. Head is crescent-shaped and dark brown. Body color is light tan or yellow brown with five black, dark grey or dark brown stripes. The median and marginal strips are narrow and have distinct edges while the lateral stripes (between the median and marginal stripes) are wider and have diffuse edges. The lateral and marginal stripes converge behind the head to form an incomplete dark band or collar and the median stripe does not reach the head.

This species is native to Vietnam and Cambodia but has been spread worldwide via plant pots and other material. They were first discovered and described from the Kew Gardens in London, England (hence the species name kewense), although they are not native to there. They are widespread in the eastern United States and can be found as far north as Pennsylvania and Illinois.

Bipalium kewsense feed exclusively on earthworms. Most individuals in North America reproduce via binary fission and the production of coccoons is uncommon.

Three-lined Land Planarian (Bipalium pennsylvanicum)

Large species, up to at least 5.1 inches (130 mm) in length. Head is dark brown and crescent-shaped but the tips are not recurved as in B. kewense. Body color is light tan or yellow brown with three dark or light brown stripes. The median stripe is dark brown, narrow, and has distinct edges; it is sometimes broken into a dashed line. The lateral stripes are lighter brown, wider, and have diffuse or indistinct edges. The median stripe reaches the head and there is not dark brown band or collar behind the head.

This species is native to Asia, possibly Japan, although its native range is not entirely certain. It is widespread but patchily distributed in eastern North America and is most commonly recorded in Pennsylvania.  

Bipalium pennsylvanicum feed primarily or exclusively on earthworms (in laboratory studies, they will feed on dead, mashed slugs but not living slugs). They reproduce primarily via cocoons, most of which are produced during the spring, and were not observed to reproduce spontaneously via binary fission, although individuals that were injured by being cut in half survived and grew new heads or tails.

Mollusc-eating Hammerhead Worm (Bipalium vagum)

Small species, up to 1 inch (25 mm) in length. Head is dark brown to black and crescent-shaped. Body color is light tan or yellow brown with three brown stripes. The median stripe is dark brown and has distinct edges. The lateral stripes are lighter brown and have diffuse or indistinct edges. The median and lateral stripes reach a dark brown to black band or collar behind the head. The brown or black coloration of the collar and head are separated by a thin light brown stripe, although this may be indistinct and difficult to see.

Like other Bipalium, this species is likely native to Asia, although like B. adventitium it has not been found in its native habitat yet. Bipalium vagum were first described from Bermuda in 2005. They have since been discovered on numerous Caribbean islands, Italy, and North America. In the Untied States, the species is known primarily from Florida and the Gulf Coast, although specimens have been found as far north as West Virginia and Ohio. Specimens in northern areas have been found in greenhouses and other indoor spaces, so it is unclear if they can survive outdoors in Pennsylvania.

Unlike other Bipalium, mollusc-eating hammerhead worms feed exclusively on molluscs (snails and slugs) rather than earthworms.

Diversibipalium multilineatum (no common name)

Large species, up to 12 inches (300 mm) in length. Body color is yellow-brown with five dark brown to black stripes. The head is crescent-shaped and the same color as the body. This species is similar in color to B. kewense except that it lacks a dark collar behind the head and the median strip extends onto and is somewhat expanded towards the apex of the head. The median stripe is often broken or discontinuous.

This species is native to Japan. Outside of their native range, they were first discovered in Italy in 2014 and reported in 2016, although earlier records from 2010 in France were later reported in 2018. They have since been reported from wide areas of Italy, France, and Switzerland.

Diversibipalium multilineatum has not officially been reported from North America. However, numerous photographs of the species have been posted to iNaturalist, primarily from North and South Carolina, Tennessee, and Georgia, although a few have been found around Washington, D.C., and a single specimen was found in southeastern Pennsylvania. The earliest North American record on iNaturalist is from 2016.

The preferred prey of this species has not been reported, nor has any information about its ecology. Mature sexual organs have not been observed, so it is suspected that they only reproduce asexually via binary fission.

