Tapeworms can reproduce by tearing themselves in half
When planar tapeworms want to mate, some have sex. Others, more frankly, are tearing themselves in two.
This last option is quick and violent. The planarian begins as a small, flattened, slug-like creature with a spade-shaped head and two googly eyes. After a few minutes of stretching and tearing, it separates into two halves: a head and a tail. In a few days, the head piece grows a tail. And even more miraculously, the tail pushes back the head. “It’s just mind-blowing,” Eva-Maria Collins of Swarthmore College, who studies these animals, told me. Breeding them is a snap: with enough food, the planarians will repeatedly double themselves by halving. And if Collins needs more animals fast, she can do with a scalpel what worms do with their own muscles. As naturalist John Graham Dalyell wrote in 1814, planarians could “almost be called immortals by the edge of a knife.”
There are thousands of species of planarians, and they are all very different from the more familiar worms like earthworms. Their bodies are weaves of muscle and connective tissue, without internal cavities filled with soft organs. The mouth sits in the middle of the underside and also serves as a anus. They release liquid waste through the pores on their backs. They receive oxygen by diffusion and lack lungs, gills, heart and blood vessels. They have kinds of brains – two groups of neurons in the head. These lead to a ladder-like nervous system of two nerve cords that run through the body and are connected by crossed bars.
This unusual anatomy is even stranger because it can tolerate bisection. The feat has puzzled people since at least the ninth century, but it is difficult to observe. Planarians only self-fragment once a month and the process is complete in minutes. They also prefer to go their separate ways in the dark and will stop if disturbed. To study them, Collins and his team filmed a species, Dugesia japonica, continuously for months. They saw that the creature begins its self-dissection by contracting its midsection to create a waist, changing its shape from a cigar to an hourglass. It then anchors its head and tail – to a laboratory Petri dish, but usually to an underwater rock in the wild – and contracts the intervening muscles, repeatedly stretching the flesh from the waist up that it breaks. (The process varies among species; in Schmidtea mediterranea, the bigger the worm, the more pieces he can tear off his cock.)
Once a worm has split open, the fragments don’t stay there. As soon as they are released from each other, the two parts appear to “possess the properties of a perfect animal, moving in the water in the same slippery manner as before the separation.” naturalist James Rawlins Johnson wrote in 1822. Two centuries later, Collins showed that this autonomy runs deeper than previously suspected. His team (comprising undergraduates Dylan Le and Ziad Sabry and a high school student, Aarav Chandra) have shown that an intact planarian will spin if pushed into the head, stretch if pushed in the middle, and twist. will contract if pushed into the tail. But if this planar is cut into three sections – head, trunk and tail –each room behaves like the full creature. The front end of the trunk will turn as if it were a head, and the rear end will contract as if it were a tail.
Collins believes that the neural circuits controlling these behaviors repeat themselves along the planar, so that every part of the body is able to act like a head, trunk, or tail. The presence of an actual head normally prevents downstream regions from acting as such. But when decapitating, the foremost part of the remaining planarian can take over the functions of the now missing head. Collins views this extreme adaptability as a survival strategy. This means that each shard can flee from danger, giving it enough time for its extraordinary regenerative powers to kick in.
Not all planarians can regenerate, but those that can tend to be spectacular there. When food is scarce, they can “decreaseâ€By destroying their own cells, to regain volume when conditions improve. They challenge the aging process by replace their old tissues and organs. They can recover from almost any physical injury (although some are more delicate; an incision between the eyes can prompt a planarian to regenerate two heads). Biologist Thomas Hunt Morgan once estimated that a complete planar could regenerate from just one 279th of its body. “Few animals can regenerate their nervous system, and I’m not aware of it all others that can regrow a brain, â€says Oné Pagán of West Chester University in Pennsylvania, who wrote The first brain: the neuroscience of planarians.
