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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.”

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Watch Christmas Island’s Annual Crab Migration on Google Street View
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Every year, the 45 million or so red crabs on the remote Australian territory of Christmas Island migrate en masse from their forest burrows down to the ocean to mate, and so the female crabs can release their eggs into the sea to hatch. The migration starts during the fall, and the number of crabs on the beach often peaks in December. This year, you don’t have to be on Christmas Island to witness the spectacular crustacean event, as New Atlas reports. You can see it on Google Street View.

Watching the sheer density of crabs scuttling across roads, boardwalks, and beaches is a rare visual treat. According to the Google blog, this year’s crabtacular finale is forecasted for December 16, and Parks Australia crab expert Alasdair Grigg will be there with the Street View Trekker to capture it. That is likely to be the day when crab populations on the beaches will be at their peak, giving you the best view of the action.

Crabs scuttle across the forest floor while a man with a Google Street View Trekker walks behind them.
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Google Street View is already a repository for a number of armchair travel experiences. You can digitally explore remote locations in Antarctica, recreations of ancient cities, and even the International Space Station. You can essentially see the whole world without ever logging off your computer.

Sadly, because Street View isn’t live, you won’t be able to see the migration as it happens. The image collection won’t be available until sometime in early 2018. But it’ll be worth the wait, we promise. For a sneak preview, watch Parks Australia’s video of the 2012 event here.

[h/t New Atlas]

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25 Species That Have Made Amazing Comebacks
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Conservationists can’t afford to become complacent. When it comes to rescuing endangered species, progress is an ongoing effort. Still, we can take comfort in the knowledge that many life forms which were once on the brink of extinction or endangerment have made tremendous comebacks with our help. Just look at what happened to these 25 plants and animals.

1. THE BALD EAGLE

A profile of a bald eagle on a black background
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For much of the twentieth century, this American icon was in jeopardy. Habitat loss, overhunting, and the widespread use of DDT—an insecticide which weakens avian eggshells—once took a major toll on bald eagles. By 1963, the species population in the lower 48 states had fallen from an estimated 100,000 individuals to just 417 wild pairs. To turn things around, the U.S. government passed a series of laws, including a 1973 ban on DDT that was implemented by the Environmental Protection Agency (EPA). These efforts paid off; today, approximately 10,000 wild breeding pairs are soaring around in the lower 48.

2. THE ARABIAN ORYX

arabian oryx
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The Arabian oryx is a kind of desert antelope indigenous to the Middle East. Reckless hunting devastated the species, which became essentially extinct in the wild during the early 1970s. However, a few individual animals were still alive and well in captivity. So, in the 1980s, American zoos joined forces with conservationists in Jordan to launch a massive breeding program. Thanks to their efforts, the oryx was successfully reintroduced to the Arabian Peninsula, where over 1000 wild specimens now roam (with a captive population of about 7000).

3. THE GRAY WOLF

gray wolves
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Even well-known conservationists like Theodore Roosevelt used to vilify America’s wolves. Decades of bounty programs intended to cut their numbers down to size worked all too well; by 1965, only 300 gray wolves remained in the lower 48 states, and those survivors were all confined to remote portions of Michigan and Minnesota. Later, the Endangered Species Act enabled the canids to bounce back in a big way. Nowadays, 5500 of them roam the contiguous states.

4. THE BROWN PELICAN

Brown pelican perched on a dock piling
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Louisiana’s state bird, the brown pelican, is another avian species that was brought to its knees by DDT. In 1938, a census reported that there were 500 pairs of them living within the Pelican State’s borders. But after farmers embraced DDT in the 1950s and 1960s, these once-common birds grew scarce. Things got so bad that, when a 1963 census was conducted, not a single brown pelican had been sighted anywhere in Louisiana. Fortunately, now that the era of DDT is over, the pelican’s back with a vengeance on the Gulf Coast and no longer considered endangered.

5. ROBBINS’ CINQUEFOIL

Robbins' Cinquefoil

Wikimedia Commons // Public Domain

Noted for its yellow flowers, Robbins’ cinquefoil—or Potentilla robbinsiana—is an attractive, perennial plant that’s only found in New Hampshire’s White Mountains and Franconia Ridge. Collectors once harvested the cinquefoil in excessive numbers and careless backpackers trampled many more to death. In response, the U.S. Fish and Wildlife Service re-routed hiking trails away from the flower’s wild habitats. This, along with a breeding program, rescued the Robbins' cinquefoil from the brink of extinction.

6. THE AMERICAN ALLIGATOR

Alligator emerging from swamp
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With its population sitting at an all-time low, the American alligator was recognized as an endangered species in 1967. Working together, the Fish and Wildlife Service and governments of the southern states the reptiles inhabit took a hard line against gator hunting while also keeping tabs on free-ranging communities. In 1987, it was announced that the species had made a full recovery.