Other Terrestrial Flatworm Species

Blue Garden Flatworm (Caenoplana coerulea)

Medium-sized species, up to 4.7 inches (120 mm) in length. The coloration of this species is distinctive amongst terrestrial flatworms in North America. The upper surface is dark grey to black with a creamy median line while the underside is blue. The head is pinkish and narrow, lacking the expanded crescent-shape found in Bipalium.

Blue garden flatworms are native to eastern Australia and New Zealand but have been introduced around the world, including Argentina, France, the Canary and Balearic Islands, and North America. In the North America, this species was first discovered in California in 1943. Blue garden flatworms prefer a warm climate and most specimens are known from California and the Southeast, although some have been found outdoors as far north as Iowa in the west and Massachusetts, Ohio, and Illinois in the east.

Blue garden flatworms are generalist predators that feed on small insects as well as snails.

Diporodemus indigenus (no common name)

Small species, up to 1 inch (25 mm) in length. Dorsum black to purple-black in color, with slightly lighter colored sides. This species is easily confused with Microplana terrestris. However, M. terrestris is darker black in color and lacks the lighter colored sides seen in D. indigenus. Other diagnostic characters require dissection and/or microscopic examination.

This species found in the Appalachian Mountains from Quebec and New York south to Virginia, as well as Kentucky, Indiana, and Illinois. Some authors state that it is native to North America while others that it was introduced from Central or South America. Specimens have not been collected in Pennsylvania although it likely occurs here.

Apparently nothing has been published about their feeding habits or ecology.

Diporodemus merridithae (no common name)

Tiny species, up to 0.4 inches (11 mm) in length. The coloration of this species is distinctive amongst terrestrial flatworms in North America. The dorsum is dark gray with a darker grey or black median stripe, a white collar, and a white tail. Two eye spots are visible on the head. The head and tail are bluntly rounded.

This species is native to North America. They are known only from a 0.8 km area of Clingman's Dome in Great Smokey Mountains National Park. Nothing is known about their feeding habits ecology.

Dolichoplana striata (no common name)

Medium-sized species, up to 4.7 inches (150 mm) in length. Dorsum is yellow to dark brown-yellow with six dark brown stripes: paired median, marginal, and lateral stripes. In some specimens, especially young individuals, some stripes may be faint or entirely absent. Somewhat similar in color pattern to Bipalium kewense, which is yellow with five stripes (the median stripe is unpaired); however, D. striata has a rounded head rather than the expanded crescent-shaped head of B. kewense. Two eyes can be seen on the head in light-colored specimens.

Originally Southeast Asia (variously reported as Malaysia, Indonesia, or Sri Lanka by various authors), D. striata has been transported around the world and has been found in Ireland and mainland Europe, Barbados, and French Guyana as well as North America. In the United States it was originally reported from Missouri in 1943 and has since been found throughout the Southeast as far west as Texas. Interestingly, the Missouri record appears to be the northernmost record in North America, with other specimens found in Tennessee and North Carolina.

This species is reported to either feed on earthworms or have unknown feeding preferences. It is suspected to reproduce via binary fission.

Geoplana arkalabamensis (no common name)

Small species, up to 2.5 inches (65 mm) in length. The upper surface of the body is black, with a thin, lighter colored median stripe. The underside of the body is dark brown. Both ends of the body are tapered. Other diagnostic characters require dissection and/or microscopic examination.

This species is reported from Alabama, Mississippi, Arkansas, and Texas and is only known from fewer than a few dozen specimens. It is unknown if this species is native or introduced from South America. Preferred prey items have not been reported for this species.

Microplana rufocephalata (no common name)

Small species, up to 1.3 inches (35 mm) in length. Body black with a reddish brown, pointed head.

This species is only know from the two specimens collected on Pine Mountain, near Harland, Harland County, Kentucky in 1952. It is unknown if it is native to North America or introduced from Europe like other Microplana species, although the remote locality suggests it is likely native. Nothing is known about their ecology.

