Planarians are flatworms in the class Turbellaria that can be found in a variety of ecosystems: freshwater, marine, and occasionally terrestrial. Biologically, they’re quite straightforward. They have no anus, they have a simple central nervous system, and their body is made up entirely of three basic germ layers. So why have these seemingly simple organisms captured the curiosity of scientists around the globe? 

The short answer to this question is that even the most unassuming organisms have valuable evolutionary adaptations that have gotten them this far. One of the planarians’ being regeneration. For most organisms, being cut in half, split in two, or having your head bit off is a death sentence, but not for planarians. They are able to fully regenerate 1/200th of a piece when separated from the main body.

This adaptation is incredibly beneficial. Besides protecting them from predators, it serves as a form of asexual reproduction. Planarians undergo a process called “fission.” They attach themselves to a rock and physically pull themselves apart, creating a genetic duplicate. The ability to survive being split in half makes planarians a particularly dangerous invasive species. Bipalium adventitium, or the broadheaded planarian, is a terrestrial flatworm that was introduced to the United States and began devastating native worm and snail populations. The problem worsened when people began attempting to kill them. Simply stepping on the worms only allowed them to clone themselves. Instead, scientists recommend pouring hot water over the flatworm.

The invasive quality of some planarians is certainly relevant to humans; however, it turns out there is a much more direct correlation. Regeneration is achieved by a plethora of organisms, ranging from deer that can regrow their antlers to starfish that can regrow arms. The question then becomes what allows some organisms to regenerate limbs, while we cannot. One way biologists hope to answer this question is by gaining a better understanding of planarians. Despite their simple central nervous system, planarians have some traits in common with humans. For one, we share many of the same neurotransmitters. With this in mind, in some cases, planarians are proving better suited for accurate drug trials than some mammals.

Although outwardly unassuming, the regenerative capabilities of planarians have proven to have caught people’s attention. Be it ecologists worried about the their ability to invade ecosystems and efficiently asexually reproduce, geneticists eager to discover how humans might learn from planarians ability, pharmacologists who are excited to use planarian to get accurate results quicker, or simply everyday people fascinated by a small, unassuming organism whose core biology goes against our definition of “humanly possible”. And yet, the more we learn about them, the more we learn that they’re more similar than we thought possible.

Photo of a planaria, a genus of planarians. From Holger Brandl, HongKee Moon, Miquel Vila-Farré, Shang-Yun Liu, Ian Henry, and Jochen C. Rink – PlanMine – a mineable resource of planarian biology and biodiversity. Nucleic Acids Res. 2016 Jan 4; 44(Database issue): D764–D773. CC BY 4.0