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== Description ==
== Description ==
''Bipalium'', also known as the hammerhead worm, land planarian, or shovel-head garden worm, is a genus of invasive flatworm found in moist terrestrial environments in the northeastern United States [2]. The hammerhead worm is native to southeast Asia, and was introduced in 1891 [1]. This is easily identifiable by its distinct flattened body and oblong hammerhead. The worms are most often yellow, orange, or light brown, with a varying number of darker brown stripes running the length of the body [2]. Hammerhead worms pose a threat to natural ecosystems, as there are at least three species that have been found to feed exclusively on earthworms. They also pose a potential threat to agricultural operations, especially those operating in warm, moist environments [3].
''Bipalium'', also known as the hammerhead worm, land planarian, or shovel-head garden worm, is a genus of invasive flatworm found in moist terrestrial environments in the northeastern United States [2]. The hammerhead worm is native to Southeast Asia and was introduced in 1891 [1]. This is easily identifiable by its distinct flattened body and crescent-shaped head. The worms are most often yellow, orange, or light brown, with a varying number of darker brown stripes running the length of the body [2]. Hammerhead worms pose a threat to natural ecosystems, as there are at least three species that have been found to feed exclusively on earthworms. They also pose a potential threat to agricultural operations, especially those operating in warm, moist environments [3].




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=== Habitat and Range ===
=== Habitat and Range ===
''R. cathartica'' contains a number of secondary compounds, present in both the fruit, leaves, root exudates, and other tissue of the plant<ref name="Seltzner"></ref> A specifically noted compound is that of emodin, which can impact germination of other plant seeds.<ref name="Knight"></ref> Emodin is present both in the root exudate of ''R. cathartica'' and in the fruit; due to the fact that the drupes often fall beneath the parent tree, both have a significant impact on germination. The presence of emodin also may contribute to the purgative effects of ''R. cathartica'' fruit, meaning that those unripe fruits which any animal may ingest will cause either regurgitation or very quick passing of the fruit.<ref name="Knight"></ref> This is significant as birds readily eat the fruit available.
Hammerhead [[flatworms]] thrive in moist environments, including forests and agricultural fields. They are found amongst the upper [[soil]] layers and in leaf litter [2]. Although these worms prefer warm, wet environments, they are highly adaptive to different environments, including temperate forests in midlatitudes. This element allows hammerhead worms to invade higher latitudes than those to which they are native. In their native range of Southeast Asia, hammerhead flatworms live in soils.  
 


=== Life Cycle ===
=== Life Cycle ===
 
The hammerhead worm is hermaphroditic like most terrestrial worms, however they also have the ability to reproduce asexually via binary fission [7]. This allows ''Bipalium'' to reproduce quickly in stressful environments, and also contributes to their ability to invade new areas across the United States [8].


== Impact as an Invasive Species ==
== Impact as an Invasive Species ==


[[File:invasionmap.jpg|right|600px|thumb|A map of where ''Bipalium'' has been found in the United States [6]]]
[[File:invasionmap.jpg|right|600px|thumb|A map of where ''Bipalium'' has been found in the United States [6]]]
Since its introduction to the United States in the 1800s, ''R. cathartica'' has established itself in much of the lower 48 states.<ref name="Map"></ref> It has significant impact as an invasive species due to its allelopathic compounds, as discussed. In addition to this, buckthorn has an advantage when it comes phenology: extended leaf phenology. Buckthorn tends to flush out much earlier than native plants in its introduced range, allowing it to have photosynthetic advantage and to shade out native understory plants in the process, encouraging the establishment of a monoculture.<ref name="Knight"></ref> Additionally, the drupes on the shrub remain long past other native berries, providing a singular food source for birds which disperse the seeds as a result.<ref name="Knight"></ref>
Hammerhead worms disrupt the natural soil ecosystem the occurs below the leaf litter in forest and agricultural environments. The worms mainly consume earthworms and other soil macroinvertebrates, which has negetive effects on the lower trophic levels of [[organisms]] in the soil.  


