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Steve Begin, Flickr
Planarians, free-living flatworms inhabiting terrestrial, marine, and freshwater environments globally, are remarkable creatures with a rich natural history and a complex role in biomedical research. Known for their ability to regenerate lost body parts, including their heads, these worms are pivotal in regeneration studies. Beyond that, they are gaining renewed attention in fields like pharmacology and neuroscience. Plus, they’re undeniably adorable. Here are some intriguing facts about them you might not be aware of.
1. Certain species engage in chemical warfare.
Several marine planarian species, along with at least one terrestrial species, produce tetrodotoxin, a highly toxic compound with no known antidote.
2. Their mouth is situated in the middle of their body and serves multiple purposes beyond just eating.
A.G. Pagán. Adapted from The First Brain: The Neuroscience of Planarians. right Oxford University Press.
Planarians are carnivorous, often preying on smaller creatures or scavenging for food. When hunting, they coil around their prey, resembling a constrictor snake. Their feeding process is both fascinating and eerie: they extend a tubular organ called a proboscis or pharynx from the center of their body. This structure not only functions as a mouth but also doubles as an anus.
3. They nearly became the cornerstone of genetic research in the 20th century.
In the early 1900s, Thomas Hunt Morgan, the pioneer of modern genetics, debated between planarians and fruit flies (Drosophila melanogaster) as the ideal animal model for his studies. Ultimately, he chose Drosophila, shaping the course of genetic research.
4. The majority of planarian species possess two eyes that appear perpetually crossed, a phenomenon with no clear explanation.
O.R. Pagán.
Some planarian species have multiple eyes scattered across their bodies, while others have just one or none at all. Interestingly, even a decapitated planarian can sense light. Additionally, many species feature ear-like structures on their heads that, instead of detecting sound, are used to sense chemicals—essentially allowing them to 'taste' and 'smell' with these organs.
5. A comic book named Planarian Man actually exists.
Neal Obermeyer
Planarian Man, a unique superhero blending human and planarian traits, was created by Neal Obermeyer, a journalist and editorial cartoonist from Omaha, Nebraska. His origin story involves a high school experiment gone awry, where a boy accidentally fused with a piece of planarian after cutting his finger. Over time, the boy transformed into the crime-fighting Planarian Man.
6. Planarians exhibit behaviors strikingly similar to human addiction when exposed to drugs commonly abused by people.
For decades, scientists used planarians to study physiology by exposing them to various drugs and chemicals, but not specifically addiction. This changed in 2001 when Dr. Robert Raffa and his team at Temple University published findings showing planarians displaying withdrawal-like symptoms after cocaine exposure. This discovery sparked a surge in research on planarian pharmacology. Since then, numerous substances, including nicotine, amphetamines, and cannabinoids, have been found to trigger addiction-like behaviors in these worms.
7. Planarians have made an appearance on The Big Bang Theory.
In the episode "The Monster Isolation," which aired on February 21, 2013, Amy Farrah Fowler, a neuroscientist played by Mayim Bialik, lamented the challenges of working with nicotine-addicted monkeys. She nostalgically remarked, “... This makes me miss my marijuana-abusing flatworms; those guys were so chill!”
8. Two popular TV shows referenced planarians on the same day and at the same time!
On March 16, 2014, both The Walking Dead and the rebooted Cosmos series, hosted by Neil deGrasse Tyson, featured planarians. In The Walking Dead episode “The Grove,” a character named Mika reminisces, “I miss science class, except for the gross parts like dissecting planaria worms.” Meanwhile, Cosmos showcased planarians in a segment about evolution during the episode “Some of the things that molecules do.” However, the animation mistakenly depicted a freshwater planarian in a marine setting, and its movements were inaccurate. Still, it’s commendable that planarians were included at all.
9. Numerous planarian species possess the remarkable ability to regenerate lost body parts, including their heads, which house simple yet fully functional brains.
A.G. Pagán. Adapted from The First Brain: The Neuroscience of Planarians. right Oxford University Press.
When a planarian is cut into multiple pieces, each fragment can regenerate into a complete worm. If decapitated, the head survives and continues to move, eventually growing a new body. Similarly, the headless body regenerates a new head, complete with a brain and nervous system.
Unlike most bilateral animals, which have a single nerve cord, planarians possess two nerve cords running along their bodies. The reason for this dual-cord system remains a mystery.
10. By cutting planarians in specific ways, scientists can induce them to grow multiple heads.
The phenomenon becomes even more fascinating (or bizarre!). Typically, when a planarian is split in half, the head regenerates a tail, and the tail regenerates a head. However, researchers have recently discovered methods to make the tail grow another tail instead of a head and the head grow another head instead of a tail. Here are a few examples:
Dr. Junji Morokuma, Levin lab, Tufts University
11. Planarians have the ability to learn, and even after decapitation, their regenerated heads retain the memories of what they previously learned.
During the 1950s and 1960s, experimental psychologist James V. McConnell and his team conducted groundbreaking experiments with planarians to study memory. Their findings suggested that trained planarians retained their learned behaviors even after decapitation and head regeneration. Remarkably, the regenerated worms appeared to remember their training.
Due to various complex reasons, many in the scientific community questioned these experiments, citing issues like inadequate controls, observer bias, and other criticisms. However, in 2013, a team led by Dr. Mike Levin at Tufts University published compelling evidence confirming that planarians can learn and that their tails retain memories.
12. Transplanting a planarian’s brain into another planarian’s body can result in the brain taking control of its new host.
Researchers have successfully transplanted planarian brains between worms, often observing partial functional recovery. In some cases, brains were even transplanted between different species. In even stranger experiments, scientists dissected and reinserted marine planarian brains at odd angles, sometimes backward, and still observed partial recovery. For a deeper dive into these experiments, check out my book, The First Brain: The Neuroscience of Planarians.
13. DARWIN STUDIED PLANARIANS.
Charles Darwin conducted studies on certain land planarians and, while not the first to document it, he observed their remarkable regenerative capabilities. He described this phenomenon in his own words:
“After cutting one of them transversely into two nearly equal parts, within two weeks, both sections developed into complete organisms. I had divided the body in such a way that one half contained both lower orifices, while the other had none. Within twenty-five days, the more complete half became indistinguishable from any other specimen.”
