For many sea creatures, regrowing a lost limb is routine. But when a young jellyfish loses a tentacle or two to the jaws of a sea turtle, for example, it rearranges its remaining limbs to ensure it can still eat and swim properly, according to a new study published June 15 in Proceedings of the National Academy of Sciences. The discovery should excite marine enthusiasts and roboteers alike, the authors say, because the jellyfish’s strategy for self-repair may teach investigators how to build robots that can heal themselves. “It’s another example of nature having solved a problem that we engineers have been trying to figure out for a long time,” says John Dabiri, a biophysicist at Stanford University who had discussed the project with the study investigators but was not involved with the research.
The surprise of symmetrization
The discovery happened almost by accident, says Michael Abrams, a PhD student at California Institute of Technology and the study’s lead author. Abrams, along with Lea Goentoro, his advisor and an assistant professor of biology at Caltech, had initially set out to study Turritopsis dohrnii, a species of chandelierlike jellyfish that has achieved biological immortality with its ability to transform back into a polyp at any stage of its life. But these largely unstudied jellyfish proved difficult to acquire so the lab began experimenting with a different species, Aurelia aurita, while waiting for the immortals to arrive. Aurelia, also called the moon jellyfish, are incredibly common and easily identified by the four crescent-shaped gonads on their umbrellas. “I started doing old-school type experiments from 150 years ago, where you just cut things up and see what happens,” Abrams says. He selected some of the ephyra, the free-swimming, larvae of the moon jellyfish, which resemble tiny starbursts, each with eight symmetrical arms radiating out of a disk-shaped body. Then he amputated two limbs from an anesthetized ephyra—fully expecting the its limbs to regrow, as is the case with many marine invertebrates and even the polyp stage of the moon jellyfish.
Instead, he saw something else entirely. Rather than regenerating its amputated arms, the young jellyfish rearranged its remaining limbs over the course of the next 18 hours until they were equally distributed around its body. By re-creating a semblance of its original symmetry, the animal recovered its ability to survive in the ocean.
The particular arrangement of a jellyfish's tentacles is critical for it to swim and eat properly. Jellyfishes have what’s called radial symmetry, which can look anything like a snowflake to a disk, as long as it can be divided like a pie to produce identical pieces. Jellyfishes move by flapping their arms to propel themselves through the water, creating a pulse in their bodies. In each pulse the jellyfish’s body deforms for propulsion, taking in nutrient-rich water then returning to its original, saucerlike shape in the recovery. These pulses require perfect radial symmetry for the jellyfish to bob in balance. Lop off a few limbs and the animal will spiral and meander through the water and become an alarmingly easy target for lunch.
A simple matter of physics
These jellyfish put themselves back together in a process that the Caltech team called “symmetrization.” Unlike regrowing a limb, this self-repair mechanism neither creates nor destroys cells. Rather, it relies on the sheer power of jellyfish muscle—which is stronger than it sounds.
The researchers found that the muscle contractions exerted by each pulse of the jellyfish with a missing limb forced its other arms to space out equally. The sudden crowding sensation of its remaining arms caused the jellyfish to push its limbs away from one another and toward the empty space, thus forming a more stable configuration. “In one pulse, it may look like it’s going back to its original form,” Abrams says. “But that pulse over thousands of times makes symmetry.”
Imagine a wagon that has lost a front wheel. Without changing the placement of the remaining three wheels, the wagon would be stuck. Centering the remaining front wheel, however, would rebalance the vehicle as a fully functional wheelbarrow. Same function, different body.
To determine the driving force behind symmetrization, the investigators turned to the muscles themselves. When the researchers added muscle relaxants to the seawater, the muscle contractions of the jellyfish slowed down, as expected. After amputation the creatures also took longer to reorganize their appendages. Conversely, the jellyfish symmetrized faster when researchers reduced the amount of magnesium, a common mineral that also relaxes muscles, in the seawater.
Aurelia’s ability to push its limbs into order is embedded in what Abrams calls “the gooeyness” of its body. Both squishy and elastic, jellyfishes can snap like a rubber band over a short periods of time and ooze like goo over longer periods. When jellyfishes move, their tissue alternates between these states of tautness and gooeyness. The technical term for a material’s dual expression of fluid and elastic properties is viscoelasticity. These taut/goo cycles essentially push the jellyfish’s limbs away from one another until they are equally spaced apart.
This shortcut to healing is a lifesaver for a creature literally made of gelatin. At any given time, 33 to 47 percent of seafloor invertebrates are injured, according to a 2010 study in Integrative and Comparative Biology. In other words, it’s hard to be soft.
The discovery explains a long-standing, if relatively minor mystery in the world of jellyfish research. Scientists have long reported seeing “freak” jellyfishes with fewer than eight limbs but could never tell whether these anomalies were the product of accident, mutation or something else entirely, says Jack Costello, a professor of biology at Providence College who specializes in jellyfish and was not involved with the study. But these “accidents” may help investigators prevent a different kind of body—this time an artificial one—from accidents of its own.
Molding the field of soft robotics
Symmetrization may provide valuable insight to research in soft robotics, a relatively new field that draws inspiration from “soft” organisms like octopuses and worms, which lack rigid structures.
Inspired by that dexterity, designers aim to create much greater freedom of movement in soft robots. This mobility would endow soft robots with the ability to solve delicate tasks in unstructured environments. In 2013 researchers at Harvard University built a locomotive soft robot inspired by a starfish that could navigate obstacles by stretching and compressing its malleable body.
This characteristic softness, however, makes these robots uniquely vulnerable to cuts and punctures from sharp objects. Designing a soft robot able to recover from damage would solve one of the field’s greatest challenges, according to Dabiri. Before the PNAS study, researchers had sought to tackle this problem by inventing a creative way to regrow lost tissue. But symmetrization may offer a better shortcut. “Jellyfish have shown us maybe a simpler and more elegant solution: redistributing tissue to maintain function in a different body than you started off with,” Dabiri says.
The quest for suitable materials, which poses another major challenge to the field of soft robotics, may also draw inspiration from the moon jellies. The viscoelasticity of a jellyfish’s body makes it remarkably muscular and efficient. Jellyfishes, which are 95 percent water, have “the highest miles per gallon of any animal we know of,” Dabiri says. Their gooey and elastic pulses propel them through the water using very little energy, according to a study published two years ago in PNAS.
In 2012 Dabiri and biophysicist Kit Parker of Harvard University bioengineered an artificial jellyfish that looked and swam like the real thing, despite being built entirely out of cells from a rat’s heart. Dabiri says the latest findings give him insight into taking his creation out of the controlled environment of his lab and up against the sharp objects of the real world.