When planarians divide naturally, the head fragment is usually larger and contains the brain, eyes, smell and taste sensors, and the mouth. “The head piece just needs to heal the wound and move on,” Collins told me. The tail, on the other hand, has to regenerate everything else. Without a mouth, it has no way of acquiring nutrients. Instead, some of its cells self-destruct to provide the raw material to make new flesh. Slowly, the isolated tail undergoes “massive remodeling,” Alejandro Sánchez Alvarado, a planar expert at the Stowers Institute, told me, “and you end up with a tiny version of the original animal.” Tail pieces are about 10 times more likely to die than head pieces, Collins added, but still, about seven in eight survive.
These powers depend on special cells called neoblasts, which have only been found in planarians. They are distributed throughout the creature’s body, constituting approximately 25 to 30 percent of its cells. In 2014, a team led by Peter Reddien from MIT bombed a planar with a lethal dose of radiation and transplanted a single neoblast–just one–of a second individual on the tail of the condemned animal. When the recipient died from head down, the transplanted neoblast began to produce new tissue from the tail up. The new cells ended up replacing all the dying, as if the donor planaria, through a single cell, had taken over and revitalized the recipient’s corpse. After two weeks, a complete and healthy animal — a planar of Theseus– walked away.
Of course, most animals grow from a single fertilized egg. But as this egg becomes an embryo, the cells it contains become more rigid. A skin cell does not turn into a neuron. Stem cells are more flexible, but in adult animals they even have their limitations: a blood stem cell cannot make liver or heart cells. Neoblasts of adult planarians have no such restrictions. They are pieces of limitless possibilities, capable of producing any tissue or organ.
Neoblasts do not work in isolation. The one Reddien transplanted didn’t start making eyes or brains; he created fabrics appropriate to his location. This is because the concentrations of certain molecules change along the planar, back and forth, and up and down. This creates a sort of coordinate system, which tells cells in each section where they are in the overall plane of the body. They can use this information during the regeneration process to regrow what is needed. Buds regenerate tails, not extra buds (although errors can occur). Tails make heads. The trunks grow heads and tails. And whatever their origins, new animals seem to remember something from their past existence.
In the 1950s and 1960s, biologist James V. McConnell has shown that the headless planarians who were forced to regrow their brains could still remember the behaviors they had learned before their beheading. He even published results suggesting that untrained planarians could adopt behaviors that the trained had learned if the elders cannibalized crushed pieces of the latter. Skeptics criticized these experiments and argued that McConnell simply saw the behavior he wanted to see. But decades later, Mike Levin and Tal Shomrat of Tufts University developed a machine which could automatically form and track planarians without any human interference or bias. They showed that worms trained to recognize the texture of a rough petri dish could still do so after being beheaded and growing new buds.
At a minimum, Levin argues, it shows that memories can indeed be stored outside the brain. It also reinforces her feeling that the classical view of memory – that it is encoded by the strength of synaptic connections between different neurons – is wrong. Instead, Levin suspects that nervous systems may have evolved to to interpret memories and not encode them; they are stored elsewhere, in an aspect of our cells that no one has yet identified. This is, to be clear, highly speculative. “We only have one study and it’s far from definitive,†Levin told me. “But this is one of the many data suggesting that we don’t really understand memory at all.”
Planarians also complicate other seemingly simple concepts. Consider a question that Levin and his colleagues installed in 2016: After a planar cut in half regenerates into two new animals, would the planar that grew from the head consider the one that grew from the tail as its double, his brother and sister, his child, Where himself? The answer is not obvious, as these words have been defined by humans – a species which, the last time I checked, cannot reproduce by tearing itself apart. “The things that are weird are exactly the things you need to watch out for,†Levin told me. “They tell you that your model of the world is incomplete in many ways. You must cherish the exceptions.
Planarians are certainly exceptional, but they are not unique in their talents. Many other animals can regenerate missing body parts, including salamanders, lizards, and starfish. Several can reproduce by dividing into two, including sea anemones and tentacles. Hydra. Many scientists are studying these creatures in hopes of finding medical breakthroughs that can restore damaged organs and lost limbs. But the most immediate price is realizing how incomplete our understanding of nature is, and how our language and concepts have been constrained by our own inflexible and indivisible bodies.