Microplana scharffi (no common name)

Medium-sized species, up to 3.5 inches (90 mm) in length. Body color varies from cream/yellow to pink or greyish, depending on recent prey. The head is pointed.

This species is native to Europe. It was first discovered in North America in Seattle, Washington in 1989 and most specimens are known from the West Coast. In eastern North America, multiple photographs posted of different specimens in Hamilton, Ontario, which suggests the species may be established there.

Microplana scharffi is a generalist predator on earthworms and small insects.

Microplana terrestris (no common name)

Small species, up to 1.2 inches (30 mm) in length. Dorsum dark grey to nearly black in color, although some specimens may be purplish, and lacks stripes or other patterning. The underside is pale in color. The head is pointed.

This species is native to Europe, where it is the most widespread terrestrial flatworm species. In North America, they were first reported from Wisconsin in 1939. They are now widespread found throughout eastern North America as well as the Pacific Northwest.

Microplana terrestris is a generalist predator on molluscs, earthworms, and small insects.

Obama nungara (no common name)

Small species, up to 2.7 inches (70 mm) long. Dorsum light brown with black lines; a pale median line may be present. Some specimens are black and may be confused for other small, darkly colored flatworms. The head is bluntly rounded.

This species is native to South America. It was discovered in Europe in 2008, where it has become the most common terrestrial flatworm in France and spread as far as Slovakia. It has not officially been reported from North America but specimens identified as O. nungara on iNaturalist have been photographed in North and South Carolina, Louisiana, Texas, and California as far as 2020.

Obama nungara is a generalist predator that has been recorded to feed on earthworms, slugs, and other terrestrial flatworms. They readily produce cocoons, which contain 4–6 offspring. No references to binary fission were found, but presumably this species can regenerate from broken pieces the way other terrestrial flatworms can.

This species has quickly (in just a decade) become the most common terrestrial flatworm in France and was ranked as a medium-risk species in The Netherlands due to their ability to predate and depopulate earthworms. However, across Europe they have been found almost entirely in disturbed urban and suburban areas, with a single record from an undisturbed natural area. So the potential impact of this species in North America is unclear and will depend on how quickly they spread and if they remain synanthropic (restricted to human-disturbed areas) or spread into forests and other natural areas.

New Guinea Flatworm (Platydemus manokwari)

Medium-sized species, up to 2.6 inches (65 mm) in length. The dorsum is dark brown with a light brown median line. The underside is light brown or tan. The head and tail are pointed.

New Guinea flatworms are native to Australia, New Guinea, and the Solomon Islands. This species has spread across many islands in the Pacific. They have also been found in Southeast Asia, mainland Europe, Hong Kong, several Caribbean islands, and Florida.

This species are snail specialists and are considered highly invasive – indeed, they are the only land planarian included in the top 100 most invasive organisms. They have been implicated in the mass extinction of Pacific Island snails.

This species is known only from tropical and subtropical regions of the world except for specimens from France that were discovered in a hothouse. In North America, they will likely have a negative impact on snails in Florida but may not be able to spread and establish further north outdoors (although may eventually be found in greenhouses and other heated structures).

Snake-headed Flatworm (Rhynchodemus sylvaticus)

Tiny species, up to 0.7 inches (18 mm) in length. Body color light brown to dark brown to nearly black. The dorsum has two dark lines and sometimes a brown spot or saddle in the middle of the body, which are especially visible in lighter-colored specimens. The head is pointed with two small eyes, usually dark like the body but sometimes light in color. Other diagnostic characters require dissection and/or microscopic examination.

This species is widespread in eastern North America, from Nova Scotia and Florida west to Nebraska and Texas; it is also known from the West Coast. In addition, it is recorded from Ireland and Great Britain, New Zealand, and South America. Once through to be a native species, it is now thought to be introduced, possibly from South America, although this has not been confirmed through thorough morphological or molecular studies.

Snake-headed flatworms are generalist predators that have been recorded to feed on small molluscs and arthropods.

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