13 Things You Didn't Know About Planarians

Steve Begin, Flickr

Planarians—free-living flatworms found around the world in terrestrial, marine, and freshwater environments—are fascinating organisms with an interesting natural history ... and a rather complicated relationship with biomedical research. Many species of these worms can regrow their body parts, including a new head, upon decapitation, and are therefore one of the organisms of choice in regeneration research. But they're also being “rediscovered” in several other research areas, including pharmacology and the neurosciences. And they're also pretty cute. Here are some things you probably didn’t know about them.

1. Some species practice chemical warfare.

There are several marine planarian species and at least one terrestrial planarian species that produce tetrodotoxin, one of the most lethal substances known. There is no known antidote.

2. Their mouth is located at the middle of their body—and it’s not only used for eating.

A.G. Pagán. Adapted from The First Brain: The Neuroscience of Planarians. Copyright Oxford University Press.

All planarians are meat-eaters (cannibals, even); most are active predators or, at the very least, scavengers. When hunting small prey, like water fleas, planarians wrap themselves around their prey just like a constrictor snake. Mealtime plays out like a scene from a horror movie: To eat, the worms extend a tube-like organ, called a proboscis or pharynx, which is located at the center of their body. Their mouth is located at the pharynx’s tip, which also acts as an anus.

3. They almost were the animal model that defined genetics in the 20th century.

In the early 20th century, Thomas Hunt Morgan —the father of modern genetics— considered planarians and the fruit fly (Drosophila melanogaster) as potential animal models. He chose Drosophila, and the rest is scientific history.

4. Most planarian species have two eyes, which always look “crossed”; nobody knows why.

O.R. Pagán.

There are also species with many eyes, distributed throughout their bodies; species with only one eye; and eyeless species. Weirdly, a decapitated planarian body can detect light, and many planarian species have ear-like structures on their heads that don't detect sound but chemicals; you could say that they taste and smell with their ears.

5. There was an actual comic book titled Planarian Man.

Neal Obermeyer

Planarian Man is the creation of Neal Obermeyer, a journalist and editorial cartoonist from Omaha, Nebraska. As expected, Planarian Man is a superhero who is half planarian and half human. His origin story states that he came to be when a young boy cut his finger while slicing a planarian as part of a high school experiment. A small piece of the worm got into the cut and eventually the boy began changing, until he became the crime-fighting Planarian Man.

6. Planarians display behaviors very similar to addiction when given many of the same drugs that humans abuse.

For quite a long time, scientists gave planarians a wide variety of drugs and other chemicals to explore aspects of physiology, but not to study specific areas like addiction. That changed in 2001, when a group led by Dr. Robert Raffa of Philadelphia's Temple University published a paper describing planarian behaviors resembling “withdrawal symptoms” upon exposure to cocaine. This paper triggered a renewed interest on the systematic research on planarian pharmacology. Since then, many other drugs have joined the list of substances that can induce addiction-like behaviors in these worms, including nicotine, amphetamines, and cannabinoids, among many other drugs. Now, speaking of pot…

7. Planarians Have been mentioned on The Big Bang Theory.

One of Theory's main characters is neuroscientist Amy Farrah Fowler, who is played by actual neuroscientist Mayim Bialik. In the episode "The Monster Isolation," which first aired on February 21, 2013, Amy was complaining about some nicotine-addicted monkeys that were going through withdrawal in a rather rough way, then said, “... This makes me miss my marijuana-abusing flatworms; those guys were mellow!”

8. Two other TV shows also mentioned planarians... Both in the same day and the same time!

On March 16, 2014, both The Walking Dead and the reimagined Cosmos documentary series, hosted by Neil deGrasse Tyson, talked about planarians. In The Walking Dead episode “The Grove”, a girl named Mika says, “I miss science class. Except when we had to do gross stuff like cut up planaria worms.” And in the episode “Some of the things that molecules do,” Cosmos used planarians in a segment about evolution. Unfortunately, the animation featured a freshwater planarian in a marine environment, and the animation moved nothing like a freshwater planarian. I guess I should be happy that they showed planarians at all.

9. Many species of planarians can regenerate lost parts—including their heads, which contain rudimentary but fully-functional brains.

A.G. Pagán. Adapted from The First Brain: The Neuroscience of Planarians. Copyright Oxford University Press.

If you cut a planarian into several pieces, over time each piece will regenerate into a complete worm. And if you decapitate a planarian, it will not die. The head will keep on living and moving, and will eventually regenerate a new body. The headless body eventually will regenerate a new head, including its brain and nervous system.

Speaking of nervous systems, in contrast to most bilateral animals (those that display unambiguously right and left sides), planarians have not one, but two nerve cords that run along their bodies. It's unclear why they need two of them.

10. If you cut planarians in a particular way, you can make them grow multiple heads.

Yes, you read that right: Like the mythical hydra, planarians can sprout several heads—in some cases, as many as 10. The phenomenon was first reported in 1814, in John Graham Dalyell's book, Observations of Some Interesting Phenomena in Animal Physiology, Exhibited by Several Species of Planariae. (Interestingly, Dalyell was not a trained scientist, but a lawyer; you can read more about the amateur naturalist in this book.) More recently, scientists have begun to unravel some of the molecular mechanisms that can trigger the development of multiple heads in planarians. This paper, though quite technical, has some really nice pictures of multi-headed worms.

And it gets better (or weirder!). If you cut a planarian in half, normally the head portion develops a new tail and the tail portion develops a new head. Very recently, scientists have learned how to make a tail portion grow another tail in place of a head and an anterior portion to develop another head instead of a tail. Here are a couple of examples:

Dr. Junji Morokuma, Levin lab, Tufts University

11. Planarians are capable of learning, and upon decapitation, the bodies with newly regenerated heads will remember what they learned.

In the 1950s and 1960s, experimental psychologist James V. McConnell and collaborators did a series of experiments using planarians to explore memory processes. Some of these experiments seemed to indicate that if you trained planarians to respond to certain stimuli, not only did they remember, but if you cut their heads off and allowed the bodies to regenerate a new head, many of the regenerated worms actually remembered their training!

For a series of complex reasons, a significant fraction of the scientific community did not trust these experiments, citing problems with the use of appropriate controls, observer bias, and other less polite reasons. But in 2013, a group led by Dr. Mike Levin at Tufts University published a really interesting paper where they conclusively demonstrated that planarians can indeed learn, and that the tail will remember.

12. If you transplant the brain of a planarian to the body of another planarian, the transplant will hold and eventually control its new body.

There are several examples of experiments where researchers removed a planarian’s brain and “installed” it in another worm’s body, with at least partial recovery of function. As if this were not remarkable enough, sometimes these brain transplants were successful even when the two worms belonged to different species. Even more bizarre experiments explored what would happen if they dissected a marine planarian brain and put it back in the same worm at an angle, sometimes even backwards. Again, the worms displayed at least some partial recovery. You can find a more detailed account of these experiments my book, The First Brain: The Neuroscience of Planarians.

13. DARWIN STUDIED PLANARIANS.

Charles Darwin himself studied certain land planarians and, although not the first to observe it, he noted their regenerative abilities. In his own words:

“Having cut one of them transversely into two nearly equal parts, in the course of a fortnight both had the shape of perfect animals. I had, however, so divided the body, that one of the halves contained both the inferior orifices, and the other, inconsequence, none. In the course of twenty-five days from the operation, the more perfect half could not have been distinguished from any other specimen.”