7. THE NORTHERN ELEPHANT SEAL

Elephant seal winking
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Due to its oil-rich blubber, the northern elephant seal became a prime target for commercial hunters. By 1892, some people were beginning to assume that it had gone extinct. However, in 1910, it was discovered that a small group—consisting of less than 100 specimens—remained at large on Guadalupe Island. In 1922, Mexico turned the landmass into a government-protected biological preserve. From a place of security, that handful of pinnipeds bred like mad. Today, every single one of the 160,000 living northern elephant seals on planet Earth are that once-small group’s descendants.

8. THE HUMPBACK WHALE

humpback whale
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Did you know that the world’s humpback whale population is divided into 14 geographically-defined segments? Well, it is—and in 2016, the National Ocean and Atmospheric Administration (NOAA) informed the press that nine of those clusters are doing so well that they no longer require protection under the U.S. Endangered Species Act. The cetaceans’ comeback is a huge win for the International Whaling Commission, which responded to dwindling humpback numbers by putting a ban on the hunting of this species in 1982. (That measure remains in effect.)

9. THE RED WOLF

red wolf
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After the red wolf was declared “endangered” in 1973, the U.S. Fish and Wildlife Service rounded up every wild member of the species they could find and put them all into captivity. By then, the canid’s formerly wide geographical range had been reduced to a small portion of coastal Texas and Louisiana. FWS officials only managed to locate 17 wolves—14 of whom helped kick off a successful breeding program. Meanwhile, the red wolf was declared extinct in the wild in 1980. But thanks to those original 14 animals, we now have a captive red wolf population of 200. The FWS has also used their stock to release additional wolves into national wildlife refuges.

10. THE WHITE RHINO

rhino with birds
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Make no mistake: The long-term survival of Earth’s largest living rhino is still very uncertain because poachers continue to slaughter them en masse. Nevertheless, there is some good news. Like black-footed ferrets and northern elephant seals, white rhinos were once presumed to be extinct. But in 1895, just under 100 of them were unexpectedly found in South Africa. Thanks to environmental regulations and breeding efforts, more than 20,000 are now at large.

11. THE WILD TURKEY

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It’s hard to imagine that these poultry birds were ever in any real trouble, and yet they looked destined for extinction in the early 20th century. With no hunting regulations to protect them, and frontiersmen decimating their natural habitat, wild turkeys disappeared from several states. By the 1930s, there were reportedly less than 30,000 left in the American wilderness. Now, over 6 million are strutting around. So what changed? A combination of bag limits set by various agencies and an increase in available shrublands.

12. THE BLACK-FOOTED FERRET

black-footed ferret

USFWS Mountain-Prairie, Flickr // CC BY 2.0

North America’s only indigenous ferret is a prairie dog-eater that was written off as “extinct” in 1979. But the story of this animal took a surprising twist two years later, when a Wyoming pooch gave a freshly-dead one to its owner. Amazed by the canine’s find, naturalists soon located a wild colony. Some of these ferrets were then inducted into a breeding program, which helped bring the species’ total population up to over 1000.

13. THE CALIFORNIA CONDOR

California condor
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Since 1987, the total number of California condors has gone up from 27 birds to about 450, with roughly 270 of those being wild animals. With its 10-foot wingspan, this is the largest flying land bird in North America.

14. THE GOLDEN LION TAMARIN

two tamarins
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A flashy, orange primate from Brazil’s Atlantic Forest, the golden lion tamarin has been struggling to cope with habitat destruction. The species hit rock-bottom in the early 1970s, when fewer than 200 remained in the wild. A helping hand came from the combined efforts of Brazil’s government, the World Wildlife Federation, public charities, and 150 zoos around the world. There’s now a healthy population of captive tamarins tended to by zookeepers all over the globe. Meanwhile, breeding, relocation, and reintroduction campaigns have increased the number of wild specimens to around 1700—although urban sprawl could threaten the species with another setback. But at least the animal doesn’t have a PR problem: Golden lion tamarins are so well-liked that the image of one appears on a Brazilian banknote.

15. THE ISLAND NIGHT LIZARD

island night lizard

Native to three of California’s Channel Islands, this omnivorous, four-inch reptile was granted federal protection under the Endangered Species Act in 1977. The designation couldn’t have come at a better time, as introduced goats and pigs were decimating the night lizard’s wild habitat in those days. But now that wild plants have been reestablished under FWS guidance, more than 21 million of the reptiles are believed to be living on the islands.

16. THE OKARITO KIWI

Photo of an Okarito kiwi at a rearing facility at West Coast Wildlife Centre in Franz Josef, New Zealand.

Small, flightless, island birds usually don’t fare well when invasive predators arrive from overseas. (Just ask the dodo.) New Zealanders take great pride in the five kiwi species found exclusively in their country, including the Okarito kiwi, which is also known as the Okarito brown or rowi kiwi. These animals have historically suffered at the hands of introduced dogs and stoats. But recently, there’s been some cause for celebration. Although there were only about 150 Okarito kiwis left in the mid-1990s, conservation initiatives have triggered a minor population boom, with about 400 to 500 adult birds now wandering about—and that population is growing by two percent a year. Taking note of this trend, the International Union for Conservation of Nature (IUCN) has just declared that the Okarito kiwi is no longer endangered.