Another advantage is the preference that [[insects]] and mammalian herbivores have for native plants over ''R. cathartica'' in North America.<ref name="Knight"></ref> White-tailed deer, a significant herbivorous population, preferentially eat native plants due to the fact that ''R. cathartica'' causes them some illness, resulting in increased herbivorous pressure on those native plants. This also applies to small mammals which forage in the relevant environments. The presence of thick monospecific thickets which ''R. cathartica'' form tends to encourage small mammal [[foraging]], due to an increase in available refuge; this limits the germination of native trees or other highly foraged plants.<ref name="Utz">Ryan M. Utz, Alysha Slater, Hannah R. Rosche, Walter P. Carson. "Do dense layers of invasive plants elevate the foragingintensity of small mammals in temperate deciduous forests? A case study from Pennsylvania, USA". ''NeoBiota 56: 73–88 (2020)'' doi: 10.3897/neobiota.56.4958. Retrieved 5/16/2022.</ref>
''Bipalium'' has the ability to reproduce through binary fission. This means that a single worm can spontaneously split into two, and each half will regrow the missing half in approximately a week.


The result of the success of ''R. cathartica'' as a competitor means that when introduced in an area, buckthorn tends to form dense monospecific thickets which exclude any native competitors and often reducing understory plants to a minimum.<ref name="Knight"></ref> With monospecific thickets such as these, there is also the alteration of [[Nutrient Cycling|nutrient cycling]], as the leaves of buckthorn are extremely high in Nitrogen. The change in litter types has significant impact on the [[Soil Ecology|soil ecology]] as it will rapidly increase high-N litter pools and then rapidly decrease them as longer lasting leaf litter is eradicated.<ref name="Knight"></ref> When this is combined with invasive earthworms such as ''L. terrestris'' the alteration of [[Soil Ecology|soil ecology]] in an area is notable, where quickly [[decomposing]] leaves and the high loss of detritus in the presence of invasive earthworms intersect and create rapidly cycling nutrient pools.
There is potential for invasive hammerhead flatworms to have natural predators in the Northeastern United States. Carnivorous snails have been shown to have positive impacts on decreasing ''bipalium'' populations in a lab setting. With more research, there is potential for these snail species to help reduce populations.


The allelopathic nature of ''R. cathartica'' also falls under the hypothesis of the ''novel weapons hypothesis''; the hypothesis that non-native plants have competitive traits which native plants in the introduced range do not have a defense against.<ref name="Pinzone">Pinzone, P., Potts, D., Pettibone, G. et al. "Do novel weapons that degrade mycorrhizal mutualisms promote species invasion?". ''Plant Ecol 219, 539–548 (2018)''. https://doi-org.gate.lib.buffalo.edu/10.1007/s11258-018-0816-4. Retrieved 5/16/2022.</ref> The root exudates and secondary compounds of ''R. cathartica'' are hypothesized to inhibit not only the growth and germination of competitor plants, but also possibly the mutualist fungi which other plants form symbiosis with. As has been established, buckthorn contains emodin, a powerful allelochemical; in some cases this has been found to impact the mutualisms of other plants. The root exudates of ''R. cathartica'' were found to reduce arbuscular and vesicular colinization in ''Ulmus'' sp. despite both plants employing [[Arbuscular Mycorrhizal Fungi|arbuscular mycorrhizal fungi]] in their mutualist relationships.<ref name="Pinzone"></ref> It is possible that there are other native competitors with fungal mutualisms which ''R. cathartica'' may indirectly degrade due to its [[allelopathy]].
== References ==


<p>[1] Ducey, P. K., J. Cerqua, L.-J. West, and M. Warner. 2006. Rare Egg Capsule Production in the Invasive Terrestrial Planarian Bipalium kewense on JSTOR.


== References ==
<p>[2] Skvarla, M. J. 2022, May 2. Hammerhead Flatworms and Other Land [[Planaria]] of Eastern North America. https://extension.psu.edu/hammerhead-flatworms-and-other-land-planaria-of-eastern-north-america.
 
<p>[3] Canello, R., V. S. Lemos, and Leal-Zanchet. 2012. Carnivore mollusks as natural enemies of invasive land flatworms - Lemos - 2012 - Annals of Applied Biology - Wiley Online Library.
 
<p>[4] County of Brant. 2024, April 30. Hammerhead Worm. https://www.brant.ca/en/recreation-and-parks/hammerhead-worm.aspx.
 
<p>[5] Jones, H., and J.-L. Justine. 2020, November 9. Land flatworms are invading the West Indies. http://theconversation.com/land-flatworms-are-invading-the-west-indies-149691.
 
<p>[6] EDDMapS. 2025. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed April 17, 2025.
 
