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The hammerhead worm (Bipalium sp.) is a terrifying and toxic terrestrial flatworm. It's both a predator and a cannibal, and is basically a large planarian that lives on land. While the distinctive-looking worms don't pose a direct threat to human beings, they are an invasive species that eradicates earthworms.
Fast Facts: Hammerhead Worm
- Scientific Name: Bipalium sp.
- Other Names: Broadhead planarian, "landchovy"
- Distinguishing Features: Large terrestrial planarian with a spade-shaped head and ventral foot or "creeping sole"
- Average Size: Over 20 cm in length (B. kewense)
- Diet: Carnivorous, known to eat earthworms and each other
- Lifespan: Potentially immortal
- Habitat: Distributed worldwide, preferring humid, warm habitats
- Conservation Status: Not evaluated
- Kingdom: Animalia
- Phylum: Platyhelminthes
- Class: Rhabditophora
- Order: Tricladida
- Family: Geoplanidae
- Fun Fact: The hammerhead worm is one of very few terrestrial invertebrates known to produce tetrodotoxin.
The most distinctive features of the hammerhead worm are its fan-shaped or spade-like head and long, flattened body. The underside of the planarian has a large "creeping sole" used for locomotion. Species are differentiated by the shape of the head, size, coloration, and stripe pattern. The terrestrial planarians are earth-colored, found in shades of gray, brown, gold, and green. Small hammerhead worms include B. adventitium, which ranges from 5 to 8 cm (2.0 to 3.1 in) in length. In contrast, adult B. kewense worms can exceed 20 cm in length.
Distribution and Habitat
Hammerhead worms are native to tropical and subtropical regions, but have become invasive worldwide. It is believed the planarians were accidentally distributed on rooted horticultural plants. Because hammerhead worms require humidity, they are uncommon in desert and mountain biomes.
Bipalium worms are carnivores, known to prey on earthworms, slugs, insect larvae, and each other. The worms detect prey using chemoreceptors located under the head or ventral groove. A hammerhead worm tracks its prey, pushes it against a surface, and entangles it in slimy secretions. Once the prey is mostly immobilized, the worm extends is pharynx out from its body and secretes digestive enzymes. It sucks liquefied tissue into its branched gut using cilia. When digestion is complete, the worm's mouth also serves as its anus.
Hammerhead worms store food in vacuoles in their digestive epithelium. A worm can survive several weeks on its reserves and will cannibalize its own tissues for food.Bipalium kewense capturing an earthworm. Researchers believe the planarian secretes a toxin to immobilize its prey. Jean-Lou Justine, Leigh Winsor, Delphine Gey, Pierre Gros, and Jessica Thévenot
While some types of worms are edible, the hammerhead worm is not among them. The planarian contains the potent neurotoxin, tetrodotoxin. The toxin is found in pufferfish, the blue-ringed octopus, and rough-skinned newts, but not in a terrestrial invertebrate prior to its discovery in the hammerhead worm. The worm can use the toxin to immobilize prey and deter predators.
Hammerhead worms have been mistakenly called hammerhead slugs because they move in a slug-like fashion. They use cilia on their creeping sole to glide over a strip of mucus. The worms have also been observed lowering themselves down a string of mucus.
Land planarians are photo-negative and need high humidity. So, the animals usually move and feed at night. They prefer cool, damp places, typically residing under rocks, logs, or shrubs.
The worms are hermaphrodites, with each individual possessing both testes and ovaries. A hammerhead worm can exchange gametes with another worm via its secretions. Fertilized eggs develop inside the body and are shed as egg capsules. After about three weeks, the eggs hatch and the worms mature. In some species, juveniles have different coloration from adults.
However, asexual reproduction is much more common than sexual reproduction. Hammerhead worms, like other planaria, are essentially immortal. Usually, a worm reproduces via fragmentation, leaving behind a tail tip stuck to a leaf or other substrate, which then develops into an adult. If the worm is cut into pieces, each section can regenerate into a fully-developed organism within a few weeks. Injured worms rapidly regenerate damaged tissue.
None of the species of hammerhead worm have been evaluated for the IUCN Red List, but there is no evidence their numbers are threatened. Land planarians are widely distributed in their natural tropical and subtropical habitats and have extended worldwide. Once established in a greenhouse, the animals disperse into the surrounding region. In cold areas, the worms are able to survive freezing by seeking protected locations.
At one time, researchers were concerned terrestrial planarians might damage plants. Over time, they were deemed harmless to greenery, but then a more insidious threat appeared. Hammerhead worms can exterminate earthworm populations. Earthworms are important because they aerate and fertilize soil. While some methods used to control slugs also work on the flatworms, hammerhead worms are considered a threatening invasive species. Their effect on ecosystems has yet to be fully understood.
- Ducey, P. K.; Cerqua, J.; West, L. J.; Warner, M. (2006). Eberle, Mark E, ed. "Rare Egg Capsule Production in the Invasive Terrestrial Planarian Bipalium Kewense". The Southwestern Naturalist. 51 (2): 252. doi:10.1894/0038-4909(2006)51252:RECPIT2.0.CO;2
- Ducey, P. K.; West, L. J.; Shaw, G.; De Lisle, J. (2005). "Reproductive ecology and evolution in the invasive terrestrial planarian Bipalium adventitium across North America". Pedobiologia. 49 (4): 367. doi:10.1016/j.pedobi.2005.04.002
- Ducey, P. K.; Messere, M.; Lapoint, K.; Noce, S. (1999). "Lumbricid Prey and Potential Herpetofaunal Predators of the Invading Terrestrial Flatworm Bipalium adventitium (Turbellaria: Tricladida: Terricola)". The American Midland Naturalist. 141 (2): 305. doi:10.1674/0003-0031(1999)1410305:LPAPHP2.0.CO;2
- Ogren, R. E. (1995). "Predation behaviour of land planarians". Hydrobiologia. 305: 105-111. doi:10.1007/BF00036370
- Stokes, A. N.; Ducey, P. K.; Neuman-Lee, L.; Hanifin, C. T.; French, S. S.; Pfrender, M. E.; Brodie, E. D.; Brodie Jr., E. D. (2014). "Confirmation and Distribution of Tetrodotoxin for the First Time in Terrestrial Invertebrates: Two Terrestrial Flatworm Species (Bipalium adventitium and Bipalium kewense)". PLoS ONE. 9 (6): e100718. doi:10.1371/journal.pone.0100718