17. THE BROWN BEAR

brown bear
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Let’s clear something up: The famous grizzly bear technically isn’t its own species. Instead, it is a North American subspecies of the brown bear (Ursus arctos), which also lives in Eurasia. Still, grizzlies are worth mentioning here because of just how far they’ve come within the confines of Yellowstone National Park. In 1975, there were only 136 of them living inside the park. Today, approximately 700 of them call the place “home,” a turn of events that led to the delisting of Yellowstone’s grizzlies as a threatened species under the Endangered Species Act earlier this year.

18. THE THERMAL WATER LILY

thermal water lily

With pads that can be as tiny as one centimeter across, the thermal water lily is the world’s smallest water lily. Originally discovered in 1985, it was only known to grow in Mashyuza, Rwanda, where it grew in the damp mud surrounding the area’s hot spring. Or at least it did. The thermal water lily seems to have disappeared from its native range. Fortunately, before the species went extinct in the wild, some seeds and seedlings were sent to London’s Royal Botanic Gardens. There, horticulturalists figured out a way to make the lilies flower in captivity, and managed to saved the species.

19. THE PEREGRINE FALCON

Peregrine falcon flying
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When a peregrine falcon dives toward its airborne prey, the bird-eating raptor has been known to hit speeds of up to 242 miles per hour. The species endured a plummet of a different sort when DDT dropped America’s population. In the first few decades of the 20th century, there were around 3900 breeding pairs in the United States. By 1975, the number of known pairs had been whittled down to 324. Things got better after the insecticide was banned, and according to the FWS, somewhere between 2000 and 3000 peregrine falcon couples currently patrol the skies in the United States, Canada, and Mexico.

20. PRZEWALSKI’S HORSE

Photo of a a wild Przewalski's horse on a snow covered field in the Chernobyl exclusions zone
GENYA SAVILOV/AFP/Getty Images

There are a few different subspecies of wild horse, all of which are endangered. One variant is the Przewalski’s horse (Equus ferus perzewalskii) from Mongolia. It completely vanished from that nation during the 1950s, but by then assorted zoos around the world had started breeding them. From 1992 to 2004, some 90 captive-born horses were released into Mongolia. They thrived and around 300 are living out there today.

21. THE NORTH AMERICAN BEAVER

North American beaver
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No one knows how many of these buck-toothed rodents were living on the continent before European fur traders showed up. But after two centuries of over-trapping, incentivized by the lucrative pelt trade, the number of North American beavers had shrunk to an abysmal 100,000 in 1900. Their fortunes reversed when restocking programs were implemented in the U.S. and Canada. Nowadays, somewhere between 10 and 15 million beavers live in those countries. Given their landscaping talents, many property owners have come to see the furballs as pests.

22. THE CAFÉ MARRON

Cafe Marron tree

Rodrigues Island in the Indian Ocean once gave biologists a chance to raise the (near) dead. This landmass is the home of a small tree with star-shaped flowers called the café marron. It was thought that the plant had long since died out when a single specimen was found by a schoolboy named Hedley Manan in 1980. As the only surviving member of its species known to mankind, that lone plant assumed paramount importance. Cuttings from the isolated café marron were used to grow new trees at England’s Royal Botanical Gardens. Right now, there are more than 50 of these plants—and all of them can have their ancestry traced straight back to that one holdout tree.

23. THE WEST INDIAN MANATEE

Manatee with fish

A docile, slow-moving marine mammal with a taste for sea grasses, the Floridian subspecies of the West Indian manatee is a creature that does not react well to razor-sharp propellers. Collisions with boats are a significant threat, and the danger won’t go away altogether. Still, the passage of tighter boating regulations has helped the Sunshine State rejuvenate its manatee population, which has more than tripled since 1991.

24. THE BURMESE STAR TORTOISE

Burmese star tortoise
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The pet trade did a number on these guys. Beginning in the 1990s, wildlife traffickers harvested Burmese star tortoises until they effectively became “ecologically extinct” in their native Myanmar. Luckily, conservationists had the foresight to set up breeding colonies with specimens who’d been confiscated from smugglers. The program started out with fewer than 200 tortoises in 2004; today, it has more than 14,000 of them. “Our ultimate objective is to have about 100,000 star tortoises in the wild,” Steve Platt, a herpetologist who’s been taking part in the initiative, said in a Wildlife Conservation Society video.

25. THE GIANT PANDA

panda in tree
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Here we have it: the poster child for endangered animals everywhere … except that the giant panda is no longer endangered. Last year, the IUCN changed its status from “endangered” to “vulnerable.” There’s still a chance that we could lose the majestic bamboo-eater once and for all someday, but the last few years have offered a bit of hope. Between 2004 and 2014, the number of wild pandas saw a 17 percent increase. The welcome development was made possible by enacting a poaching ban and seeing an explosion of new panda reserves. It’s nice to know that, with the right environmental policies, we can make the future brighter for some of our fellow creatures.

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