<p>[7] Ducey, P. K., L.-J. West, G. Shaw, and J. De Lisle. 2005. Reproductive [[ecology]] and evolution in the invasive terrestrial planarian Bipalium adventitium across North America. Pedobiologia 49:367–377.


[6] EDDMapS. 2025. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed April 17, 2025.
<p>[8] Ogren, R. E., and J. K. Sheldon. 1991. Ecological Observations on the Land Planarian Bipalium Pennsylvanicum Ogren, with References to Phenology, Reproduction, Growth Rate and Food Niche. Journal of the Pennsylvania Academy of Science 65:3–9.

Latest revision as of 18:02, 17 April 2025

Scientific Classification
caption
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Turbellaria
Order: Tricladida
Family: Geoplanidae
Genus: Bipalium
Source: County of Brant [4]

Description

Bipalium, also known as the hammerhead worm, land planarian, or shovel-head garden worm, is a genus of invasive flatworm found in moist terrestrial environments in the northeastern United States [2]. The hammerhead worm is native to Southeast Asia and was introduced in 1891 [1]. This is easily identifiable by its distinct flattened body and crescent-shaped head. The worms are most often yellow, orange, or light brown, with a varying number of darker brown stripes running the length of the body [2]. Hammerhead worms pose a threat to natural ecosystems, as there are at least three species that have been found to feed exclusively on earthworms. They also pose a potential threat to agricultural operations, especially those operating in warm, moist environments [3].


Bipalium preying upon an earthworm [5]

Behaviors

Habitat and Range

Hammerhead flatworms thrive in moist environments, including forests and agricultural fields. They are found amongst the upper soil layers and in leaf litter [2]. Although these worms prefer warm, wet environments, they are highly adaptive to different environments, including temperate forests in midlatitudes. This element allows hammerhead worms to invade higher latitudes than those to which they are native. In their native range of Southeast Asia, hammerhead flatworms live in soils.


Life Cycle

The hammerhead worm is hermaphroditic like most terrestrial worms, however they also have the ability to reproduce asexually via binary fission [7]. This allows Bipalium to reproduce quickly in stressful environments, and also contributes to their ability to invade new areas across the United States [8].

Impact as an Invasive Species

A map of where Bipalium has been found in the United States [6]

Hammerhead worms disrupt the natural soil ecosystem the occurs below the leaf litter in forest and agricultural environments. The worms mainly consume earthworms and other soil macroinvertebrates, which has negetive effects on the lower trophic levels of organisms in the soil.

Bipalium has the ability to reproduce through binary fission. This means that a single worm can spontaneously split into two, and each half will regrow the missing half in approximately a week.

There is potential for invasive hammerhead flatworms to have natural predators in the Northeastern United States. Carnivorous snails have been shown to have positive impacts on decreasing bipalium populations in a lab setting. With more research, there is potential for these snail species to help reduce populations.

References

[1] Ducey, P. K., J. Cerqua, L.-J. West, and M. Warner. 2006. Rare Egg Capsule Production in the Invasive Terrestrial Planarian Bipalium kewense on JSTOR.

[2] Skvarla, M. J. 2022, May 2. Hammerhead Flatworms and Other Land Planaria of Eastern North America. https://extension.psu.edu/hammerhead-flatworms-and-other-land-planaria-of-eastern-north-america.

[3] Canello, R., V. S. Lemos, and Leal-Zanchet. 2012. Carnivore mollusks as natural enemies of invasive land flatworms - Lemos - 2012 - Annals of Applied Biology - Wiley Online Library.

[4] County of Brant. 2024, April 30. Hammerhead Worm. https://www.brant.ca/en/recreation-and-parks/hammerhead-worm.aspx.

[5] Jones, H., and J.-L. Justine. 2020, November 9. Land flatworms are invading the West Indies. http://theconversation.com/land-flatworms-are-invading-the-west-indies-149691.

[6] EDDMapS. 2025. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed April 17, 2025.

[7] Ducey, P. K., L.-J. West, G. Shaw, and J. De Lisle. 2005. Reproductive ecology and evolution in the invasive terrestrial planarian Bipalium adventitium across North America. Pedobiologia 49:367–377.

[8] Ogren, R. E., and J. K. Sheldon. 1991. Ecological Observations on the Land Planarian Bipalium Pennsylvanicum Ogren, with References to Phenology, Reproduction, Growth Rate and Food Niche. Journal of the Pennsylvania Academy of Science 65:3–9.

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