Saturday, 22 November 2014

Hammerhead Slug: World's Largest Flatworm

Bipalium kewense, the weird hammerhead slug
Bipalium kewense
Notice the distinctive hammer-like head
By Ajaykuyiloor (Own work) [CC-BY-SA-3.0],
via Wikimedia Commons
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Turbellaria
Order: Tricladida
Suborder: Continenticola
Family: Geoplanidae
Subfamily: Bipaliinae
Genus: Bipalium
Species: Bipalium kewense
Common Names: Hammerhead slug, Greenhouse Planarian

Nicknamed as the "hammerhead slug" due to its half-moon shaped head, Bipalium kewense is not your everyday flat worm. Not only does it hold the record for world's largest flatworm but it's also one of the few flatworms that live on land. Oh, did I mention that it defecates from its mouth?

Distribution & Habitat
The hammerhead slug is believed to originate from Southeast Asia. However, it appears that the species has become cosmopolitan with recordings coming from many different tropical and subtropical countries. It's especially common in greenhouses, thus its second common name, the "greenhouse planarian".

The species has been found spanning the entire southern portion of North America. Verified recordings include:
  • Encanto, California 
  • Jersey City, New Jersey 
  • Nashua, New Hampshire 
  • New Orleans & Baton Rouge, Louisiana 
  • Puerto Rico, near Silver Springs 
  • Savannah, Georgia 
  • Urbana, Ohio 
  • Washington DC

Bipalium kewense is also common in the Hawaiian Islands and in the tropical parts of South America. It has also been sighted in the UK, China, Japan, New Zealand and many other countries. This widespread occurrence is the result of horticultural practices, mainly the commercial dispersion of potted plants.

Like earthworms, hammerhead slugs prefer to burrow in moist soil.

Fully mature adults routinely reach 40 cm (10 in) in length, with the maximum recorded length being about 60 cm (23 in). The body is covered by a layer of mucus that prevents it from losing too much water to the environment. The mucus is also important for locomotion.

They usually come in dark colors, like gray, brown and black and have two distinctive dorsal stripes that run the length of the body.

One of the species' weirdest traits is the half-moon shaped head. The mouth is located mid-way down the body (on the ventral side) which also serves as the.. anus since they don't have one. Yum! They also have no respiratory and circulatory system, skeleton and legs.

Hammerhead slug (Bipalium kewense) in Hawai
Hammerhead slug, crossing a road near Hilo, Hawaii.
By Dick Culbert from Gibsons, B.C., Canada (Bipalium kewense, a Hammerhead Worm.) [CC-BY-2.0], via Wikimedia Commons

The Hammerhead slug is predatory, primarily feeding on earthworms although it will turn to cannibalism when food is scarce. The species dietary patterns have not been extensively studied, and it possibly feeds on other organisms, like slugs and insect larvae.

To eat, it will follow trails left behind by earthworms until it finds one. When prey is caught, it will lay atop of it, as the sticky slime helps to hold it down to the soil. Then it protrudes its pharynx and sucks out the body fluid of the earthworm. Surely, not a good way to die..

Hammerhead slug attacking an earthworm

Bipalium kewense is hermaphroditic (like all Bipalium species) and capable of both asexual and sexual reproduction. However, the latter has rarely been observed and apparently fragmentation is the preffered form of reproduction.

This is done by chipping off about 1cm of the tail. The tip first attaches itself to something in the soil, and then the parent worm pulls away. The new worm can move immediately and develops a head within 10 days.

As for sexual reproduction, they lay eggs in a bright red cocoon. After one day the cocoon turns black and the eggs hatch about 20 days later, depending on temperature and moisture conditions.

Is it dangerous?
Over half of all known flatworm species (Platyhelminthes) are parasitic and some do enormous harm to humans and their livestock. However, this is not the case with the majority of the flatworms in the Turbellaria class, including B. kewense.

Production of Tetrodotoxin  
Tetrodotoxin (or TTX) is a potent neurotoxin that among others induces paralysis. Recent research revealed that Bipalium kewense and the closely related B. adventitium have small amounts of it in their body, most probably used during predation to subdue large prey items. As of 2014, they remain the only known terrestrial invertebrates capable of producing this toxin.

Interesting and Weird Facts Sum-Up
- Half-moon shaped head
- Mouth also serves as anus
- All individuals are hermaphroditic and capable of sexual and asexual reproduction. They usually reproduce by chipping a small part of the tail
- It is considered a pest to farmers because they predate on earthworms
- Non-parasitic, harmless to humans
- Along with the closely related B adventitium, the only known terrestrial invertebrate to produce the Tetrodotoxin toxin
References & Further Reading
- L. Winsor (1981). The taxonomy, zoogeography and biology of Bipalium kewense Moseley, 1878 (Tricladida, Terricola) Hydrobiologia, 84 (1), 17-17 DOI: 10.1007/BF00026158
- Yasuko Shirasawa & Naoya Makino (1991). Pharyngeal regeneration in the land planarian Bipalium kewense Hydrobiologia, 227 (1), 57-57 DOI: 10.1007/BF00027581
- Daly JJ, Farris HE Jr, & Matthews HM (1976). Pseudoparasitism of dogs and cats by the land planarian, Bipalium kewense. Veterinary medicine, small animal clinician : VM, SAC, 71 (11), 1540-2 PMID: 1049475
- Stokes, A., Ducey, P., Neuman-Lee, L., Hanifin, C., French, S., Pfrender, M., Brodie, E., & Brodie Jr, E. (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) DOI: 10.1371/journal.pone.0100718
- Winsor, L. 1983. A revision of the cosmopolitan land planarian Bipalium kewense Moseley, 1878 (Turbellaria: Tricladida: Terricola). Zool. J. of the Linnean Soc. 79: 61-100.
- Ducey, P. K., J. Cerqua, L-J West, and M. Warner. 2006. Rare egg capsule production in the invasive terrestrial planarian Bipalium kewense. Southwest Naturalist 51(2):252-254.


  1. Replies
    1. You have my permission.

  2. I live in Murrieta California and found one yesterday.. I thought it was a baby snake.. so put it over the fence to the front yard so my dog wouldn't get it..

  3. Just found one in Pomona, CA @ the Fairplex!

  4. One of these was in my garden in Agoura Hills, CA about ten years ago. Always thought it was some kind of weird slug transformation. Happy to have this resolved. Also horrified.

  5. There was one on my pants after sitting at a bus stop in Santa Ana, CA on Wednesday night. It was my first time coming across one so to be safe, I grabbed it with a plastic bag and threw it in the dumpster. Then, I used pine sol to clean up the fragments it left behind on my bathroom floor after I had frantically kicked off my pants.

  6. Found one in Atlanta, Ga.

  7. We have Hammerhead worms in March every year for the past three year, in our house. My husband has sealed everything he can seal off, and we still have them every March then they are gone. We hate when they come in our house, and are so glad when they stop coming in. Is there a spray or powder that would keep them out of our home?

  8. I just found one in my KITCHEN in Destin, Florida! It's now outside where it belongs!

  9. Found one this morning in my bathroom in Arlington, Texas

  10. Found one on my doorstep in riverdale Georgia i am concerned does this mean there are many more to come? Is it their season? Do i need to break contract and move my family ? I have small kids that like to play outside is it harmful to humans?

  11. Found one in la porte,Texas

  12. Saw one in my yard yesterday in Dacula, Ga

  13. My husband and I found one in West TN (Oakland,TN) today.

  14. My son found one in Seagrove, NC